It’s the boat bottom, so quit thinking whatever you were thinking. Painting the boat bottom requires just as much work as painting the topsides, but work of a different kind. Fiberglass boat bottoms develop osmotic blisters when seawater intrudes and reacts with the fiberglass resin. These blisters are usually localized and small, and dealt with by grinding out to dry fiberglass, then re-glassing and/or filling with epoxy. Antifouling paint then goes over all. The bad news: it’s a lot of work. The good news: the bottom is underwater, so it doesn’t need to be as pristinely smooth as the topsides.