The invention relates to a primer containing a solvent-containing two-component polyurethane binder comprising I) a curing component (A), comprising an addition product of at least one organic polyisocyanate (B) with an average NCO functionality of 2.5 to 5.0 and an isocyanate content of 8 to 27 wt. % and an alkoxysilane (C) of formula (I) with at least one isocyanate-reactive group, Q—Z—SiXaY3−a  (I), in which Q represents an isocyanate-reactive group, Z represents a linear or branched C1-C12 alkylene group, X represents a hydrolyzable group, Y represents identical or different C1-C4 alkyl groups and a is an integer from 1 to 3, and II) a lacquer resin (D) which can react with isocyanate groups as adhesion promoter.