Blocked polyisocyanates containing aliphatically and/or cycloaliphatically bound isocyanate groups blocked by a mixture of blocking agents containing A) 30 to 70 equivalent % of 1,2,4-triazole, B) 30 to 70 equivalent % of 3,5-dimethylpyrazole, and C) 0 to 30 equivalent % of other blocking agents, a method of preparing these blocked polyisocyanates by blocking the isocyanate groups of a (cyclo)aliphatic polyisocyanate, wherein up to 20 NCO equivalent % of the isocyanate groups may optionally be reacted with compounds containing carboxylic acid hydrazide groups and isocyanate-reactive groups, and the use of the blocked polyisocyanates as crosslinking agents for organic polyhydroxyl compounds in one-component polyurethane stoving compositions.

An aqueous binder mixture comprising substantially an aqueous solution and/or dispersion of a combination of a) at least one NCO prepolymer which is soluble and/or dispersible in water in the presence of component b), exhibits urethane groups and has blocked isocyanate groups, and b) a polyamine component comprising at least one (cyclo)aliphatic polyamine having at least 2 primary and/or secondary amino groups, and the use of the aqueous binder mixture for the preparation of coatings on heat-resistant substrates.