Aqueous coating agent including a water-dilutable mixture of binding agents ofI. 50 to 88 wt. % of a neutralised water-soluble reaction product including(A) 10 to 80 wt. % of a polyurethane resin with an acid number of 70 to 160, at least one terminal blocked isocyanate group devoid of free hydroxyl groups and fatty acid residues with more than 12 C-atoms and with a limiting viscosity number of 6.5 to 12.0 ml/g, measured in DMF at 20.degree. C., and(B) 20 to 90 wt. % of a polyester resin with an OH-number of 50 to 500, an acid number of less than 20, a limiting viscosity number of 8.0 to 13.0 ml/g, measured in DMF at 20.degree. C., with the reaction product having a limiting viscosity number of 13.5 to 18.0 ml/g measured in DMF at 20.degree. C.II. 10 to 40 wt. % of one or several blocked polyisocyanates not capable of being diluted with water, andIII. 2 to 30 wt. % of at least one amine resin by way of additional cross-linking agent.