The present invention relates to a method for forming a multilayer coating onto an object to be coated by applying at least three coating layers and jointly curing said coating layers. The pigments contained in the basecoat composition are selected such that each density difference between the pigments is 2 g/ml or less to prevent undesired color shifts of basecoat layers prepared from such basecoat materials after storage of said basecoat materials without mixing prior to their application to a substrate. A transparent or semi-transparent coating layer containing color and/or effect pigments is applied on the topmost basecoat layer to achieve high chroma with excellent depth of color and glamour. The present invention moreover relates to a coated object comprising a multilayer coating prepared according to the inventive method.

State of the art

Vehicles, in particular land vehicles such as automobile, motorcycle and truck bodies, are treated with multiple layers of coatings which enhance the appearance of the vehicle and also provide protection from corrosion, scratch, chipping, ultraviolet light, acid rain and other environmental conditions. Multicoat paint systems comprising basecoat and clearcoat layer(s) for automobiles and trucks have been commonly used over the past two decades.

Producing these multicoat paint systems generally involves electrophoretically depositing or applying an electrocoat material on the metallic substrate, such as an automobile body. The metallic substrate may undergo various pretreatments prior to the deposition of the electrocoat material - for example, known conversion coatings such as phosphate coatings, more particularly zinc phosphate coats, may be applied. Following application of the electrocoat material, the coated substrate is optionally rinsed and subjected to flashing and/or intermediate drying before the applied electrocoat material is cured. Film thickness of the cured electrocoating should be around 15 to 25 micrometers. Afterwards, a filler or primer-surfacer material may be applied to the cured electrocoat, optionally subjected to flashing and/or intermediate drying, and thereafter cured. If present, typical film thicknesses of the cured filler or primer-surfacer layer are in the range of 25 to 45 micrometers. In case such a layer is present, at least one basecoat material comprising color and/or effect pigments is applied to said cured layer. However, it is also possible to apply at least one basecoat material directly to the cured electrocoating layer. The applied basecoat material(s) is/are optionally subjected to flashing and/or intermediate drying. The at least one basecoat film or the topmost basecoat film thus produced is then coated with a clearcoat material without separate curing. The clearcoat film can be subjected to flashing and/or intermediate drying before any basecoat and clearcoat film(s) present are jointly cured (so-called 2 coat 1 bake (2C1 B) or 3 coat 1 bake (3C1 B) method). The cured basecoat layer preferably has film thicknesses of 10 to 60 micrometers, while film thicknesses of 30 to 60 micrometers are used for the cured clearcoat to achieve scratch resistance, mar resistance, environmental protection, gloss, and distinctness of image (DOI). With regard to environmental regulations, aqueous filler, primer-surfacer and basecoat compositions are preferably used.

The desire for unique and attractive coatings has led the auto industry to utilize a basecoat/ tinted clearcoat process, whereby a lightly pigmented clear coat is applied over a pigmented basecoat comprising color and/or effect pigments in a wet on wet application, to provide a multilayer coating of high optical quality and appearance with excellent color saturation, depth of color and glamour. Usually, this process involves applying a lightly pigmented clearcoat over a regular pigmented basecoat in the same color area, i.e., red over red, blue over blue, yellow over yellow, to significantly enhance the individual basecoat color shade and to provide very deep, clean, vibrant, high end colors.

Unfortunately, the durability of these tinted clear coat layers has left much to be desired, since the pigments contained in said layer are subject to UV degradation. Often, chalking, cracking and flaking of such multilayer coatings is observed after relatively short periods of exposure to weathering, thus resulting in the need for costly refinishing processes. Various ideas have been proposed to solve these durability problems. A first approach is directed to increase the UV resistance of said tinted clearcoat layer. A second approach is to apply an additional layer of a regular clearcoat on top of the basecoat/tinted clearcoat multilayer, in order to increase the UV resistance. This approach, however, significantly increases manufacturing costs, since the vehicle must be sent through the painting process a second time. Moreover, tinted clearcoat compositions are used only for a limited number of high-end colors while the rest of the color range in a basecoat/clearcoat process still requires the application of a regular untinted clearcoat, thus requiring separate circulation systems for such tinted clearcoat compositions in order to avoid contamination of the regular clearcoat lines. Such lines are, however, extremely expensive and occupy valuable floor space.

Yet another approach is to use a transparent or semi-transparent basecoat layer instead of the tinted clearcoat layer described in the second approach. Such basecoat materials can contain small amounts of opaque, semi-transparent or transparent effect pigments. Since already existing basecoat circulation lines can be used for the transparent or semi-transparent basecoat composition, this process does not require the installation of separate circulation systems. The previously described coating compositions may be applied via pneumatic and/or electrostatic spray application gear known to the skilled person. In auto manufacturer's paint shops such application gear is normally connected via a so-called tap line with a circulation line. Said circulation line is connected to the paint storage room and is used to keep the coating material in constant movement to ensure a homogenous composition of the coating material upon application to the substrate. Said homogenous composition is essential to reliably achieve the required optical and/or mechanical properties of the resulting coating layer.

In case of color providing basecoat materials, a separate circulation line is needed for each differently colored basecoat material. Since various differently colored basecoat materials are used in an auto manufacturer's paint shop to provide the desired color range, a particular basecoat material may not be applied during the coating process for a longer period in time. Because circulation is only ensured in the circulation lines but not in the tap lines connecting the circulation lines with the application gear, separation of pigments being present in the basecoat material into different pigment phases can occur inside said tap lines. This separation is particularly occurring in light colored basecoat materials, especially white basecoat materials, which are tinted with color pigments and/or effect pigments to obtain desired color shades and/or effects and to compensate for the variation in color resulting from quality differences of different batches of titanium dioxide. Said separation, however, results in basecoat layers not achieving the predefined color quality, such as the light-dark flop, the color flop or the color values, and thus renders it necessary to remove the separated basecoat material from the tap line by fresh, i.e. circulated, basecoat material, prior to its application to the automotive body. This process, however, increases the manufacturing costs due to the waste of basecoat material as well as the additional time needed for the cleaning of tap lines.

There is therefore a need to reduce or avoid defects in the optical quality, especially the color quality, of prepared multilayer coatings without increasing the manufacturing costs.

Object

Accordingly, the object of the present invention is a process to prepare multilayer coatings reliably achieving the required excellent optical and mechanical properties, especially having a constant highly saturated color appearance with excellent depth of color and desired glamour, without increasing the time and/or costs for the preparation of these multilayer coatings. The occurrence of defects in color of the prepared multilayer coatings should be reduced or completely avoided, even if the basecoat materials used to prepare the multilayer coatings are not circulated to provide a homogenous composition prior to their application. The process should be designed to be run in existing basecoat/clear coat painting facilities, such as continuous in-line or modular batch facilities located at vehicle assembly plants, without the need to reconfigure the existing painting equipment.

Technical solution

The objects described above are achieved by the subject matter claimed in the claims and also by the preferred embodiments of that subject matter that are described in the description hereinafter. A first subject of the present invention is therefore a method for forming a multilayer coating (MC) onto an object to be coated, said method comprising:

(i) producing a basecoat layer (BL) or at least two directly consecutive basecoat layers (BL-x) directly on the object to be coated by applying an aqueous basecoat material (BC) directly to the object to be coated or by applying directly in succession at least two aqueous basecoat materials (BC-x) directly to the object to be coated;

(ii) producing a further coating layer (CL) directly on the basecoat layer (BL) or on the topmost basecoat layer (BL-z) by applying a coating material (CM) comprising at least one pigment PCM directly to the basecoat layer (BL) or to the topmost basecoat layer (BL-z);

(iii) producing a clearcoat layer (CCL) directly on the further coating layer (CL) by applying a clearcoat material (CC) directly to the further coating layer (CL); and

(iv) jointly curing the basecoat layer (BL), the further coating layer (CL) and the clearcoat layer (CCL) or the basecoat layers (BL-x), the further coating layer (CL) and the clearcoat layer (CCL); wherein the at least one basecoat material (BC) or at least one of the basecoat materials (BC-x) comprises

• a first pigment P,

• at least one further pigment P _x being different from pigment P, and

• at least one binder B, wherein each difference Ap _x between the pigment density p(P) of the first pigment P and each pigment density p(P _x ) of each at least one further pigment P _x fulfills equation (1 )

Ap _x = p(P) - p(P _x ) < 2 g/ml (1 ) and wherein the further coating layer (CL) - when cured - has a luminous transmittance of at least 4 percent as measured at a film thickness of 15 to 18 micrometers according to ASTM D 1003-00 (procedure A) using a CIE standard illuminant D65.

The above-specified method is hereinafter also referred to as method of the invention and accordingly is a subject of the present invention. Preferred embodiments of the method of the invention are apparent from the description hereinafter and also from the dependent claims. In light of the prior art it was surprising and unforeseeable for the skilled worker that the object on which the invention is based could be achieved by using at least one specific aqueous basecoat material (BC, BC-x) comprising at least two pigments and in which the pigments are selected such that each difference between the pigment density of the first pigment P and each pigment density of each further pigment P _x is 2 g/ml or less. The exclusive use of pigment mixtures fulfilling equation (1 ) prevents the formation of different pigment fractions upon storage of the basecoat material without circulation, for example in tap lines, and thus ensures reliable excellent optical and mechanical properties, especially a constant highly saturated color appearance with excellent depth of color and desired glamour, of the multilayer coatings comprising basecoat layers being prepared from such basecoat materials without the need to guarantee a constant circulation and homogenization of the basecoat material prior to its application to the substrate. The present method moreover eliminates the use of tinted clearcoats and their associated problems without sacrificing durability and weatherability. The method of the present invention can be run in a batch or continuous process and is designed to be run in existing basecoat/clear coat painting facilities, such as continuous in-line or modular batch facilities located at vehicle assembly plants or plants producing tier parts (also called mounting parts) to be used during vehicle assembly, without the need to reconfigure or slow down the line or extend the painting time. Since the desired color enrichment can be obtained from a basecoat layer having a certain degree of transparency instead of a tinted clearcoat, there is no need for an extra clearcoat paint circulation system. Instead, the existing basecoat paint circulation systems accommodating multiple colors can be used.

A further subject of the present invention is a coated substrate produced by the inventive process.

Detailed description

Definitions:

First of all, a number of terms used in the context of the present invention will be explained. A “binder” in the context of the present invention and in accordance with DIN EN ISO 4618:2007-03 is the nonvolatile component of a coating composition, without pigments and fillers. Hereinafter, however, the expression is used principally in relation to particular physically and/or thermally curable polymers, examples being polyurethanes, polyesters, polyethers, polyureas, polyacrylates, polysiloxanes and/or copolymers of the stated polymers. A copolymer in the context of the present invention refers to polymer particles formed from different polymers. This explicitly includes both polymers bonded covalently to one another and those in which the different polymers are bound to one another by adhesion. Combinations of the two types of bonding are also covered by this definition. The nonvolatile fraction may be determined by the method described in the Examples section.

The term “(meth)acrylate” shall refer hereinafter both to acrylate and to methacrylate.

The term “color pigment” or “coloring pigment” refers to pigments producing an optical effect based on selective light absorption in conjunction with light scattering. Examples of color pigments include inorganic and organic color pigments. The term “inorganic color pigment” refers to natural and synthetically produced pigments based on inorganic compounds and includes white pigments, inorganic colored pigments and black pigments. The term “organic color pigment” refers to coloring agents which are practically insoluble in the application medium and includes azo pigments and polycyclic pigments, i.e. organic non-azo pigments characterized by at least one aromatic and/or heteroaromatic ring system.

