The present invention relates to a copolymer obtainable by copolymerization of at least three ethylenically unsaturated monomers each different from one another, the copolymer having at least three structural units (S1) and (S2) and (S3) different from one another, to the use of this copolymer as an adhesion-promoting additive in an aqueous coating composition, to an aqueous coating composition comprising at least one polymeric resin (A1), at least one pigment (B), and at least one copolymer of the invention, used as adhesion-promoting additive, as component (C), and also to methods for at least partly coating a substrate with a basecoat and for at least partly coating a substrate with a multicoat paint system.