A process for the production of coated planar moldings of thermoplastics is disclosed. The process entails extrusion of a molded body, cooling of the molding, applying the aqueous-based coating agent and drying. The coating agent, having a pH value of less than 6, contains a salt of sulfondicarboxylic diester, a water-insoluble oxide of metal or semi-metal and an acid/water mixture. It is applied to at least one surface of the molding immediately after the production of the molding and with the aid of an application roll.