If you have the patience and time, remove the old stuff with a 1/2 inch or so chisel, flipped over so the beveled edge is down. Then sand the aluminum with a corse grit to get a good profile. Wipe the bottom down with tolulol (sp?), mask off the sides and transom and recoat. You will definately notice a difference in water drag. One gallon will do it all, about 80$. Other wise chip off the lose stuff sand and patch or recoat over the whole thing. Just my 02 cents worth, good luck.