Just coming to the end of a hard week in dry dock with Badger.  I worked out that a full length boat has about 700 square feet of side to be blacked.  Owners of modern boats have the luxury  of comparatively smooth sides which readily accept paint roller application.  Not so with the "patina" of an old boat. Try using a roller and the surface is covered but all the little (and large) dimples are still bare and that is just where the paint needs to be to do its job.  An old boat requires old techniques and I have found the best method of application is a "Turks Head" brush.  Admittedly not as economical on the paint as a roller but, then again, the best place for the paint is on the boat, not in the tin!