In contrast, the term “effect pigment” or “visual effect pigment” refers to pigments producing an optical effect, such as a color or lightness effect, based primarily on light reflection, in particular on angle dependent light reflection. Examples of effect pigments include luster pigments, such as metal effect pigments, pearlescent pigments and interference pigments, flaky graphene, flaky iron oxide and micronized titanium dioxide.

The application of a coating composition to a substrate, or the production of a coating film on a substrate, are understood as follows: the respective coating composition is applied in such a way that the coating film produced therefrom is arranged on the substrate. However, a direct contact with the substrate is not necessary. Thus, other layers can be present between the coating film and the substrate. In contrast, the application of a coating composition directly to a substrate, or the production of a coating film directly on a substrate, results in a direct contact of the produced coating film and the substrate. Thus, more particularly, no other layer is present between the coating film and the substrate. Of course, the same principle applies to directly successive application of coating compositions or the production of directly successive coating films, for example in step (i) of the present invention.

The term “flashing off” denotes the vaporization of organic solvents and/or water present in a coating composition after application, usually at ambient temperature (i.e. room temperature), for example 15 to 35 °C for a period of, for example, 0.5 to 30 minutes. Since the coating composition is still free-flowing at least directly after the application in droplet form, it can form a homogeneous, smooth coating film by leveling. After the flash-off operation, the coating film, however, is still not in a state ready for use. For example, it is no longer free-flowing, but is still soft and/or tacky, and in some cases only partly dried. More particularly, the coating film still has not cured as described below.

In contrast, intermediate drying takes place at, for example, higher temperatures and/or for a longer period, such that, in comparison to the flash-off, a higher proportion of organic solvents and/or water evaporates from the applied coating film. Thus, intermediate drying is usually performed at a temperature elevated relative to ambient temperature, for example of 40 to 90 °C, for a period of, for example, 1 to 60 minutes. However, the intermediate drying does not give a coating film in a state ready for use either, i.e. a cured coating film as described below. A typical sequence of flash-off and intermediate drying operations would involve, for example, flashing off the applied coating film at ambient temperature for 5 minutes and then intermediately drying it at 80 °C for 10 minutes.

Accordingly, curing of a coating film is understood to mean the conversion of such a film to the ready-to-use state, i.e. to a state in which the substrate provided with the respective coating film can be transported, stored and used as intended. More particularly, a cured coating film is no longer soft or tacky, but has been conditioned as a solid coating film which does not undergo any further significant change in its properties, such as hardness or adhesion on the substrate, even under further exposure to curing conditions as described below.

In the context of the present invention, “physically curable” or the term “physical curing” means the formation of a cured coating film through release of solvent from polymer solutions or polymer dispersions, the curing being achieved through interlooping of polymer chains.

In the context of the present invention, “thermally” or the term “thermal curing” means the crosslinking, initiated by chemical reaction of reactive functional groups, of a paint film (formation of a cured coating film), it being possible to provide the activation energy for these chemical reactions through thermal energy. In the case of a purely physically curing coating composition, curing is performed preferably between 15 and 90 °C over a period of 2 to 48 hours. In this case, curing may thus differ from the flash-off and/or intermediate drying operation merely by the duration of the curing step.

In principle, and within the context of the present invention, the curing of thermally curable, especially preferably thermally curable and externally crosslinking, one- component systems is performed preferably at temperatures of 80 to 250 °C, more preferably 80 to 180 °C, for a period of 5 to 60 minutes, preferably 10 to 45 minutes. Accordingly, any flash-off and/or intermediate drying phase which precedes the curing is performed at lower temperatures and/or for shorter periods. In principle, and within the context of the present invention, the curing of thermally curable, especially preferably thermally curable and externally crosslinking, two-component systems is performed at temperatures of, for example, 15 to 90 °C, preferably 40 to 90 °C, for a period of 5 to 80 minutes, preferably 10 to 50 minutes. This of course does not rule out curing of a two-component system at higher temperatures. If, for example, both one- component and two-component systems are present within the films formed according to the inventive process, the joint curing is guided by the curing conditions needed for the one-component system, thus resulting in the use of higher curing temperatures as described for one-component systems. Accordingly, any flash-off and/or intermediate drying phase which precedes the curing is performed at lower temperatures and/or for shorter periods. A filler layer (primer-surfacer layer) is describing an intermediate layer used to fill out the irregularities of the substrate, to support corrosion resistance and adhesion as well as to provide protection from mechanical exposure such as stone chipping. To allow the cured filler layer to fulfill the objectives identified above, film thicknesses of 25 to 45 micrometers are commonly used.

A primer layer is describing the first layer of a multilayer coating which is applied onto the substrate and is used to provide improved adhesion for the multilayer coating. Moreover, the primer layer can provide improved corrosion protection, for example on metallic substrates.

The measurement methods to be employed in the context of the present invention for determining certain characteristic variables can be found in the Examples section. Unless explicitly indicated otherwise, these measurement methods are to be employed for determining the respective characteristic variable. Where reference is made in the context of the present invention to an official standard without any indication of the official period of validity, the reference is implicitly to that version of the standard that is valid on the filing date, or, in the absence of any valid version at that point in time, to the last valid version.

All film thicknesses reported in the context of the present invention should be understood as dry film thicknesses. It is therefore the thickness of the cured film in each case. Hence, where it is reported that a coating material is applied at a particular film thickness, this means that the coating material is applied in such a way as to result in the stated film thickness after curing.

All temperatures elucidated in the context of the present invention should be understood as the temperature of the room in which the substrate or the coated substrate is located. It does not mean, therefore, that the substrate itself is required to have the temperature in question. Inventive Method:

The process of this invention is suitable for coating a variety of metallic and non- metallic substrates in a batch or continuous process. In a batch process, also referred to as a modular process, the substrate is stationary during each treatment step of the process, whereas in a continuous process the substrate is in continuous movement along the paint line in an assembly line fashion.

Suitable objects to be coated according to the method of the invention include (i) uncoated metal substrates or metal substrates being coated with a cured electrocoat layer and/or a cured filler layer and/or a cured multilayer coating; (ii) plastic substrates optionally being coated with a cured primer layer; and (iii) substrates comprising metallic and plastic parts and optionally being coated with a cured electrocoat layer and/or a cured filler layer and/or a cured primer-surfacer layer and/or a cured primer layer, preferably from metal substrates being coated with a cured electrocoat layer and/or a cured filler layer, very preferably from metal substrates being coated with a cured electrocoat layer.

Suitable metal substrates are selected from the group comprising or consisting of steel, iron, aluminum, copper, zinc and magnesium substrates as well as substrates made of alloys of steel, iron, aluminum, copper, zinc and magnesium.

Coated and uncoated metal substrates can be pretreated in a manner known per se, i.e. , for example, cleaned and/or provided with known conversion coatings. Cleaning can be effected mechanically, for example by means of wiping, grinding and/or polishing, and/or chemically by means of etching methods, such as surface etching in acid or alkali baths using, for example, hydrochloric acid or sulfuric acid, or by cleaning with organic solvents or aqueous detergents. Pretreatment by application of conversion coatings, especially by means of phosphation and/or chromation, preferably phosphation, may likewise take place. Preferably, the metallic substrates are at least conversion-coated, especially phosphated, preferably by a zinc phosphation.

Metal substrates being coated with a cured electrocoat are produced by electrophoretic application of an electrocoat material to the substrate and subsequent curing of the applied electrocoat material. The electrocoat material may be a cathodic or anodic electrocoat material, preferably a cathodic electrocoat material. Electrocoat materials are aqueous coating materials comprising anionic or cationic polymers as binders. These polymers contain functional groups which are potentially anionic, i.e. can be converted to anionic groups, for example carboxylic acid groups, or functional groups which are potentially cationic, i.e. can be converted to cationic groups, for example amino groups. The conversion to charged groups is generally achieved by the use of appropriate neutralizing agents (organic amines (anionic), organic carboxylic acids such as formic acid (cationic). The electrocoat materials generally comprise typical anticorrosion pigments. The cathodic electrocoat materials preferred in the context of the invention comprise preferably cationic polymers as binders, especially hydroxyfunctional polyether amines, which preferably have aromatic structural units. These polymers are especially used in combination with blocked polyisocyanates known per se. The application of the electrocoating material proceeds by electrophoresis. For this purpose, the metallic workpiece to be coated is first dipped into a dip bath containing the coating material, and an electrical DC field is applied between the metallic workpiece and a counterelectrode. The workpiece thus functions as an electrode; the nonvolatile constituents of the electrocoat material migrate, because of the described charge of the polymers used as binders, through the electrical field to the substrate and are deposited on the substrate, forming an electrocoat film. For example, in the case of a cathodic electrocoat, the substrate is thus connected as the cathode, and the hydroxide ions which form there through water electrolysis neutralize the cationic binder, such that it is deposited on the substrate and forms an electrocoat layer. After the electrolytic application of the electrocoat material, the coated substrate is removed from the bath, optionally rinsed off with, for example, water-based rinse solutions, then optionally flashed off and/or intermediately dried, and finally cured. The dry film thickness of the cured electrocoat is, for example, 10 to 40 micrometers, preferably 15 to 25 micrometers.

Metal substrates being coated with a cured filler layer are produced by applying a filler coating composition to the substrate, optionally flashing off and/or intermediately drying said applied composition and finally curing said composition. Suitable filler coating compositions are known in the state of the art. The dry film thickness of the cured filler layer is, for example, 10 to 40 micrometers, preferably 25 to 30 micrometers. Preferred plastic substrates are basically substrates comprising or consisting of (i) polar plastics, such as polycarbonate, polyamide, polystyrene, styrene copolymers, polyesters, polyphenylene oxides and blends of these plastics, (ii) synthetic resins such as polyurethane RIM, SMC, BMC and (iii) polyolefin substrates of the polyethylene and polypropylene type with a high rubber content, such as PP-EPDM, and surface-activated polyolefin substrates. The plastics may furthermore be fiber- reinforced, in particular using carbon fibers and/or metal fibers.

Preferably, the objects to be coated according to the method of the present invention are used as components to fabricate vehicles, preferably automotive vehicles, including but not limited to automobiles, trucks, and tractors. The objects can have any shape, but are usually in the form of automotive body components such as bodies, hoods, doors, fenders, bumpers and/or trims for automotive vehicles. The invention is most useful in the context of coating automotive bodies and components thereof traveling in continuous movement along an automotive assembly line.

The at least three different coatings employed herein are then applied consecutively over the object to be coated in the manner described below.

The at least three different coating compositions that are used in the method of the present invention include: at least one first pigmented basecoat material (BC, BC-x) which is formulated as either a solid color or as an effect color composition based on flake and/or other effect pigments; a further coating layer (CL) having a certain degree of transparency, i.e. the color and/or effect of the underlying basecoat layer is visible beneath the further coating layer (CL), is used to enrich the color appeal of the basecoat layer (BL) or the basecoat layers (BL-x); and a regular untinted clearcoat (CC) as the topmost layer (CCL).

Step (i):

Step (i) of the inventive process either comprises production of exactly one basecoat layer (BL) (denoted as step (i)(a) hereinafter) or production of at least two directly successive basecoat layers (BL-a) and (BL-z) (denoted as step (i)(b) hereinafter). According to the first alternative of step (i), exactly one basecoat layer (BL) is produced by (a) applying exactly one aqueous basecoat composition (BC) directly to the object to be coated. After having been produced, therefore, the basecoat layer (BL) according to step (i)(a) is disposed directly on the object to be coated.

