ZEP MOLD STAIN & MILDEW STAIN REMOVER The industrial strength formula of Mold Stain & Mildew Stain Remover quickly dissolves mold and mildew stains on toilet bowls, sinks, ceramic tile, grout and other unpainted surfaces. The bleach-based product is intended for use on fiberglass, tile and porcelain surfaces and can be used both inside and outside. Its unique, ready-to-use formula removes stains without the need to scrub.