Disclosed is a polymer which is prepared by reacting (a) dimer fatty acids with (b) at least one polyether of general structural formula (I) where R is a C3 to C6 alkylene radical and n is selected accordingly such that said polyether has 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