Abstract: A two-package polyurethane coating system is provided which consists of a paint component comprising an OH-containing film-forming resin, such as a polyacrylate polyol or an alkyd resin, and a thinner-curing agent component which is a solution, adjusted to insure a predetermined final application viscosity of the coating, of trimerized isophorone diisocyanate which functions as a curing agent for the resin in an active hydrogen-free organic solvent which functions as a thinner, the two components being supplied in independent cans for mixing and application at a coating site without need for viscosity adjustment.