Thermally curable coating composition

A composition is disclosed which includes a urethane resin containing reactive ethylenically unsaturated groups and hydroxyl groups, which is preferably derived by reacting a polyisocyanate and an isocyanate reactive group-containing unsaturated monomer selected from the group consisting of hydroxyalkyl acrylates, hydroxyalkyl methacrylates, hydroxyalkyl acrylamides, hydroxy functional allylic compounds, and mixtures thereof to form an isocyanate group-containing reaction product which is then reacted with an organic polyol wherein the equivalent number of hydroxyl groups is in excess compared with the equivalent number of isocyanate groups. The composition also includes an aminoplast resin for reaction with hydroxyl groups on the urethane resin in an amount of less than about 10% by weight based on solids of the composition which is effective in thin film curing of the composition. The composition can also optionally include a thermally activated free radical initiator and a cure-promoting catalyst different from the free radical initiator; such as a metal soap. A method is also disclosed of applying a layer of the composition in which a portion of the layer is a thin film. The method is particularly useful in applying the composition to a substrate which is coated with a cationically electrodeposited primer. The composition and method of the invention are particularly useful for making a chip resistant coating and obtaining good cure at thin film layers of the composition.