The present invention is directed to a process for the preparation of air drying binders containing free isocyanate groups, having thixotropic properties and prepared by reacting aromatic diamines which have at least one alkyl substituent in the ortho position to each amino group with either prepolymers or semi-prepolymers containing free isocyanate groups and based on polyisocyanate mixture of the diphenylmethane series containing at least 20% by weight of 2,4'-diisocyanatodiphenylmethane and organic polyhydroxyl compounds or mixtures of these prepolymers or semi-prepolymers with prepolymers or semi-prepolymers based on other organic polyisocyanates, provided that the mixture contains at least 3% of the prepolymers or semi-prepolymers based on polyisocyanate mixtures of the diphenylmethane series. The invention further relates to the binders produced by this process and their use for the preparation of coating and sealing compounds which harden under the influence of atmospheric moisture.