Re: Aluminum boat corrosion
If it's only a cosmetic spot at this point, I would simply clean and seal the aluminum. Aluminum isn't like steel, it doesn't rust but can react to corrosives. Usually once the corrosive element is removed, the problem won't progress. Chances are it's salt corrosion or the result of the original anodization being removed. I would clean it and neutralize any corrosives left behind and most likely consider painting it with something to prevent further damage.
Price would determine whether or not I'd want to fool with it or not. If it's not badly pitted with perforation of the hull, its probably more cosmetic at this point than anything else.
Someone may have also tried to remove lettering or decals with the wrong stuff as well, using certain automotive wheel cleaners will turn aluminum white as well.