An in-mold coating composition is disclosed which includes (a) an isocyanate-terminated material containing an excess of reactive isocyanate groups and (b) a material selected from the group consisting of a composition containing reactive hydrogen atoms, generally hydroxyl groups, and a catalyst capable of trimerizing the terminal isocyanate groups. The coating composition is applied to a molded substrate, such as an FRP part, by placing a charge of the composition in the mold on the substrate, the charge being sufficient to provide a coating having a thickness less than about 20 mils. Appropriate pressure is then applied to the charge, such as by closing the mold, so that the coating composition is forced to cover substantially the entire surface area of the substrate, filling any surface defects. The composition is cured to yield a crosslinked, isocyanate-based coating which adheres tightly to the substrate.