I have used a product on many boats with rust spots, its called OSPHO, get it at the local ACE hardware store, DON,T BREATH IT, it looks like green kool aid, put it in a spray bottle, spray it on your engines, it turns the rust into an iron oxide, the next day just repaint, this product will remove rust from paint and fiberglass,takes a few hours or overnite to work, do not spray it on rubber hoses on your engine.----Another trick if you want to strip down a fiberglass boat to gelcoat is to put some MURATIC aCID,like you use in swimming pools in a spray bottle,stand upwind and spray in on the boat,stains and wax disappear!!! Great for removing the $hit ring you get just above the boot strip on boats Also takes off old oxidation, wash off with a hose and rewax--Got SS handrail?? get some COLINITE , comes in a plastic bottle and is brown in color, this stuff is amazing-rub it on the rust and just wipe it OFF, it also puts on a protective film so the salt water wont restain the metal. I buy boats and clean them up rather quickley with these products. Have FUN!! Sorrydog