In the novel process, a precondensate solution is firstly prepared in a manner known from EP-B 0 355 760, by mixing the starting materials, for example melamine and aqueous formaldehyde solution, and, if desired, paraformaldehyde, in a molar ratio melamine: formaldehyde of from 1:1.5 to 1:3, preferably from 1:2 to 1:3, if desired adding known modifiers and additives which do not disturb the conden