If you're dealing with Metals or minerals in pool water coming out of solution, Rust emerging through the wall, floor, or around fittings, Organic material like leaves, dirt, oils, worms, or algae pool stains , the Pool Plaster Hand Disc NON-CORROSIVE is the ideal solution. This foam block fits perfectly in your hand, allowing you to apply the necessary pressure for stain removal. Unlike other sanding blocks, our diamonds are bonded with NON-CORROSIVE material, ensuring that no residuals will remain in the pool. With a medium grit, this product removes all small topical plaster stains and lasts three times longer than Pool Stone. Use it above or below the waterline, and don't forget to wear a mask to make it easier to see. Every plaster swimming pool should have one of these hand discs for stain removal.