Adhesion promoter for coating compositions suitable for producing surfacer coats

A mixture (M) comprising at least two components (M1) and (M2), which are different from one another and comprise in each case independently of one another free and/or blocked isocyanate groups, and which mixture optionally comprises a further component (M3) different from (M1) and (M2);

• wherein component (M1) comprises at least one aliphatic polyisocyanate, wherein component (M1) has an NCO content in a range from 8 to 25 wt. % based on total weight of component (M1),
• component (M2) comprises at least one aliphatic polyisocyanate having polyether units, the relative weight ratio of components (M1) and (M2) in the mixture (M) to one another is in a range from 25:1 to 3:1, based in each case on the solids content of components (M1) and (M2), and
• wherein the mixture (M) is obtained by mixing components (M1) and (M2) and optional component (M3) and then partly reacting the isocyanate groups of components (M1) and (M2) with at least one hydrolyzable silane compound which has at least one functional group that is reactive toward isocyanate groups such that 5 to 30 mol % of all the isocyanate groups originally present in components (M1) and (M2) are reacted with the at least one hydrolyzable silane compound, such that each of components (M1) and (M2) contain a hydrolyzable silane group after reaction.