PURPOSE: To provide the compsn. which forms an undercoating film having adhesion hardly lowering and properties, such as corrosion resistance, hardly being degraded even when application and baking have bean repeated several times to build a thick film.

CONSTITUTION: The compsn. contains a polyester polyol in an amt. of 10-20wt.% of the total resin component (solid base) contained in the compsn. Even when the compsn. is applied and baked several time, molecules of the polyester polyol P control or prevent the increase of crosslinking density between main chains A of an acrylic resin and molecules M of a melamine resin and thus moderate the rigidity of the resulting undercoating film.