The only way to remove oxidation is to grind off enough gel coat to get down to unaffected gel. If the gel is oxidized badly enough, you'll grind off all the gel, leaving a fairly rough surface on the boat. Go slowly and try a buffer with a fine polishing compound on a small area to see how it goes. I'd check in the boat restoration forum on Bass Boat Central at Bassboat Restoration: BBC Boards for the best advice.