Antimicrobial guanidine compounds are used, either alone or with at least
one other antimicrobial compound, to make mildew-resistant wallboard.
Particularly useful co-additives are o-phenyl phenol and sodium
methyldithiocarbamate. The antimicrobial compound(s) may be added to the
covering, the core body, an adhesive at the interface of the core body
and the covering, or all three of these sites. A preferred guanidine
compound is dodecyl guanidine hydrochloride.