Thanks Dave. I always appreciate your posts and responses. I think I am getting "pretty much" all of the old paint off. What is left on the gel coat  after the stripper has a light blue mottled pattern. That essentially goes away with moderate sanding. Although I am sure there are plenty of old anti-fouling paint molecules hanging around, my sense is they are few and far between. Unless those molecules act like a wax or something, I wouldn't think there would be an issue applying a non-anti-fouling paint where anti-fouling paint used to be, but that is certainly why I am asking the question. There must be some point where new non-anti-fouling finish can be applied. Would that require removing all the gel coat too? I'll look into that product you mention. Does it make sense that anti-fouling paint will always stick to anti-fouling paint even if completely different compositions? If there isn't concern for matching anti-fouling paints I wonder how strictly the "only anti-fouling after anti-fouling paint" applies. Don't take any of this as doubting your advice, I am just trying to understand better! Message: 13 Date: Mon, 13 Apr 2015 10:25:32 -0600 From: Dave Scobie <scoobscobie@gmail.com> To: For and about Montgomery Sailboats     <montgomery_boats@mailman.xmission.com> Subject: Re: M_Boats: Plan to Refinish Montgomery 15 Bottom Message-ID:     <CAGjBOA6DrC6xaS-V=xt9eezJfjeODzRLY150S8GPH7NzFC3-hw@mail.gmail.com> Content-Type: text/plain; charset=UTF-8 Once boat bottom painted the boat is  'committed' to the treatment. Unless you know for sure what is currently on the boat you need to get it all off to avoid incompatible paint combinations. Besides barrier coating you can paint with a durable antifouling paint for trailering such as VC17 (which is good in most lakes ... But not be n Cali.) ViViD is a good hard ablative and has many color choices including mixing colors to make a custom match to your boat. :: Dave Scobie :: M17 #375 SWEET PEA :: Former M15 owner