I do not understand why you want to varnish the bottom of the boat ? I have restored many classics over the years and have a 1960 # 16 ft Century resorter that i am restoring at the moment and over the summer i have put on all new coverboards ,aft deck ,one transom board , and two new boards on both sides of the hull and planning on more every few weeks .But nun the less let me know if you are talking hull side's or the running surface because i have used many different makes of varnish over the years including Epifanes.