Y-10 rapidly "absorbs" yellow/brown waterline and transom stains on white or light-coloured fibreglass boats. Will not damage gelcoat. Y-10 was specifically formulated to remove the stubborn stains that have exasperated boat owners because they were so hard to clean off without damaging the hull surface. To use, spread a thin coating of Y-10 over the stain, wait a few minutes while a mild chemical reaction causes the stain to vanish, then wash it off. The gel spreads easily and adheres to vertical surfaces. Also effective against rust stains and may be used on painted boats. Y-10 comes in 340g jars containing enough gel to remove waterline stains on a 26-footer twice!