A coating composition for coating plastic substrates is disclosed which comprises at least one hydroxyl-functional film forming polymer, at least one polyisocyanate compound; and a novel adhesion promoter additive. The adhesion promoter additive comprises (A) at least one modified polyolefin; and (B) at least one component selected from the group consisting of monohydric alcohols, epoxy functional silanes, and mixtures thereof. The invention also relates to coated plastic substrates and method of coating such substrates.