For bottom paint stains you can take bronze wool and scrub them away...it also scrubs the paint away. If you want to use tidy bowl types look for the ones with phosphoric acid. The safe way to use them is apply for a minute or less, scrub a little and rinse with fresh water. If that doesn't do it apply again and leave on longer. It usually takes me several times to get rid of hard stains on bare gel coat. If left on long it will etch...if done in short sessions it cleans without etching but always has to be rinsed well. On Off is the strongest (and most expensive) of the cleaners if you want to go that route. However, I would just paint over bottom paint stains and not worry about it. Over the yrs I've done all of the above many times on different boats with good results. Bill P. M10 dink ************************************** See what's free at http://www.aol.com.