Coating method with crosslinked coatings from two coat-one bake systems

In a method for forming coatings on a substrate, comprising the steps of applying to the substrate a base coating consisting essentially of (i) a coloring pigment and/or a metallic pigment and (ii) a vehicle component in a solvent to produce a base coat, appying a topcoating containing a hydroxyl group-containing resin and a polyisocyanate compound over the base coat, and curing the coats at a temperature range between room temperature and about 140.degree. C., the improvement wherein the vehicle component is selected from the group consisting of:

• (a) a resin having a hydroxyl group and a carboxyl group in the molecule;
• (b) a combination of a resin having a hydroxyl group in the molecule and a catalyst for promoting the reaction between the hydroxyl group of the resin and an isocyanate group of the polyisocyanate compound in the topcoating;
• and
• (c) a combination of a resin having a hydroxyl group and a carboxyl group in the molecule and a catalyst for promoting the reaction between the hydroxyl group of the resin and an isocyanate group of the polyisocyanate compound in the topcoating;
• wherein the resin contained in the base coating has a number-average molecular weight of about 5,000 to about 100,000 when the solvent of the base coating is an organic solvent and a number-average molecular weight of about 5,000 to about 2,000,000 when the solvent of the base coating is an aqueous solvent.