According to the second alternative of step (i), at least two basecoat layers (BL-a) and (BL-z) are produced by directly successively applying at least two basecoat compositions (BC-a) and (BC-z) to the object to be coated. In connection with this alternative, the basecoat compositions and basecoat layers are generally designated by (BC-x) and (BL-x), wherein the x is replaced by other appropriate letters in the naming of the specific individual basecoat compositions and basecoat layers.

The directly successive application of at least two, i.e. a plurality of, basecoat compositions to the object to be coated is thus understood to mean that a first basecoat material (BC-a) is applied directly to the object to be coated and then a second basecoat material (BC-b) is applied directly to the layer of the first basecoat material. This operation can then be repeated analogously for further basecoat materials (i.e. a third, fourth, fifth, etc. basecoat material). The uppermost basecoat layer obtained after step (i)(b) of the inventive method is denoted as basecoat layer (BL-z). The first basecoat layer (BL-a) is thus arranged directly on the object to be coated.

The terms “basecoat composition” and “basecoat layer” in relation to the coating compositions applied and coating films produced in step (i) of the inventive process are used for the sake of better clarity. The basecoat layer or layers is/are cured together with the clearcoat material, the curing is thus achieved analogously to the curing of so-called basecoat compositions used in the standard method described in the introduction. More particularly, the coating compositions used in step (i) of the process of the invention are not cured separately, like the coating compositions referred to as fillers or primer-surfacers in the context of the standard methods.

According to a preferred embodiment of step (i), exactly one basecoat material (BC) is applied directly onto the object to be coated.

The basecoat material (BC) or the basecoat materials (BC-x) employed in step (i) of the inventive method is/are a pigmented composition which may be formulated as a solid color (straight shade) or effect color coating. "Effect color coatings” generally contain at least one effect pigment and optionally other colored pigments or spheres which give the desired color and effect. "Straight shade" or "solid color coatings” primarily contain colored pigments and exhibit no visible flop or two tone metallic effect. With particular preference, the basecoat material (BC) used in step (i) of the present invention is a solid color or straight shade basecoat material.

With particular preference, the basecoat layer (BL) or at least one basecoat layer (BL-x) is opaque, i.e. it has a visible light transmission of less than 4 %, preferably of 0 to 3 %, very preferably 0 to 2 %. With “visible light transmission”, a light transmission in the range of 395 nm to 800 nm is meant. Thus, the basecoat layer (BL) or at least one basecoat layer (BL-x) formed in step (i) fully hides the underlying object to be coated, i.e. the object to be coated is no longer visible beneath the formed basecoat layer (BL) or the formed at least one basecoat layer (BL-x).

Preferably, as indicated above, the basecoat material (BC) (effect or solid shade) used is an aqueous coating composition in order to meet the current low overall solvent emission requirements. The expression "aqueous coating composition" is known to the skilled person. It refers fundamentally to a coating composition which is not based exclusively on organic solvents. Indeed, any such coating composition based on organic solvents contains exclusively organic solvents and no water for dissolving and/or dispersing the components, or is a coating composition for which no water is added explicitly during its production, water entering the composition instead only in the form of contaminant, atmospheric moisture and/or solvent for any specific additives employed. Such a composition, in contrast to an aqueous coating composition, would be referred to as being solvent borne or "based on organic solvents". "Aqueous" in the context of the present invention should be understood preferably to mean that the coating composition comprises a water fraction of at least 20 wt.%, preferably at least 25 wt.%, very preferably at least 50 wt.%, based in each case on the total amount of the solvents present (that is, water and organic solvents). The water fraction in turn is preferably 40 to 100 wt.%, more particularly 45 to 90 wt.%, very preferably 50 to 85 wt.%, based in each case on the total amount of the solvents present. Said basecoat material (BC) or at least one of the basecoat materials (BC-x), preferably all basecoat materials (BC-x), contain a first pigment P and at least one further pigment P _x being different from pigment P. The term “first pigment P” refers to the pigment being present in the highest amount in each basecoat material (BC-x), i.e. the pigment being responsible for the main color shade of each basecoat material (BC- x). In contrast, the term “further pigment P _x ” refers to the pigment(s) being present in lower amounts compared to the first pigment, i.e. the pigments used for adjusting the color of each basecoat material (BC-x) by tinting said basecoat materials (BC-x) with tinting pastes containing said further pigment(s) P _x . Upon storage of the basecoat material without agitation, ingredients having a relatively high density form a sediment while ingredients having a relatively low density are present in the upper phase, i.e. the phase above the sediment. Depending on the pigment density of pigment P and further pigment(s) P _x , pigment P and further pigment(s) P _x are either present in the sediment or in the upper phase. In order to prevent the formation of different pigment phases upon storage of the basecoat material, it is essential according to the invention that each difference Ap _x between the pigment density p(P) of the first pigment P and each pigment density p(P _x ) of each at least one further pigment P _x fulfills equation (1 )

Ap _x = p(P) - p(P _x ) < 2 g/ml (1 ).

The pigment density of pigment P and pigment(s) P _x may be determined according to DIN EN ISO 787-10:1995-10. The term “pigment phase” refers to a visually distinct phase comprising pigment P and/or further pigment(s) P _x . In case more than one visually distinct phase is present within the basecoat material upon storage and at least two of these phases contain pigment P and/or further pigment(s) P _x , different pigment phase are present. In contrast, only one pigment phase is present in case the pigment P and all further pigment(s) P _x are present in the same visually distinct phase.

Values for Ap _x may therefore range from positive values up to 2 in case the pigment density of the at least one further pigment P _x is smaller than the pigment density of pigment P. However, values for Ap _x which fulfill equation (1 ) can also be zero or negative in case the pigment density of the at least one further pigment P _x is equal to or larger than the pigment density of pigment P. If a mixture of pigments is used in which each pigment difference Ap _x does not fulfill equation (1 ), i.e. each pigment difference Ap _x is greater than 2, an undesired formation of different pigment phases is observed due to the high difference in pigment densities. Thus, all further pigments P _x being present in the basecoat material (BC) or (BC-x) apart from pigment P must be selected such that equation (1 ) is fulfilled for all further pigments P _x . If, for example, 2 further pigments Pi and P2 are present apart from pigment P, pigments Pi and P2 must be selected such that the following equations are fulfilled:

Api = p(P) - p(Pi) < 2 g/ml for pigment P1 and

Ap2 = p(P) - p(P2) < 2 g/ml for pigment P2.

Each difference Ap _x between the pigment density p(P) of the first pigment P and each pigment density p(P _x ) of each at least one further pigment P _x may be < 1.5 g/ml, preferably < 1 .0 g/ml. Moreover, each difference Ap _x between the pigment density p(P) of the first pigment P and each pigment density p(P _x ) of each at least one further pigment P _x may be < 1.5 to -5.0 g/ml, preferably < 1.0 to -2.5 g/ml. Use of a pigment mixture fulfilling the aforementioned pigment density differences guarantees that all pigments being present in the aqueous basecoat material (BC) are contained within the same pigment phase after storage of said material without agitation and thus reduces undesired color shifts upon application of said stored basecoat materials without prior homogenization. Thus, the basecoat material being present in the tap lines for longer time periods can be used without a negative impact on the achieved color quality and thus renders cleaning of said tap lines after longer periods of non-use redundant.

In a first embodiment, the first pigment P has a pigment density p(P) of > 2.5 g/ml, preferably of 3 to 15 g/ml, more preferably of 3.5 to 10 g/ml, very preferably of 4 to 6 g/ml, determined in each case according to DIN EN ISO 787-10:1995-10. Such pigments have a relatively high density and tend to sediment during the storage of the basecoat material.

The first pigment P of this embodiment may be selected from a color pigment and/or an effect pigment. With particular preference, the pigment P is a color pigment. Suitable color pigments include pigments having a white, yellow, green, red, blue or black color. The choice of the color pigment is depending on the main color of the basecoat material. Suitable color pigments include inorganic and organic color pigments, preferably inorganic color pigments and organic metal complex pigments, such as titanium oxide, bismuth vanadium oxide, iron oxide, preferably a-Fe2Os, chromium oxide and C.l. pigment blue 76.

The at least further pigment P _x of this embodiment may be selected from color pigments and/or effect pigments, preferably color pigments, such as inorganic and organic color pigments. The basecoat material (BC) or (BC-x) of this embodiment is thus preferably a solid color basecoat material comprising no effect pigments, i.e. the amount of effect pigments is 0 wt.%, based on the total weight of the aqueous basecoat material. Suitable color pigments include iron oxide, preferably a-Fe2Os or FesO4, copper chromite, chromium iron oxide, chromium-ill oxide, tin antimony cassiterite, cobalt aluminate blue spinel, aluminum chromium cobalt oxide, C.l. pigment blue 76, chromium oxide, chromium titanite green spinel, C.l. pigment green 36, titanium dioxide, bismuth vanadium oxide or mixtures thereof.

According to a second, alternative embodiment, the pigment density p(P) of the first pigment P is < 2.5 g/ml, preferably 0.5 to < 2.5 g/ml, more preferably 1.2 to 2 g/ml, determined in each case according to DIN EN ISO 787-10:1995-10.

The first pigment P of this embodiment may be selected from a color pigment and/or an effect pigment. With particular preference, the pigment P is a color pigment. Suitable color pigment may have a black, brown, red, yellow or orange color, in particular a black color. The choice of the color pigment is depending on the main color of the basecoat material. Suitable color pigments include carbon black, azo pigments, polycyclic pigments, and mixtures thereof. Azo pigments may include monoazo pigments and diazo pigments, such as monoazo yellow pigments, !3>-naphthol pigments, naphthol AS pigments, salts of BONS pigments, salts of naphthalenesulfonic acid pigments, benzimidazolone pigments, pyrazoloquinazolone pigments, diaryl yellow pigments, bisacetylacetoacetic acid arylide pigments, disazopyrazolone pigments and disazocondensation pigments. Suitable polycylic pigments may include metal complex pigments, such as phthalocyanine pigments, Azomethine metal complex pigments and isoindolinone metal complex pigments; azomethine pigments; bisazomethine pigments; isoindolinone pigments; isoindoline pigments; quinophthalone pigments; diketo-pyrrolopyrrole pigments; aniline black; chinacridone pigments; triarylcarbenium pigments; triphenylmethane pigments; perylene pigments; naphthalenetetracarboxylic acid pigments; thioindigo pigments; anthraquinone pigments; aminoanthraquinone pigments; anthrapyrimidine pigments; indanthrone pigments; flavanthrone pigments; pyranthrone pigments and anthanthrone pigments. With preference, the first pigment P of this embodiment is carbon black or a polycyclic pigment. With particular preference, the first pigment P is carbon black or a perylene.

The at least further pigment P _x of the second embodiment may be selected from color pigments and/or effect pigments. With particular preference, the at least one further pigment P _x is selected from color pigments or a mixture of color pigments and effect pigments listed hereinafter. In case the further(s) pigment P _x is/are selected from color pigments, the basecoat material is a solid color basecoat material. Such basecoat materials (BC) preferably comprise no effect pigments, i.e. 0 wt.%, based on the total weight of the aqueous basecoat material, of effect pigments. In case the further pigment(s) is/are selected from a mixture of color pigments and effect pigments, the basecoat material is an effect color basecoat material. Suitable color pigments may include indanthrone blue, C.l. pigment blue 27, C.l. pigment red 179, C.l. pigment red 254 and mixtures thereof. Suitable effect pigments may include flake aluminum pigments, gold bronzes, fire-colored bronzes, iron oxide aluminum pigments, pearlescent pigments, metal oxide mica pigments, flake graphite, flake iron oxide, multilayer effect pigments from PVD films and mixtures thereof, in particular flake aluminum pigments.

