The invention relates to a method for bonding metal or plastic substrates, in which the substrate is freed from adhering impurities, a dual-component polyurethane adhesive is directly applied onto at least one substrate without prior application of a primer and the substrates are joined and cured. According to the invention, the dual-component polyurethane adhesive is composed of a component A, containing 1 to 98% by weight with reference to the component A of an oleochemic polyol, 1 to 10% by weight of at least one three-, four- or five-functional polyol with a molecular weight of 90 to 750 g/mol, 2.5 to 60% by weight of at least on aldehyde resin, keton resin and/or keton/aldehyde resin with a molecular weight of 250 to 25000 g/mol, and 0 to 70% by weight of other additives and a component B, containing at least one polyisocyanate, the NCO/OH ratio of the isocyanate component to the polyol component ranging between 1.0 and 2.0:1.