Those things are incredible. I used to work for a company called Aquaglass. They make the fiberglass tub/showers that Lowe's sells. We would use them to get all of the black scuffs off of the gelcoat on the displays from customers standing in them. I don't know what they are made out of, but they work. You just use it until it pretty much disenegrates. Oh, be sure to wet them first before you use em.