This invention relates to a process for coating a substrate which comprises contacting the substrate with a low volatility organic (so-called "low VOC") coating composition having a viscosity as measured by ZAHN cup 2 of less than about 200 seconds and consisting essentially of at least one polyisocyanate prepolymer which is the reaction product of a polyisocyanate with an active hydrogen-containing compound selected from the group consisting of monoahls, polyols, imines, oxazolidines, and combinations thereof, a solvent in an amount of between 0% and 45% by weight based upon the amount of said polyisocyanate in said composition, and a tertiary amine catalyst, said composition being essentially free of any volatile mono- and di-isocyanates, and said composition being moisture curable upon exposure to atmospheric moisture. Also claimed is the coating composition itself.