A heat-curable binder based on an aqueous polymer dispersion comprising an emulsion polymer (EP), a polymer composed of at least 5% by weight of an ethylenically unsaturated monocarboxylic acid, dicarboxylic acid or dicarboxylic anhydride (acid polymer SP for short), and monofunctional or polyfunctional epoxide compounds as curatives. The monofunctional or polyfunctional epoxide compound is stirred, preferably in the liquid state, into the aqueous polymer dispersion. Besides the epoxide compound, the heat-curable binder may further comprise a polyol or an alkanolamine as hardener.