I have used the same method for the past ten years and never have more than one or two barnacles on my prop or shaft. I clean the stainless steel
shaft and bronze prop down to clean metal with a wire brush on an electric
drill. Wipe with solvent (acetone) and apply two coats of a non-metallic primer. I have used Interlux
Primocon or Interprotect successfully but other non-metallic primers work as well – even Rustoleum primer. I then apply two coats of any available copper based bottom paint
and splash the boat. Don't prime or paint under or over the zincs. As I said, little to no fouling after six+ months. I use the boat almost every weekend and motor
perhaps for 30-60 minutes total each time we take the boat out. By the end of the season, the paint has mostly worn off the tips and parts
of the prop blades but is intact elsewhere.
I have tried this method without the primer and always got bad fouling of the prop and shaft. Interlux
outdrive spray works well if used with their primer but is very expensive. I have heard others theorize that the electrolysis current
set up between the metal shaft/prop and plain copper paint somehow keeps the copper inactive and thus allows barnacle fouling. A non-metallic barrier coat of primer stops this interaction and allows the copper to inhibit growth.
I sail in the salt
waters of Barnegat Bay NJ and keep the boat in Forked River - a somewhat brackish river feeding the bay.