Sounds like you're talking ablout deck hardware which i'm guessing means that the plates are on fiberglass. If that's the case heat won't be good. Forget paint, that's easy to replace, but your deck is another story. Are you able to get to the nut from the underside? If no try drilling them out. Another though might be to chill the bolt. You could try something like (I know this will sound funny, but...) Dr. Sholl's wart remover which should allow you to freeze only the bolt. That may be enough to break it free.