The aqueous basecoat material applied in step (i) comprises at least one binder B. Suitable binders B may be selected from (i) poly(meth)acrylates, more particularly hydroxy-functional and/or carboxylate-functional and/or amine-functional poly(meth)acrylates, (ii) polyurethanes, more particularly hydroxy-functional and/or carboxylate-functional and/or amine-functional polyurethanes, (iii) polyesters, more particularly polyester polyols and polycarbonate polyols, (iv) polyethers, more particularly polyether polyols, (v) copolymers of the stated polymers, and (vi) mixtures thereof, preferably from hydroxy-functional poly(meth)acrylates, hydroxy-functional polyurethanes, hydroxy-functional polyesters, hydroxy-functional polyethers and copolymers of said polymers. With particular preference, the at least one binder is selected from polyurethane-poly(meth)acrylate copolymers.

The at least one binder B may be present in the aqueous basecoat material(s) (BC) or (BC-x) in a total amount of 55 to 90wt.%, preferably of 65 to 85 wt.%, based in each case on the total solid content of resins present in the aqueous basecoat material(s). The term “resins” refers herein to all binders and crosslinkers being present in the aqueous basecoat material.

The aqueous basecoat material(s) (BC) or (BC-x) may comprise a pigment to binder ratio of 0.04 to 2, preferably of 0.05 to 1 .8.

The aqueous basecoat material(s) (BC) or (BC-x) may further comprise at least one crosslinker CL. Suitable crosslinkers are selected from am inoplast resins, blocked and/or free polyisocyanates, polycarbodiimides and mixtures thereof. With particular preference, the crosslinker is selected from am inoplast resins such as melamine formaldehyde resins. The at least one crosslinker CL may be present in the aqueous basecoat material(s) (BC) or (BC-x) a total amount of 10 to 45 wt.%, preferably of 15 to 35 wt.%, based in each case on the total solid content of resins being present in the aqueous basecoat material(s).

The aqueous basecoat material(s) (BC) or (BC-x) may further comprise at least one additive AD. Suitable additives AD may be selected from salts which can be broken down thermally without residue or substantially without residue, binders being different from binder B and which are curable physically, thermally and/or with actinic radiation, organic solvents, reactive diluents, transparent pigments, fillers, dyes soluble in a molecular dispersion, nanoparticles, light stabilizers, antioxidants, deaerating agents, emulsifiers, slip additives, polymerization inhibitors, initiators of free-radical polymerizations, adhesion promoters, flow control agents, film-forming assistants, sag control agents (SCAs), flame retardants, corrosion inhibitors, waxes, siccatives, biocides, and flatting agents and mixtures thereof. They are used in the customary and known amounts. For example, the at least one additive AD may be present in a total amount of 0.1 to 20 wt.%, preferably of 1 to 20 wt.%, based in each case on the total weight of the aqueous basecoat material (BC) or (BC-x).

The solids content of the basecoat materials (BC) or (BC-x) may vary according to the requirements of the individual case. The solids content is guided primarily by the viscosity required for application, more particularly for spray application, and so may be adjusted by the skilled person on the basis of his or her general art knowledge, optionally with assistance from a few exploratory tests. The basecoat materials (BC) or (BC-x) may have a solids content of 5 to 70 wt.%, more preferably 8 to 60 wt.%, most preferably 12 to 55 wt.%. The solids content (nonvolatile fraction) refers to the weight fraction of the residue remaining after evaporation under specified conditions, such as 130 °C for 60 minutes. In the present specification, the solids content is determined according to DIN EN ISO 3251 :2019-09.

The basecoat material(s) can be applied by methods known to those skilled in the art for application of liquid coating compositions, for example by dipping, bar coating, spraying, rolling or the like. Preference is given to employing spray application methods, for example compressed air spraying (pneumatic application), airless spraying, high-speed rotation, electrostatic spray application (ESTA), optionally in association with hot-spray application, for example hot-air spraying. Most preferably, the basecoat material(s) is/are applied by means of electrostatic spray application (ESTA).

After application, the basecoat material(s) applied is/are flashed off, for example, at 15 to 35 °C for a period of, for example, 0.5 to 30 minutes and/or intermediately dried at a temperature of preferably 40 to 90 °C for a period of, for example, 1 to 60 min. Preference is given to first flashing off at 15 to 35 °C for a period of 0.5 to 10 min. The basecoat material(s) is/are not cured in step (i), i.e. they are preferably not exposed to temperatures of more than 80 °C for a period of longer than 1 min, and especially preferably are not exposed to temperatures of more than 80 °C at all.

The application of the basecoat material is preferably effected in such a way that each basecoat layer, after the curing in step (iv), has a dry film thickness of, for example, 10 to 40 micrometers, preferably 10 to 25 micrometers. Step (ii):

In step (ii) of the inventive method, a further coating layer (CL) is produced by applying a coating material (CM) comprising at least one pigment PCM directly to the basecoat layer (BL) or to the topmost basecoat layer (BL-z). This further coating layer (CL) has - when cured - a luminous transmittance of at least 4 percent as measured at a film thickness of 15 to 18 micrometers according to ASTM D 1003-00 (procedure A) using a CIE standard illuminant D65. Thus, the aqueous coating material (CM) is formulated to provide a transparent or semi-transparent coating layer (CL), i.e. the basecoat layer (BL) or at least one of the basecoat layers (BL-x) is fully or at least partially visible beneath the cured further coating layer (CL). The term “luminous transmittance” refers to the % of light, that passes through the cured further coating layer (CL) and can be calculated by dividing the luminous flux transmitted by the cured further coating layer (CM) by the flux incident upon it.

The further coating layer - when cured - may have a luminous transmittance of 10 to 100 percent, preferably 30 to 95 percent, more preferably 40 to 95 percent, even more preferably 45 to 95 percent, very preferably 50 to 75 percent, measured in each case at a film thickness of 15 to 18 micrometers according to ASTM D 1003-00 (procedure A) using a CIE standard illuminant D65. The high luminous transmittance ensures that the basecoat layer (BL) or at least one of the basecoat layers (BL-x) is at least partially visible beneath the cured further coating layer (CL). This results in a highly saturated color appearance with excellent depth of color and desired glamour of the obtained multilayer coating.

The further coating layer - when cured - may have a haze of 50 to 100 %, preferably of 60 to 100 %, more preferably of 70 to 100 %, even more preferably of 80 to 100 %, very preferably of 85 to 95 %, measured in each case at a film thickness of 15 to 18 micrometers according to ASTM D 1003-00 (procedure A) using a CIE standard illuminant D65. The term “haze” is defined as the scattering of light by the cured further coating layer responsible for the reduction in contrast of objects viewed through it. The percent of transmitted light that is scattered so that its direction deviates more than 2.5° from the direction of the incident beam. The at least one pigment PCM may be selected from effect pigments and/or color pigments, preferably effect pigments. Suitable effect pigments include lamellar aluminum pigments, aluminum pigments having a cornflake and/or silver dollar form, aluminum pigments coated with organic pigments, glass flakes, glass flakes coated with interference layers, gold bronzes, oxidized bronzes, iron oxide-aluminum pigments, pearlescent pigments, metal oxide-mica pigments, lamellar graphite, platelet-shaped iron oxide, multilayer effect pigments composed of PVD films, and mixtures thereof. With particular preference, the effect pigment is selected from lamellar aluminum pigments and/or glass flakes.

Suitable color pigments include (i) white pigments such as titanium dioxide, zinc white, zinc sulfide or lithopone; (ii) black pigments such as carbon black, iron manganese black, or spinel black; (iii) chromatic pigments such as ultramarine green, ultramarine blue, manganese blue, ultramarine violet, manganese violet, red iron oxide, molybdate red, ultramarine red, brown iron oxide, mixed brown, spinel phases and corundum phases, yellow iron oxide, bismuth vanadate; (iv) organic pigments such as monoazo pigments, bisazo pigments, anthraquinone pigments, benzimidazole pigments, quinacridone pigments, quinophthalone pigments, diketopyrrolopyrrole pigments, dioxazine pigments, indanthrone pigments, isoindoline pigments, isoindolinone pigments, azomethine pigments, thioindigo pigments, metal complex pigments, prinone pigments, perylene pigments, phthalocyanine pigments, aniline black; and (v) mixtures thereof.

In one embodiment, the coating material (CM) comprises 0 to 10 wt.%, more preferably 0 to 4 wt.%, very preferably 0 wt.%, based in each case on the total weight of the coating material (CM), of color pigments. The presence of color pigments in low amounts or their absence ensures that the desired transparency of the cured further coating layer (CL) is obtained. Use of small amounts of color pigments may be preferred if a depth in color and color saturation is to be achieved while color pigments may not be present in case the further coating layer (CL) is used to provide texture to the underlying basecoat layer(s).

The coating material (CM) may further comprise at least one binder B1 . This binder B1 may be the same binder as binder B being present in the basecoat material (BC) or (BC-x) or may be different from said binder B. Suitable binders B1 include hydroxy- functional and/or acid-functional polymers and/or carbamate-functional polymers, preferably from hydroxy-functional poly(meth)acrylates, hydroxy-functional polyurethanes, hydroxy-functional polyesters, hydroxy-functional polyethers, hydroxyfunctional polysiloxanes and copolymers of the stated polymers. With particular preference, the at least one binder B1 is selected from hydroxy-functional poly(meth)acrylate copolymers.

The at least one binder B1 is present in a total amount of 55 to 90 wt.%, preferably of 65 to 85 wt.%, based in each case on the total solid content of resins being present in the further coating material (CM). The term “resins” refers herein to all binders and crosslinkers being present in the aqueous basecoat material.

The coating material (CM) may comprise a pigment to binder ratio of 0.00001 to 0.8, preferably of 0.01 to 0.7, very preferably of 0.1 to 0.6.

The coating material (CM) may further comprise at least one crosslinker CL1 . This crosslinker CL1 may be the same crosslinker as crosslinker CL being present in the basecoat material (BC) or (BC-x) or may be different from said crosslinker CL. Suitable crosslinkers CL1 include anhydride-functional compounds, amino resins, tris(alkoxycarbonylamino)triazines and derivatives thereof, compounds having free and/or blocked isocyanate groups, epoxy-functional compounds and mixtures thereof.

The coating material (CM) may further comprise at least one additive AD1. Suitable additives AD1 include catalysts, UV absorbers; light stabilizers such as HALS compounds, benzotriazoles or oxalanilides; rheology modifiers such as sagging control agents (urea crystal modified resins), organic thickeners and inorganic thickeners; free-radical scavengers; slip additives; polymerization inhibitors; defoamers; wetting agents; fluorine compounds; adhesion promoters; leveling agents; film-forming auxiliaries such as cellulose derivatives; fillers, such as nanoparticles based on silica, alumina or zirconium oxide; flame retardants and mixtures thereof. They are used in the customary and known amounts. For example, the at least one additive AD may be present in a total amount of 0 to 20 wt.%, more preferably of 0.005 to 15 wt.% and particularly of 0.01 to 10 wt.%, based in each case on the total weight of the further coating material (CM). The further coating material (CM) may be an aqueous or solvent-based coating material. Preferably, the further coating material (CM) is an aqueous coating material in order to meet the current low overall solvent emission requirements. With respect to the term “aqueous coating material”, reference is made to the previously listed definition in connection with the aqueous basecoat material (BC) or (BC-x).

The further coating material (CM) can be applied by methods known to those skilled in the art for application of liquid coating compositions, for example by dipping, bar coating, spraying, rolling or the like. Preference is given to employing spray application methods, for example compressed air spraying (pneumatic application), airless spraying, high-speed rotation, electrostatic spray application (ESTA), optionally in association with hot-spray application, for example hot-air spraying. Most preferably, the further coating material (CM) is applied by means of electrostatic spray application (ESTA).

