Lacquers are disclosed, containing (i) a synthetic resin (A) which contains hydroxyl groups and α,β-unsaturated carbonyl groups or a mixture of such synthetic resins and/or (ii) a mixture of (a) a hydroxyl group-containing synthetic resin (B) or a mixture of such synthetic resins and (b) a cross-linking agent (C) which contains α,β-unsaturated carbonyl groups or a mixture of such cross-linking agents and (iii) a catalyst (D) that catalyzes the reaction of the hydroxyl groups with the α,β-unsaturated carbonyl groups or a mixture of such catalysts. The invention is characterized by the fact that the catalyst (D) is a Brönsted's acid.