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Boats are mostly concave surfaces. Belt sanders have flat surfaces and make flat spots on concave surfaces that can show up when gloss paint is applied. That being said I`ve used belt sanders myself on boats, but mostly on flat transoms or around areas that will be repaired with addition of new glass or bondo. Actually an angle grinder with a sandpaper disc will work alot faster than a belt sander and if used in a circular motion will not leave flat spots.