After application, the further coating material (CM) is flashed off, for example, at 15 to 35 °C for a period of, for example, 0.5 to 30 minutes and/or intermediately dried at a temperature of preferably 40 to 90 °C for a period of, for example, 1 to 60 minutes. Preference is given to first flashing off at 15 to 35 °C for a period of 0.5 to 5 min and then intermediately drying at 60 to 80 °C for a period of 1 to 5 minutes. The further coating material (CM) is not cured in step (ii), i.e. it is preferably not exposed to temperatures of more than 80 °C for a period of longer than 1 minute, and especially preferably are not exposed to temperatures of more than 80 °C at all.

The application of the further coating material (CM) is preferably effected in such a way that the further coating layer, after the curing in step (iv), has a dry film thickness of, for example, 3 to 15 micrometers, preferably 4 to 10 micrometers.

Step (Hi):

In step (iii) of the method of the invention, a clearcoat layer (CCL) is produced directly on the further coating layer (CL). This production is accomplished by corresponding application of a clearcoat material (CC). Suitable clearcoat materials are described, for example, in WO 2006042585 A1 , WO 2009077182 A1 or else WO 2008074490 A1 . Said clearcoat material (CC) does preferably not comprise any color pigments, effect pigments, fillers and matting agents, i.e. said pigments, fillers and matting agents are contained in these material in an amount of 0 wt.%, based on the total weight of the clearcoat material (CC). However, the clearcoat material (CC) may comprise dyes or transparent pigments not having any visual effect.

The clearcoat material (CC) may in principle be any transparent coating composition known to the person skilled in the art in this context. This includes aqueous or solvent borne transparent coating compositions, which may be formulated either as one- component or two-component coating compositions, or multicomponent coating compositions. In case of one-component coating compositions, crosslinking reactions during storage need to be avoided, for example by using blocked crosslinking agents which are only reactive at higher temperatures. Two-component coating compositions comprise the reactive components, i.e. binder and crosslinker, in separate containers. The components of containers 1 and 2 are then mixed prior to use, preferably shortly before application of the mixed coating composition to the substrate. In addition, powder slurry clearcoat materials are also suitable. Preference is given to solventbased 2K clearcoat materials.

The clearcoat materials (CC) used may especially be thermally and/or actinochemically curable. More particularly, they are thermally curable and externally crosslinking.

The transparent coating compositions preferably comprise at least one (first) polymer as a binder having functional groups, and at least one crosslinker having a functionality complementary to the functional groups of the binder. Preference is given to using at least one hydroxy-functional poly(meth)acrylate polymer as a binder and a polyisocyanate as a crosslinking agent. Suitable clearcoat materials are described, for example, in WO 2006/042585 A1 , WO 2009/077182 A1 or else WO 2008/074490 A1 .

The clearcoat material (CC) is applied by methods known to those skilled in the art for application of liquid coating compositions, for example by dipping, bar coating, spraying, rolling or the like. Preference is given to employing spray application methods, for example compressed air spraying (pneumatic application), and electrostatic spray application (ESTA).

After application, the clearcoat material (CC) may be flashed off and/or intermediately dried at 15 to 35 °C for a period of 10 to 30 minutes.

The application of the clearcoat material (CC) is effected in such a way that the clearcoat layer, after the curing in step (iv), has a dry film thickness of, for example, 15 to 80 micrometers, preferably 20 to 65 micrometers, especially preferably 25 to 60 micrometers.

Step (iv):

In step (iv) of the method of the invention, there is joint curing of the basecoat layer (BL), the further coating layer (CL) and the clearcoat layer (CCL), or of the basecoat layers (BL-x), the further coating layer (CL) and the clearcoat layer (CCL).

The joint curing is preferably effected at temperatures of 80 to 250 °C, preferably 80 to 180 °C, for a period of 5 to 60 min, preferably 10 to 45 min. Curing conditions of this kind apply especially to the preferred case that the basecoat material (BC) or at least one of the basecoats materials (BC-x), preferably all the basecoats materials (BC-x), is/are based on a thermally curable one-component coating composition, since these conditions are necessary to achieve curing of such a one-component coating composition.

Inventive coated object:

After the end of step (iv) of the method of the invention, the result is a coated object of the invention.

The coated object of the present invention has a constant highly saturated color appearance with excellent depth of color and desired glamour as well as excellent mechanical properties. This constant appearance is achieved even if the basecoat material(s) used to prepare the coated object are not homogenized prior to their application on the substrate. What has been said about the inventive coating composition applies mutatis mutandis with respect to further preferred embodiments of the inventive coated substrates.

The invention is described in particular by the following number clauses:

1. Method for forming a multilayer coating (MC) onto an object to be coated, said method comprising:

(i) producing a basecoat layer (BL) or at least two directly consecutive basecoat layers (BL-x) directly on the object to be coated by applying an aqueous basecoat material (BC) directly to the object to be coated or by applying directly in succession at least two aqueous basecoat materials (BC-x) directly to the object to be coated;

(ii) producing a further coating layer (CL) directly on the basecoat layer (BL) or on the topmost basecoat layer (BL-z) by applying a coating material (CM) comprising at least one pigment PCM directly to the basecoat layer (BL) or to the topmost basecoat layer (BL-z);

(iii) producing a clearcoat layer (CCL) directly on the further coating layer (CL) by applying a clearcoat material (CC) directly to the further coating layer (CL); and

(iv) jointly curing the basecoat layer (BL), the further coating layer (CL) and the clearcoat layer (CCL) or the basecoat layers (BL-x), the further coating layer (CL) and the clearcoat layer (CCL);

- wherein the at least one basecoat material (BC) or at least one of the basecoat materials (BC-x) comprises

• a first pigment P,

• at least one further pigment P _x being different from pigment P, and

• at least one binder B, wherein each difference Ap _x between the pigment density p(P) of the first pigment P and each pigment density p(P _x ) of each at least one further pigment P _x fulfills equation (1 )

Ap _x = p(P) - p(P _x ) < 2 g/ml (1 )

- and wherein the further coating layer (CL) - when cured - has a luminous transmittance of at least 4 percent as measured at a film thickness of 15 to 18 micrometers according to ASTM D 1003-00 (procedure A) using a CIE standard illuminant D65. Method according to clause 1 , wherein the object to be coated is selected from (i) uncoated metal substrates or metal substrates being coated with a cured electrocoat layer and/or a cured filler layer and/or a cured multilayer coating; (ii) plastic substrates optionally being coated with a cured primer layer; and (iii) substrates comprising metallic and plastic parts and optionally being coated with a cured electrocoat layer and/or a cured filler layer and/or a cured primer-surfacer layer and/or a cured primer layer, preferably from metal substrates being coated with a cured electrocoat layer and/or a cured filler layer, very preferably from metal substrates being coated with a cured electrocoat layer. Method according to clause 2, wherein metal substrate is selected from the group comprising or consisting of steel, iron, aluminum, copper, zinc and magnesium substrates as well as substrates made of alloys of steel, iron, aluminum, copper, zinc and magnesium. Method according to any of the preceding clauses, wherein the aqueous basecoat material (BC) comprises water in an amount of 40 to 100wt.%, preferably 45 to 90wt.%, very preferably 50 to 85wt.%, based in each case on the total amount of solvents present in the aqueous basecoat material (BC) or (BC-x). Method according to any of the preceding clauses, wherein each difference Ap _x between the pigment density p(P) of the first pigment P and each pigment density p(Px) of each at least one further pigment P _x is < 1 .5 g/ml, preferably < 1 .0 g/ml. Method according to any of the preceding clauses, wherein each difference Ap _x between the pigment density p(P) of the first pigment P and each pigment density p(P _x ) of each at least one further pigment P _x is < 1 .5 to -5.0 g/ml, preferably < 1 .0 to -2.5 g/ml. Method according to any of the preceding clauses, wherein the first pigment P has a pigment density p(P) of > 2.5 g/ml, preferably of 3 to 15 g/ml, more preferably of 3.5 to 10 g/ml, very preferably of 4 to 6 g/ml, determined in each case according to DIN EN ISO 787-10:1995-10. 8. Method according to clause 7, wherein the first pigment P is selected from a color pigment and/or an effect pigment, preferably a color pigment.

9. Method according to clause 7 or 8, wherein the color pigment is selected from a pigment having a white, yellow, green, red, blue or black color.

10. Method according to clause 8 or 9, wherein the color pigment is selected from the group consisting of titanium oxide, bismuth vanadium oxide, iron oxide, preferably a-Fe2O3, chromium oxide and C.l. pigment blue 76.

11 . Method according to any of the preceding clauses, wherein the at least further pigment P _x is selected from color pigments and/or effect pigments, preferably color pigments.

12. Method according to clause 11 , wherein the color pigment is selected from the group consisting of iron oxide, preferably a-Fe2Os or FesO4, copper chromite, chromium iron oxide, chromium-ill oxide, tin antimony cassiterite, cobalt aluminate blue spinel, aluminum chromium cobalt oxide, C.l. pigment blue 76, chromium oxide, chromium titanite green spinel, C.l. pigment green 36, titanium dioxide, bismuth vanadium oxide or mixtures thereof.

13. Method according to any of clauses 1 to 6, wherein the pigment density p(P) of the first pigment P is < 2.5 g/ml, preferably 0.5 to < 2.5 g/ml, more preferably of 1 .2 to 2 g/ml, determined in each case according to DIN EN ISO 787-10:1995-10.

14. Method according to clause 13, wherein the first pigment P is a color pigment and/or an effect pigment, preferably a color pigment.

15. Method according to clause 14, wherein the color pigment is selected from carbon black, azo pigments, polycyclic pigments, and mixtures thereof, preferably carbon black or polycyclic pigments, very preferably carbon black or perylenes.

16. Method according to any of clauses 13 to 15, wherein the at least one further pigment Px is selected from color pigments and/or effect pigments. 17. Method according clause 16, wherein the color pigment is selected from indanthrone blue, C.l. pigment blue 27, C.l. pigment red 179, C.l. pigment red 254 and mixtures thereof.

18. Method according to clause 16 or 17, wherein the effect pigment is selected from flake aluminum pigments, gold bronzes, fire-colored bronzes, iron oxide aluminum pigments, pearlescent pigments, metal oxide mica pigments, flake graphite, flake iron oxide, multilayer effect pigments from PVD films and mixtures thereof, in particular flake aluminum pigments.

19. Method according to any of the preceding clauses 1 to 15, wherein the aqueous basecoat material(s) (BC) or (BC-x) comprises 0 wt.%, based on the total weight of the aqueous basecoat material(s), of effect pigments.

20. Method according to any of the preceding clauses, wherein the at least one binder B is selected from (i) poly(meth)acrylates, more particularly hydroxy-functional and/or carboxylate-functional and/or amine-functional poly(meth)acrylates, (ii) polyurethanes, more particularly hydroxy-functional and/or carboxylate- functional and/or amine-functional polyurethanes, (iii) polyesters, more particularly polyester polyols and polycarbonate polyols, (iv) polyethers, more particularly polyether polyols, (v) copolymers of the stated polymers, and (vi) mixtures thereof, preferably from hydroxy-functional poly(meth)acrylates, hydroxy-functional polyurethanes, hydroxy-functional polyesters, hydroxy-functional polyethers and copolymers of said polymers, very preferably hydroxy-functional polyurethane- poly(meth)acrylate copolymers.

