Use of a transition metal complex a) as a drier for air drying auto-oxidative resin composition comprising a polymer b) comprising unsaturated fatty acid residues, wherein the transition metal complex a) has a cation of the following structure (I) wherein R═C1-C20 alkyl optionally substituted with heteroatoms, C6-C20 aryl optionally substituted with heteroatoms, or a polymeric residue; R1═H, C1-C20 alkyl optionally substituted with heteroatoms, C6-C20 aryl optionally substituted with heteroatoms; X═O, OC(R2)O in which a=4 when X═O, and a=3 when X═OC(R2)O, R2═C1-C20 alkyl optionally substituted with heteroatoms, C6-C20 aryl optionally substituted with heteroatoms, or a polymeric residue.