Specifically designed to quickly remove common yellow stains the tannins from fiberglass boat hulls experienced in many waterways. Removes spider droppings from boats, algae and scum lines, scale, chalking, rust and water mineral build up. Use on fiberglass boat hulls, stainless steel, concrete, sprinklers, vinyl siding and more. Excellent for removing chalking and stains within dry/indoor storage where rinsing is not permitted. Biodegradable.