21 . Method according to any of the preceding clauses, wherein the aqueous basecoat material(s) (BC) or (BC-x) comprise(s) the at least one binder B in a total amount of 55 to 90wt.%, preferably 65 to 85 wt.%, based in each case on the total solid content of resins being present in the aqueous basecoat material(s).

22. Method according to any of the preceding clauses, wherein the aqueous basecoat material(s) (BC) or (BC-x) comprise(s) a pigment to binder ratio of 0.04 to 2, preferably of 0.05 to 1 .8. Method according to any of the preceding clauses, wherein the aqueous basecoat material(s) (BC) or (BC-x) further comprise(s) at least one crosslinker CL. Method according to clause 23, wherein the at least one crosslinker CL is selected from am inoplast resins, blocked and/or free polyisocyanates, polycarbodiimides and mixtures thereof, preferably am inoplast resins, very preferably melamine formaldehyde resins. Method according to any of the preceding clauses, wherein the aqueous basecoat material(s) (BC) or (BC-x) further comprise(s) at least one additive AD. Method according to clause 25, wherein the at least one additive AD is selected from salts which can be broken down thermally without residue or substantially without residue, resins as binders being different from binder B and which are curable physically, thermally and/or with actinic radiation, organic solvents, reactive diluents, transparent pigments, fillers, dyes soluble in a molecular dispersion, nanoparticles, light stabilizers, antioxidants, deaerating agents, emulsifiers, slip additives, polymerization inhibitors, initiators of free-radical polymerizations, adhesion promoters, flow control agents, film-forming assistants, sag control agents (SCAs), flame retardants, corrosion inhibitors, waxes, siccatives, biocides, and flatting agents and mixtures thereof. Method according to clause 25 or 26, wherein the at least one additive AD is present in a total amount of 0.1 to 20 wt.%, preferably of 1 to 20 wt.%, based in each case on the total weight of the aqueous basecoat material(s). Method according to any of the preceding clauses, wherein the further coating layer - when cured - has a luminous transmittance of 10 to 100 percent, preferably 30 to 95 percent, more preferably 40 to 95 percent, even more preferably 45 to 95 percent, very preferably 50 to 75 percent, measured in each case at a film thickness of 15 to 18 micrometers according to ASTM D 1003-00 (procedure A) using a CIE standard illuminant D65. Method according to any of the preceding clauses, wherein the further coating layer - when cured - has a haze of 50 to 100 %, preferably of 60 to 100 %, more preferably of 70 to 100 %, even more preferably of 80 to 100 %, very preferably of 85 to 95 %, measured in each case at a film thickness of 15 to 18 micrometers according to ASTM D 1003-00 (procedure A) using a CIE standard illuminant D65.

30. Method according to any of the preceding clauses, wherein the at least one pigment PCM is selected from effect pigments and/or color pigments, preferably effect pigments.

31. Method according to clause 30, wherein the effect pigments are selected from lamellar aluminum pigments, aluminum pigments having a cornflake and/or silver dollar form, aluminum pigments coated with organic pigments, glass flakes, glass flakes coated with interference layers, gold bronzes, oxidized bronzes, iron oxidealuminum pigments, pearlescent pigments, metal oxide-mica pigments, lamellar graphite, platelet-shaped iron oxide, multilayer effect pigments composed of PVD films, and mixtures thereof, preferably lamellar aluminum pigments and/or glass flakes.

32. Method according to clause 30 or 31 , wherein the at least one color pigment is selected from (i) white pigments such as titanium dioxide, zinc white, zinc sulfide or lithopone; (ii) black pigments such as carbon black, iron manganese black, or spinel black; (iii) chromatic pigments such as ultramarine green, ultramarine blue, manganese blue, ultramarine violet, manganese violet, red iron oxide, molybdate red, ultramarine red, brown iron oxide, mixed brown, spinel phases and corundum phases, yellow iron oxide, bismuth vanadate; (iv) organic pigments such as monoazo pigments, bisazo pigments, anthraquinone pigments, benzimidazole pigments, quinacridone pigments, quinophthalone pigments, diketopyrrolopyrrole pigments, dioxazine pigments, indanthrone pigments, isoindoline pigments, isoindolinone pigments, azomethine pigments, thioindigo pigments, metal complex pigments, prinone pigments, perylene pigments, phthalocyanine pigments, aniline black; and (v) mixtures thereof.

33. Method according to any of clauses 1 to 31 , wherein the coating material (CM) comprises 0 to 10 wt.%, more preferably 0 to 4 wt.%, very preferably 0 wt.%, based in each case on the total weight of the coating material (CM), of color pigments. Method according to any of the preceding clauses, wherein the coating material (CM) further comprises at least one binder B1 . Method according clause 34, wherein the at least one binder B1 is selected from hydroxy-functional and/or acid-functional polymers and/or carbamate-functional polymers, preferably from hydroxy-functional poly(meth)acrylates, hydroxyfunctional polyurethanes, hydroxy-functional polyesters, hydroxy-functional polyethers, hydroxy-functional polysiloxanes and copolymers of the stated polymers, very preferably hydroxy-functional poly(meth)acrylate copolymers. Method according to clause 34 or 35, wherein the at least one binder B1 is present in a total amount of 55 to 90 wt.%, preferably of 65 to 85 wt.%, based in each case on the total solid content of resins being present in the further coating material (CM). Method according to any of the preceding clauses, wherein the coating material (CM) comprises a pigment to binder ratio of 0.00001 to 0.8, preferably of 0.01 to 0.7, very preferably of 0.1 to 0.6. Method according to any of the preceding clauses, wherein the coating material (CM) further comprises at least one crosslinker CL1 . Method according to clauses 38, wherein the at least one crosslinker CL1 is selected from anhydride-functional compounds, amino resins, tris(alkoxycarbonylamino)triazines and derivatives thereof, compounds having free and/or blocked isocyanate groups, epoxy-functional compounds and mixtures thereof. Method according to any of the preceding clauses, wherein the coating material (CM) further comprises at least one additive AD1 . Method according to clause 40, wherein the at least one additive AD1 is selected from catalysts, UV absorbers; light stabilizers such as HALS compounds, benzotriazoles or oxalanilides; rheology modifiers such as sagging control agents (urea crystal modified resins), organic thickeners and inorganic thickeners; free- radical scavengers; slip additives; polymerization inhibitors; defoamers; wetting agents; fluorine compounds; adhesion promoters; leveling agents; film-forming auxiliaries such as cellulose derivatives; fillers, such as nanoparticles based on silica, alumina or zirconium oxide; flame retardants and mixtures thereof. Method according to clauses 40 or 41 , wherein the at least one additive AD1 is present in a total amount of 0 to 20 wt.%, more preferably of 0.005 to 15 wt.% and particularly of 0.01 to 10 wt.%, based in each case on the total weight of the coating material (CM). Method according to any of the preceding clauses, wherein the coating material (CM) is an aqueous or solvent borne coating material, preferably an aqueous coating material. Method according to any of the preceding clauses, wherein the clearcoat material (CC) is selected from solvent borne 1 K or 2K clearcoat materials, preferably from solvent borne 2K clearcoat materials. Method according to any of the preceding clauses, wherein the aqueous basecoat material(s) (BC) or (BC-x) applied in step (i) is/are flashed off at 15 to 35 °C for 0.5 to 30 minutes. Method according to any of the preceding clauses, wherein the coating material (CM) applied in step (ii) is flashed off at 15 to 35 °C for 0.5 to 10 minutes and/or dried at 60 to 80 °C for 1 to 5 minutes. Method according to any of the preceding clauses, wherein the clearcoat material (CC) applied in step (iii) is flashed off at 15 to 35 °C for 0.5 to 5 minutes and/or dried at 60 to 80 °C for 5 to 15 minutes. Method according to any of the preceding clauses, wherein the joint curing in step (iv) is performed at a temperature of 15 to 250 °C, preferably 80 to 180 °C, for a duration of 5 to 60 minutes, preferably 10 to 45 minutes. 49. Coated object produced by a method as claimed according to any of clauses 1 to 48.

Examples

The present invention will now be explained in greater detail by the use of working examples, but the present invention is in no way limited to these working examples. Moreover, the terms "parts", "%" and "ratio" in the examples denote "parts by mass", "mass %" and "mass ratio" respectively unless otherwise indicated.

1. Methods of determination:

1.1 Solids content (solids, non-volatile fraction)

Unless stated otherwise, the solids content (also called proportion of solids, solid-state content, proportion of non-volatiles) was determined to DIN EN ISO 3251 : 2018-07 at 130°C; 60 min, starting weight 1 .0 g.

1.2 Evaluation of storage stability of basecoat materials (BC)

The respective basecoat material (BC) was placed into a transparent measuring cylinder of 500 ml content and a diameter of about 4 cm. The container was stored for 48 hours at room temperature without further treatment. After this time, the measuring cylinder was evaluated visually regarding the color of the different phases which formed after said storage time. A formation of different pigment phases is not observed, if the formed upper or lower phase is colorless.

1 .3 Determination of the luminous transmittance of the cured further

The luminous transmittance and haze of the cured further coating layer (CL) is determined at a film thickness of 15 to 18 micrometer using a Byk haze-Gard plus according to ASTM D 1003-00 (procedure A) with a CIE standard illuminant D65. For this purpose, the further coating material (CM) is doctor bladed on an uncoated glass plate, which has been cleaned prior to application of the coating material with water and soap, such that the resulting dry film thickness is 15 to 18 micrometer. After application, the further coating material was flashed off at 23 °C for 10 minutes, intermediate dried at 80 °C for 10 minutes and then cured at 120 °C for 20 minutes. 2. Preparation of aqueous basecoat compositions (BC)

2.1 Preparation of pigment pastes used for tinting of straight shade basecoat materials Various pigment pastes were prepared by mixing the compounds listed in Tables 1 to 4 below.

- 38 - able 1 : Ingredients of different black pigment pastes P1 to P6 (all amounts are given in wt.%, based on the respective pigment paste) ) prepared according to WO 90/15528 (resin dispersion A) ) Carbon black, available from Carbot, p(Monarch 1400) = 1.8 g/ml ) iron oxide black pigment, available from Lanxess, p(Bayferrox 318M) = 4.6 g/ml ) copper chromite black pigment, available from Dainichiseika Color & Chemicals, p(Dypyroxide black 9510) = 5.2 g/ml ) chromium iron oxide black pigment, available from BASF Colors & Effects GmbH, p(Sicopal Black K0095) = 4.6 g/ml ) chromium-lll-oxide black pigment, available from Asahi Sangyo Kaisha, p(Black 10C909A) = 5.3 g/ml ) 80:20 wt.% mixture of Titan Rutil 2310 (titanium dioxide, available from x) and Titan Rutil Dentall WK500 (tin antimony cassiterite, vailable from Moriroku Chemicals Company), p(Titan Rutil 2310) = 4.2 g/ml, p( Titan Rutil Dentall WK500) = 6.3 g/ml ) polypropylene oxide, available from BASF SE

- 39 - able 2: Ingredients of different blue pigment pastes P7 to P13 (all amounts are given in wt.%, based on the respective pigment paste) ) prepared according to WO 90/15528 (resin dispersion A) ) C.l. Pigment Blue 60, available from BASF Colors & Effects GmbH, p(Pigment Blue L6470) = 1.45 g/ml ) C.l. Pigment Blue 27, available from Trust Chem Co., Ltd., p(Pigment Blue 27 TCB02704) = 1 .8 g/ml ) C.l. Pigment Blue 36, available from BASF Colors & Effects GmbH, p(Sicopal blue K7210) = 4.5 g/ml ) C.l. Pigment Blue 28, available from Heubach GmbH, p(Heucodur blue 550) = 4.3 g/ml ) C.l. Pigment Blue 76, available from BASF Colors & Effects GmbH, p(Heliogen blue L6955) = 4.5 g/ml ) C.l. Pigment Blue 15, available from BASF Colors & Effects GmbH, p(Heliogen blue L6905F) = 1.62 g/ml ) C.l. Pigment Blue 16, available from BASF Colors & Effects GmbH, p(Heliogen blue L7560) = 1.6 g/ml ) Solution of a high molecular weight block copolymer with pigment affinic groups, available from BYK GmbH 0 ⁾ polypropylene oxide, available from BASF SE

