Abstract: The present invention relates to primer compositions comprising at least one binder which contains isocyanate groups and is prepared from a mixture of an amorphous polyester resin A which is solid at room temperature and contains at least one OH group and from at least one polyisocyanate B of the formula (I). The ratio of solid polyester resin A and polyisocyanate B is employed in this case in a ratio such that the stoichiometric ratio of the NCO groups to the OH groups is situated in the range between 50:1 and 100:1 and/or the weight ratio of polyester resin A to polyisocyanate B has a value between 1.6 and 0.75. The primer compositions are notable in particular for effective adhesion to plastics and paints, especially automobile paints.