The present invention relates to a polymer, preparable by reacting at least one polymer, preparable by reacting (a) dimer fatty acids with at least (b) one polyether having a C3 to C6 alkylene radical and a number-average molecular weight of 450 to 2200 g/mol, components (a) and (b) are used in a molar ratio of 0.7/2.3 to 1.3/1.7, and the resulting polymer has a number-average molecular weight of 1500 to 5000 g/mol and an acid number