The invention also relates to a method to increase stain resistance and penetration resistance of aqueous coating compositions which method comprises providing a coating composition which contain at least one pigment, at least one film-forming polymer in the form of an aqueous polymer dispersion and at least one anionic surfactant, wherein the anionic surfactant comprises at least 85% by weight, preferably at least 90% by weight, more preferably at least 95% by weight, based on the total weight of anionic surfactant in the coating composition, of at least one anionic surfactant S, which is selected from semi-esters of sulfuric acid or phosphoric acid with an alcohol, which alcohol carries at least one alkyl radical having from 8 to 30 carbon atoms or an alkyl substituted phenyl radical wherein alkyl has from 4 to 30 carbon atoms and which alcohol may carry an oligo-C.sub.2-C.sub.3-alkylene-ether group provided that the number of repeating units in the oligo-C.sub.2-C.sub.3-alkylene-ether group is at most 15, or a salt thereof.