This invention relates to aminoplast crosslinker resins compositions based on at least partially alkylated reaction products A of melamine, formaldehyde and an alkanol with the following parameters: the ratio of the amount of substance n(CH2O) of combined formaldehyde to the amount of substance n(Mel) of melamine is in the range of from 5.6 mol/mol to 6.2 mol/mol, the ratio of the amount of substance n(RO) of alkyl ether groups in the crosslinker resin to the amount of substance n(Mel) of melamine is in the range of from 5.0 mol/mol to 5.6 mol/mol, the mass fraction of monomer in the resin, calculated as the ratio of the mass of monomer m(1) to the mass m(A) of the reaction products A is between 35% and 55%, to a process for their preparation, and to a method of use thereof as crosslinker in combination with hydroxy-functional polymers for coating of heat-sensitive substrates.