PURPOSE: To make it possible to improve coating workability, the appearance of coating film and film properties, by diluting an isocyanate-curable synthetic resin coating material in an organic solvent containing a reaction accelerator and applying the obtained solution to a substrate.

CONSTITUTION: An isocyanate-curable synthetic resin coating material is prepared by using a polyol resin and a polyisocyanate compound (e.g., hexamethylene diisocyanate) as a crosslinking agent as a vehicle component and mixing this vehicle component with, optionally, a pigment, a pigment dispersant, a flow modifier, a plasticizer, an ultraviolet absorber, etc. The obtained coating material is diluted with 100pts.wt. organic solvent (e.g., thinner) containing 0.001W5pts.wt. reaction accelerator (e.g., dilauryltin) and the obtained solution is applied to a substrate.