- 40 - able 3: Ingredients of different red pigment pastes P14 to P16 (all amounts are given in wt.%, based on the pigment paste) ) prepared according to WO 90/15528 (resin dispersion A) ) C.l. Pigment Red 179 (Hostperm Red P2GL-WD, available from Colorants Solutions, p(Hostperm Red P2GL-WD) = 1.5 g/ml ) C.l. Pigment Red 254, available from BASF Colors & Effects GmbH, p(lrgazin Red DPP) = 1.58 g/ml ) red iron oxide pigment, available from Lanxess, p(Bayferrox 130BM) = 5.0 g/ml ) Solution of a high molecular weight block copolymer with pigment affinic groups, available from BYK GmbH ) polypropylene oxide, available from BASF SE

- 41 - able 4: Ingredients of two different green pigment pastes P17 and P18 (all amounts are given in wt.%, based on the pigment paste) ) prepared according to WO 90/15528 (resin dispersion A) ) green cobalt titanium oxide spinel pigment, available from Alfred Kochen GmbH & Co. KG, p(Daipyroxide Green 9320) = 5.0 g/ml ) green chromium oxide pigment, available from Lanxess, p(Chromium oxide green GN-M granel) = 5.2 g/ml ) Solution of a high molecular weight block copolymer with pigment affinic groups, available from BYK GmbH ) polypropylene oxide, available from BASF SE ) liquid blend of nonionic alkoxylated compounds, available from Munzing Chemie GmbH

2.2 Preparation of coloring pigment pastes

2.2.1 White pigment paste W1

The white pigment paste W1 was prepared according to example 2.3 of DE-A- 19705219. The white pigment Titan Rutil 2310 contained in said paste has a density p(TiO2) of 4.2 g/ml.

2.2.2 Yellow pigment paste Y1

The yellow pigment paste Y1 was prepared from 55 wt.% of a resin dispersion prepared according to WO 90/15528 resin dispersion A (acrylated polyurethane dispersion), 38 wt.% of a yellow bismuth-vanadatium oxide pigment (Irgazin Yellow 2GTA, available from BASF Colors & Effects GmbH, p(lrgazin Yellow 2GTA) = 5.5 g/ml), 2.0 wt.% of Pluriol P900 (polypropylene oxide, available from BASF SE) and 5.0 wt.% of demineralized water.

2.2.3 Red pigment paste R1

The red pigment paste R1 was prepared from 16.9 wt.% of a resin dispersion prepared according to WO 90/15528 resin dispersion A (acrylated polyurethane dispersion), 47.1 wt.% of a red iron oxide pigment (Bayferrox 130BM, available from Lanxess, p(Bayferrox 130BM) = 5.0 g/ml), 4.7 wt.% Pluriol P900 (polypropylene glycol available from BASF SE), 0.1 wt.% methyl-iso-butyl ketone, 2.0 w% butyl cellusolve, 0.6 wt.% dimethylethanol amine (10 w% in demineralized water), and 28.6 wt.% of demineralized water.

2.2.4 Green pigment paste G1

The green pigment paste 2 was prepared from 50.5 wt.% of a resin dispersion prepared according to WO 90/15528 resin dispersion A (acrylated polyurethane dispersion), 35.6 wt.% of a green chromium oxide pigment (Chromium oxide green GN-M granel, available from Lanxess, p(Chromium oxide green GN-M granel) = 5.2 g/ml), 4.3 wt.% of Disperbyk-184 (available from Byk Chemie GmbH), 3.2 wt.% Pluriol P900 (polypropylene glycol available from BASF SE), 0.3 wt.% of Agitan 299 (available from Muenzing Chemie), and 6.1 w% of demineralized water. 2.2.5 Blue pigment paste B1

The blue pigment paste B1 was prepared from 21.6 wt.% of a resin dispersion prepared according to WO 90/15528 resin dispersion A (acrylated polyurethane dispersion), 22 wt.% of a C.l. Pigment Blue 76 (Heliogen blue L6955, available from BASF Colors & Effects GmbH, p(Heliogen blue L6955 = 4.5 g/ml), 2.5 wt.% Disperbyk- 184 (available from Byk Chemie GmbH), 0.1 wt.% methyl-iso-butyl ketone, 1.6 wt.% dipropylene glycol monomethyl ether, 0.8 wt.% 1 ,2-propylene glycol, 4.4 wt.% of Pluriol P900 (polypropylene oxide, available from BASF SE), 0.6 wt.% dimethylethanol amine (10 wt.% in demineralized water), and 46.4 wt.% of demineralized water.

2.2.6 Black pigment paste BL1

The black pigment paste BL1 was prepared from 25.1 wt.% of an acrylated polyurethane dispersion prepared according to example A of EP-B-521928, 10 wt.% of perylene black (Paliogen® SW L0086, available from BASF Colors & Effects GmbH, p(Paliogen® SW L0086= 1.5 g/ml)), 2.0 wt.% Pluriol P900 (polypropylene oxide, available from BASF SE), 1.5 wt.% of dimethylethanol amine (10 w% in demineralized water) and 61.4 wt.% demineralized water

2.3 Preparation of tinted aqueous solid color basecoat materials

2.3.1 Aqueous white solid color basecoat tinted with different pigment pastes

The compounds listed in Table 5 were mixed in the stated order to obtain an aqueous white solid color basecoat material (BC). The material is subsequently adjusted using deionized water and dimethylethanolamine to a pH of 8.0 and to a spray viscosity of 95 mPa*s (measured at a shear rate 1000 s ^_1 using a rotary viscometer (Rheomat RM 10 instrument from Mettler-Toledo) at 23 °C). Tinting was done by using the respective pigment pastes of Tables 1 to 4 in amounts of 0.1 wt.% to 0.5 wt.% (see Table 6 for amount of respective tinting paste). An overview of all prepared aqueous white solid color basecoat materials BC1 to BC18 tinted with different pigment pastes P1 to P18 is given in Table 6. Table 5: Ingredients used to prepare an aqueous white solid color basecoat material

¹⁾ melamine formaldehyde resin, available from BASF SE

²⁾ available from BASF SE

³⁾ polypropylene oxide, available from BASF SE ⁴⁾ available from Exxon Mobile

⁵⁾ available from Shell

⁶⁾ available from Munzing Chemie

⁷⁾ prepared according to example 3 of DE-A-04028368

⁸⁾ prepared according to example A of EP-B-521928

- 45 - able 6: Overview of prepared aqueous white solid color basecoat materials tinted with different pigment pastes ) Titan Rutil 2310 (TiO2) having a density p(TiO2) = 4.2 g/ml is contained in white pigment paste W1

2.3.2 Aqueous yellow solid color basecoat tinted with different pigment pastes

The aqueous yellow solid color basecoat material was prepared as described for the aqueous white solid color basecoat material (see point 2.3.1 ) by using an equal amount of yellow pigment paste Y1 (see point 2.2.2) instead of white pigment paste W1 . Tinting was done by using the respective pigment pastes of Tables 1 to 4 or the white paste W1 (see point 2.2.1 ) in the amounts listed in Table 7.

Table 7: Overview of prepared aqueous yellow solid color basecoat materials tinted with different pigment pastes

¹⁾ Irgazin Yellow 2GTA (BiVO4) having a density p(BiVO4) = 5.5 g/ml is contained in yellow pigment paste Y1

2.3.3 Aqueous red solid color basecoat tinted with different pigment pastes

The aqueous red solid color basecoat material was prepared as described for the aqueous white solid color basecoat material (see point 2.3.1 ) by using an equal amount of red pigment paste R1 (see point 2.2.3) instead of white pigment paste W1 . Tinting was done by using the respective pigment pastes of Tables 1 to 4, the white paste W1 (see point 2.2.1 ) or the yellow pigment paste Y1 (see point 2.2.2) in the amounts listed in Table 8. Table 8: Overview of prepared aqueous red solid color basecoat materials tinted with different pigment pastes

¹⁾ Bayferrox 130BM (a-Fe2Os) having a density p(a-Fe2Os) = 5.0 g/ml is contained in red pigment paste R1

2.3.4 Aqueous green solid color basecoat tinted with different pigment pastes

The aqueous green solid color basecoat material was prepared as described for the aqueous white solid color basecoat material (see point 2.3.1 ) by using an equal amount of green pigment paste G1 (see point 2.2.4) instead of white pigment paste W1 . Tinting was done by using the respective pigment pastes of Tables 1 to 4, the white paste W1 (see point 2.2.1 ) or the yellow pigment paste Y1 (see point 2.2.2) in the amounts listed in Table 9.

Table 9: Overview of prepared aqueous green solid color basecoat materials tinted with different pigment pastes

¹⁾ Chromium oxide green GN-M granel (Cr20s) having a density p(Cr20s) = 5.2 g/ml is contained in green pigment paste G1

2.3.5 Aqueous blue solid color basecoat tinted with different pigment pastes

The aqueous blue solid color basecoat material was prepared as described for the aqueous white solid color basecoat material (see point 2.3.1 ) by using an equal amount of blue pigment paste B1 (see point 2.2.5) instead of white pigment paste W1 . Tinting was done by using the respective pigment pastes of Tables 1 to 4, the white paste W1 (see point 2.2.1 ) or the yellow pigment paste Y1 (see point 2.2.2) in the amounts listed in Table 10.

Table 10: Overview of prepared aqueous blue solid color basecoat materials tinted with different pigment pastes

¹⁾ Heliogen Blue L6955 (C.l PB 76) having a density p(C.I PB 76) = 4.5 g/ml is contained in blue pigment paste B1

2.3.6 Aqueous black basecoat tinted with different pigment pastes

The compounds listed in Table 11 were mixed in the stated order to obtain aqueous black basecoat materials (BC-B1 ) to (BC-B4). The materials are subsequently adjusted using deionized water and dimethylethanolamine to a pH of 8.0 and to a spray viscosity of 95 mPa*s (measured at a shear rate 1000 s ^_1 using a rotary viscometer (Rheomat RM 10 instrument from Mettler-Toledo) at 23 °C). Tinting was done by using the respective pigment pastes of Tables 1 to 4 or the yellow paste Y1 (see point 2.2.2) in amounts of 0.1 wt.% to 0.4 wt.% (see Tables 11 -15 for amount of respective pigment paste used for tinting). An overview of all prepared aqueous black solid color basecoat materials BC50 to BC85 tinted with different pigment pastes is given in Tables 11 to 15. Table 11 : Ingredients used to prepare aqueous black basecoat materials (all amounts are given in wt.%, based on the total weight of the respective basecoat material)

²⁾ available from BASF SE ³⁾ polypropylene oxide, available from BASF SE

⁴⁾ available from Exxon Mobile

⁵⁾ available from Shell

⁶⁾ available from Munzing Chemie

⁷⁾ prepared according to example 3 of DE-A-04028368 ⁸⁾ prepared according to example A of EP-B-521928

