Epoxy barrier coat for the bottom of a Cape Dory Typhoon sailboat
Owners of trailer-sailor boats like the Cape Dory Typhoon generally don't bother with epoxy barrier coating and instead go with bare bottoms or a coat of ablative antifouling to protect their boat bottoms from excessive marine growth. Since these boats are kept on trailers rather than in the water for extended periods of time, there is probably no need to apply barrier coat paint. However, I do plan on keeping Cantaora moored or docked for long periods in waters particularly prone to promoting growth on boat bottoms so I will be applying epoxy barrier coating as well as several coats of antifouling paint
The bottom on Cantaora was stripped and sanded clean with 80 grit on a palm sander, wiped down with alcohol and, over the course of a few days, was painted with several layers of epoxy barrier coating. The gel coat under the stripped ablative antifoulng was in decent shape, and apart from a previously patched area, and the two cockpit scupper holes I had patched myself, there were no additional holes or problem areas that would have had to be fixed and faired before applying barrier coat. She had beem sitting high and dry on a trailer for years, after all, so there was no sign of osmosis bilstering etc. I sealed the scupper thruhulls but decided to leave a drain hole I had cut into the lowest part of the bilge to allow drainage from the interior bilge while I was fixing the boat? making it much easier to just pressure wash the interior. That drain hole will be plugged permanently later (I am not a fan of garboard drains either)
I dropped the rudder before applying the barrier coat; I will cover that in another post regarding the rudder project.
I can't say exactly how many layers of epoxy barrier coating I ended-up applying to the boat bottom because I had gotten pretty tipsy from the fumes by about the third time around. This is smelly stuff, even outside. In addition to three quarts of new barrier coating paint I had opened, I also used up about another quart or two of some left-over stuff from another project. Since one quart of epoxy barrier coat seemed to cover the boat bottom about 1.5 times over, I'd guess there were at least around 6 layers of epoxy barrier coat applied in total.
The function of epoxy barrier coating on boat bottoms is two-fold: acting as a primer, it plugs micropores that can trap moisture & promotes adhesion of the anti-fouling paint that is applied over it; it also acts as a waterproof barrier to the absorbtion of moisture by the fiberglass hull under it, which can lead to the dreaded "boat pox" or osmosis blistering. Having seen my share of boats with pox blisters, I wanted to avoid that as much as possible
Boats made starting in the early 70's including Cantaora first started showing symptoms of boat pox; some say it was caused by the use of cheaper resins by manufacturers due to the rise in cost of raw material attributed to the 1973 Arab Oil Embargo that led to lines around gas stations in the US at the time too. Regardless of history, the actual proximate cause of boat pox is moisture penetrating the hull, and barrier coating is the preventative method.
The saying, "the more the better" actually applies to epoxy barrier coating paint for boat bottoms. Coating thickness is what counts. Gel coat itself is porous and can also crack/easily damaged; epoxy barrier coat is meant to be waterproof but only if applied in thick coats. I did not bother measuring the exact thickness of the barrier coating to compare it to manufacturers' recommendations; I just figure 6 coats should be enough.
Epoxy barrier coat paint for boats comes in various brands. I used two different brands but made sure I waited for the previous coating to cure overnight before applying a different brand epoxy barrier coat paint over it, just in case there was any chemical inconsistency between the two brands. (Interlux Interprotect 2000e also seems a shade lighter-colored than TotalBoat epoxy barrier coat paint.)
This isn't cheap paint, and epoxy barrier coat applied in several thick coats to boat hulls to prevent osmotic bilstering may become somewhat expensive but it is a one-time only job if done right and won't have to be touched for several years afterwards as new antifouling paint is re-applied over it. Certainly, the cost of fixing osmotic blisters is significantly higher.