Star Brite 32 oz. Hull Cleaner. This safe and easy-to-use cleaner does not contain harmful acids, making it suitable for fiberglass, metal, and painted surfaces. It quickly removes stains from the water line and is excellent for cleaning scum lines and stains on boat hulls. Instantly removes stains from grassy marine growth, rust, scum line, fish blood, etc. without the need for rubbing or scrubbing.