Zep Commercial 1-Gallon Calcium, Lime & Rust Stain Remover Concentrate eliminates stains from various surfaces such as stucco, cement, porcelain, glass, fiberglass, and brick. Use on toilets, sinks, tubs, shower heads, and glassware. To use, simply mix the concentrate with water and apply it to the desired surface using a brush, sponge, or cloth.