The invention is directed to a one-component (1K) coating system comprising an aqueous polyurethane and/or polyurethane-polyacrylate polymer dispersion and one or more compounds of elements of sub-groups 5 and/or 6 of the periodic table in which the particular element has an oxidation state of at least +4. The invention also relates to a method for improving the appearance of grained wood surfaces, the use of a coating system according to the invention for the coating of grained wood surfaces and a grained wood surface comprising a coating obtained by applying a coating system according to the invention and subsequent drying of the coating system.