The invention is directed to coating compositions comprising A) at least one hydroxyl-functional and optionally acid-functional binder, preferably with an acid number of 0 to 150 mg KOH/g binder solids, B) at least one polyisocyanate curing agent with free isocyanate groups, C) at least one thermally latent tetrahydrocarbyl tin catalyst of general formula (I) RnR′4-nSn  (I) wherein R is an alkyl group, R′ is an aryl, arylalkyl or alkenyl group and n is 1, 2 or 3, and D) optionally at least one acid-functional compound, that is different from component A), with the proviso that the acid number of a composition, consisting of the at least one binder A) and optionally of the at least one acid-functional compound D), has an acid number of at least 5 mg KOH/g solids, preferably of least 10 mg KOH/g solids, and to a process for coating substrates comprising the steps: I) applying the coating composition to an optionally pre-coated substrate, and II) curing the applied coating composition by means of thermal energy.