Disclosed is an emulsion of an internal paper size and a biocidal emulsifying agent, such as a cationic non-starchy quaternary ammonium compounds, that is introduced into a paper-making furnish to produce an anti-microbial paper. The antimicrobial paper is useful as facing for gypsum panels. A method of making the paper includes emulsifying the internal size with a biocide before introducing the size to the paper furnish. The disclosed method of making a mold-resistant gypsum panel comprises the steps of: selecting a biocidal emulsifying agent, wherein the biocidal emulsifying agent is a non-starchy quaternary ammonium compound； emulsifying an alkenyl succinic anhydride or alkyl ketene dimer internal sizing agent with the biocidal emulsifying agent to form a biocidal sizing emulsion in water； adding the biocidal sizing emulsion to a paper furnish； making a facer from the paper furnish； preparing a slurry of calcium sulfate hemihydrate and water； depositing the slurry onto the facer； forming the slurry into a microorganism&ndash；resistant gypsum panel； and allowing the slurry to set.
