disclosed is a composition and method for providing antimicrobial protection to plastic structures , such as plastic decking , planking , fencing , and panels . the method comprises applying a water - soluble biocide to the metal - containing structure , and converting the soluble biocide to a water - insoluble metal biocide salt that is adsorbed on the surface of , or into the porous structure of the plastic material . the slow release of the insoluble antimicrobial agent from the surface or from within the pores of said plastic structure provides antimicrobial protection for the plastic structure .