The present invention relates to a reaction product preparable by reacting (a) dimer fatty acids with (b) at least one compound (b) of the general structural formula (I) where R is a C3 to C6 alkylene radical and n correspondingly is selected such that the compound (b) possesses a number-average molecular weight of 120 to 445 g/mol, components (a) and (b) are used in a molar ratio of 0.7/2.3 to 1.6/1.7, and the resulting reaction product possesses a number-average molecular weight of 600 to 4000 g/mol and an acid number of less than 10 mg KOH/g. The invention further relates to a pigmented aqueous basecoat material which comprises this reaction product, and to the use of the reaction product in pigmented aqueous basecoat materials for improving adhesion. The present invention further relates to a method for producing multicoat paint systems, and also to the multicoat paint systems producible by means of said method.