⁹⁾ The aluminum slurry was prepared by mixing of 33 wt.% Alu-Stapa® Hydrolux 2154 (available from Altana-Eckart, density = 2.7 g/ml), 40 wt.% of butyl glycol (available from BASF SE) and 27 wt.% of a polyester prepared according to example D, column 16, line 37-59 of DE-A-4009858 Table 12: Overview of prepared aqueous black solid color basecoat materials prepared from black basecoat material BC-B1 tinted with different pigment pastes

¹⁾ carbon black having a density p(carbon black) = 1.8 g/ml is contained in black pigment paste P1

Table 13: Overview of prepared aqueous black solid color basecoat materials prepared from black basecoat material BC-B2 tinted with different pigment pastes

¹⁾ Bayferrox 318M having a density p(Fe2Os) = 4.6 g/ml is contained in black pigment paste P2

Table 14: Overview of prepared aqueous black solid color basecoat materials prepared from black basecoat material BC-B3 tinted with different pigment pastes

¹⁾ Paliogen SW L0086 having a density p(perylene) = 1.5 g/ml is contained in black pigment paste BL-1

Table 15: Overview of prepared aqueous black effect basecoat materials prepared from black basecoat material BC-B4 tinted with different pigment pastes

¹⁾ Monarch 1400 having a density p(carbon black) = 1.8 g/ml is contained in black pigment paste P1

2.4 Preparation of further aqueous basecoat materials

The compounds listed in Table 16 were mixed in the stated order to obtain further coating materials BC86 and BC87. The materials are subsequently adjusted using deionized water and dimethylethanolamine to a pH of 8.0 and to a spray viscosity of 95 mPa*s (measured at a shear rate 1000 s ^-1 using a rotary viscometer (Rheomat RM 10 instrument from Mettler-Toledo) at 23 °C). Table 16: Ingredients used to prepare aqueous light grey basecoat materials BC86 and BC87 (all amounts are given in wt.%, based on the total weight of the respective basecoat material)

¹⁾ melamine formaldehyde resin, available from BASF SE

²⁾ available from BASF SE

³⁾ polypropylene oxide, available from BASF SE

⁴⁾ available from Exxon Mobile

⁵⁾ available from Shell

⁶⁾ available from Munzing Chemie

⁷⁾ prepared according to example 3 of DE-A-04028368

⁸⁾ prepared according to example A of EP-B-521928

The compounds listed in Table 17 were mixed in the stated order to obtain a further coating material (CM). The material is subsequently adjusted using deionized water and dimethylethanolamine to a pH of 8.0 and to a spray viscosity of 95 mPa*s (measured at a shear rate 1000 s ^_1 using a rotary viscometer (Rheomat RM 10 instrument from Mettler-Toledo) at 23 °C). Table 17: Ingredients used to prepare an aqueous coating material (CM)

¹⁾ melamine formaldehyde resin, available from BASF SE

²⁾ available from BASF SE

³⁾ polypropylene oxide, available from BASF SE

⁴⁾ available from Exxon Mobile

⁵⁾ available from Shell

⁶⁾ available from Munzing Chemie

⁷⁾ prepared according to example 3 of DE-A-04028368

⁸⁾ prepared according to example A of EP-B-521928

⁹⁾ prepared according to example 9 - chapter [148] of EP-B-1265967

¹⁰⁾ the mica slurry was prepared analog to example 1 of EP-B-1504068 by the usage of 0.5 parts of Iriodin® 9225 SQB Rutil Perlblau (available from Merck chemicals GmbH), 4.8 parts of Iriodin® 9119 SW Polarweil3> (available from Merck chemicals GmbH), 1.2 parts of Mearlin Ext. Fine Pearl 139V (available from BASF Colors & Effects) and 17 parts of a mixture of 81.9 parts demineralized water, 2.7 parts of Rheovis AS S130 (available from BASF SE), 8.9 parts of TMDD BG 52 (available from BASF SE), 3.2 parts of Dispex Ultra FA 4437 (available from BASF SE) and 3.3 parts of 10wt.% aqueous dimethylethanolamine.

The further coating material - when cured - has a luminous transmittance of 56.1 ± 0.35 % and a haze of 89.4 ± 0.54 % (determined as described in point 1 .3 above). 4. Preparation of multilayer coatings

4.1 Preparation of multilayer coatings using a 3C1 B-process

Steel panels (30 x 50 cm, Chemetall) coated with a cured standard cathodic electrocoat material (Cathoguard 800, available from BASF Coatings GmbH) and a cured surfacer-system (HR Light Grey 121 -71245-10, dry film thickness 25 to 30 micrometers, flashing for 5 minutes at 23 °C, drying for 5 minutes at 70 °C, curing for 10 minutes at 150 °C) were coated with via electrostatic application with the agueous basecoat composition BC1 or BC2 (see Table 6) such that the film thickness of the obtained basecoat layer BL after drying was 15 micrometers. The panels were dried for 2 minutes at 23 °C and subseguently coated with the agueous further coating material (CM) via electrostatic application such that the film thickness after drying was 6 micrometers. The panels were dried for 2 minutes at 23 °C and then for 10 minutes at 70 °C. Afterwards, a ProGloss® two-component clearcoat material available commercially from BASF Coatings GmbH (FF99-0373) was applied atop the respective dried waterborne basecoat layer. The resulting clearcoat layer was flashed off at 23 °C for 20 minutes. The basecoat layer BL, the further coating layer (CL) and clearcoat layer were then jointly cured in an air circulation oven at 140 °C for 20 minutes. The film thickness of the cured clearcoat layer was constant over the whole panel (±1 micrometers), with a clearcoat film thickness of 40 to 45 micrometers.

4.2 Preparation of multilayer coatings using a 4C1 B-process

Steel panels (30 x 50 cm, Chemetall) coated with a cured standard cathodic electrocoat material (Cathoguard 800, available from BASF Coatings GmbH) were coated via electrostatic application with the agueous basecoat material BC86 or BC87 such that the film thickness of the obtained basecoat layer BL-1 after drying was 20 micrometers. The panels were flashed off for 4 minutes at 23 °C and then coated with the agueous basecoat material BC1 or BC2 (see Table 6) via electrostatic application such that the film thickness of the obtained basecoat layer BL-z after drying was 14 micrometers. The panels were dried for 2 minutes at 23 °C and subseguently coated with the agueous further coating material (CM) via electrostatic application such that the film thickness after drying was 6 micrometers. The panels were dried for 2 minutes at 23 °C and then for 10 minutes at 70 °C. Afterwards, a ProGloss® two- component clearcoat material available commercially from BASF Coatings GmbH (FF99-0373) was applied atop the dried basecoat layer BL-z. The resulting clearcoat layer was flashed off at 23 °C for 20 minutes. The basecoat layers BL-1 and BL-z, the further coating layer (CL) and the clearcoat layer were then jointly cured in an air circulation oven at 140 °C for 20 minutes. The film thickness of the cured clearcoat layer was constant over the whole panel (±1 micrometers), with a clearcoat film thickness of 40 to 45 micrometers.

5. Results

5.1 Storage stability of aqueous basecoat materials

The storage stability of aqueous basecoat materials prepared according to point 2. was determined as described in point 1.2 above. The obtained results are listed in the following tables.

Table 18: storage stability of aqueous white straight shade basecoat materials tinted with different pigment pastes Table 19: storage stability of aqueous yellow straight shade basecoat materials tinted with different pigment pastes

Table 20: storage stability of aqueous red straight shade basecoat materials tinted with different pigment pastes

Table 21 : storage stability of aqueous green straight shade basecoat materials tinted with different pigment pastes Table 22: storage stability of aqueous blue straight shade basecoat materials tinted with different pigment pastes

Table 23: storage stability of aqueous black straight shade basecoat material BC-B1 tinted with different pigment pastes

Table 24: storage stability of aqueous black straight shade basecoat material BC-B2 tinted with different pigment pastes Table 25: storage stability of aqueous black straight shade basecoat material BC-B3 tinted with different pigment pastes

Table 26: storage stability of aqueous black effect basecoat material BC-B4 tinted with different pigment pastes

The results summarized in Tables 11 to 26 demonstrate that the formation of different pigment phases and thus a reduced storage stability after 48 h without circulation of the basecoat material only occurs if a pigment P having a relatively high density is combined with further pigment(s) having a relatively low pigment density, i.e. the density difference(s) is/are greater than 2, while no formation of different pigment phases is observed if each density difference between the pigments being present in the aqueous basecoat material is 2 or less (including negative density differences). Upon storage of the basecoat material, high density ingredients sediment and form a lower phase while ingredients having a lower density are present in the upper phase. In case a pigment mixture is used which contains pigments having a relatively high density and pigments having a relatively low density, i.e. the density difference is greater than 2, the pigments having a high density will sediment while the pigments having a low density will be present in the upper phase, thus resulting in the formation of different pigment phases. In case the pigment mixture only contains pigments having a relatively high density, i.e. pigment densities of at least 4 g/ml, all pigments will sediment upon storage to the lower phase, thus preventing the formation of different pigment phases (see Tables 11 to 22). The same applies in case the pigment mixture only contains pigments having a relatively low density, i.e. pigment densities of less than 2.5 g/ml, since no phase formation is observed in these cases (see Tables 23 to 26). The formation or absence of different pigment phases is observed irrespective of the type and combination of different pigments.

5.2 Optical properties of multilayer coatings

The lightness of comparative multilayer coating MC-1 (prepared using basecoat material BC1 ) and inventive multilayer coating MC-2 (prepared using basecoat material BC2) was evaluated visually. The multilayer coatings MC-1 and MC-2 were prepared according to point 4.1 from basecoat materials BC1 and BC2 which were each stored in tap lines for 48 h prior to their application to the substrate. Additionally, reference multilayer coatings MC-R1 and MC-R2 were prepared from basecoat materials BC1 and BC2 which were homogenized prior to their application. The lightness of the multilayer coatings MC-1 and MC-2 was visually compared with the respective reference multilayer coating. The results are listed in Table 27.

Table 27: Result of visual inspection of prepared multilayer coatings inventive

The formation of different pigment phases in the basecoat material BC1 upon storage of said material in the tap line prior to its application results in a negative impact on the visual impression, i.e. an undesired color shift to the color of the tinting paste. Without being bound to this theory, the observed undesired color shift to the color of the tinting paste seems to be due to the formation of different pigment pastes upon storage of the basecoat material BC1 in the tap line. In contrast, the use of a pigment mixture in which the pigment density difference is 2 does not result in the formation of different pigment phases upon storage in the tap line and thus allows a constant visual impression even if the basecoat material BC2 is not homogenized prior to its application. 6. Discussion of the results

The formation of different pigment phases upon storage of aqueous basecoat materials is only occurring if each density difference between pigments being present in said aqueous basecoat material is greater than 2 g/ml. Application of basecoat materials in which different pigment phases have been formed after storage without circulation results in a basecoat layer having an undesired color shift. Multilayer coatings prepared from such stored basecoat materials therefore do not meet the predefined color quality.

In contrast, the selection of pigments such that each density difference between the selected pigments is 2 or less does not result in the formation of different pigment phases. Application of such basecoat materials in which no formation of different pigment phases is observed after storage results in basecoat layers not having an undesired color shift even if the basecoat materials are not homogenized prior to their application. Multilayer coatings prepared from such stored basecoat materials therefore do meet the predefined color quality.

In conclusion, the selection of pigments such that each density difference between the selected pigments is 2 g/ml or less in aqueous basecoat materials renders it possible to obtain a constant high color quality of multilayer coatings prepared from such basecoat materials irrespective of the storage time of such basecoat materials without circulation, for example in tap lines connection the application gear with the circulation line. Thus, said basecoat materials render cleaning of the tap line prior to use superfluous, resulting in reduced manufacturing costs and waste.