After the bottom has been thoroughly washed and dried you want to apply a strong primer. You want to select a good primer such as Barrier Coat Primer , Tuff Stuff High-Build Epoxy Primer orIsland Prime Bottom Maine Paint Primer. Beware of boats painted with sanding-free primer. This is primarily done when boats are new, as prep for their first coat of bottom paint. If used again years later all the bottom paint will start to “alligator” and require laborious removal.