Blisters, blisters. They used to be a much more prevalent curse than with boats built more recently. That said, and although certain construction techniques (cored hull, vacuum bagged) and materials (vinylester, epoxy) minimize the risk, it is still better to put a barrier coat on, when the boat is first launched. After that....if the hull has absorbed any water, it must be left to dry, which can take a long time. If you sand, when you completely strip the bottom paint, you take off the most water resistant part of the gelcoat
, which is the surface, and you had better put on a barrier coat to replace it, which should go into a decision to mechanically strip. Chemical strip doesn't cause this, of course.
I once had a long discussion with an Interlux chemist as to the best method of application. During the previous five years, their manual had reversed itself on some of the procedures. In his opinion, he said that the three most important things to do, in addition to drying and following the directions were 1) apply twice the amount recommended for long life. The manuals
specify the minimum, and since most people sell their boats within five years, and there is competition between the barrier coat manufacturers......their business decision is to specify a bit on the skimpy side. So, more than 20 mils is really necessary. After 40 mils thickness, the stuff can get brittle...bad. 2) Either barrier coat the bilge
or keep it dry. Not easy, in some cases, but the bilge
water will saturate as readily as the outside water. And, if the bilge water is fresh, the osmotic difference is even greater, so the blistering will be more rapid. 3) Barrier coat six inches above the waterline. I have never seen a boatyard quote for this as it gets into the bootstripe area.
When last I did a barrier coat job, in 1994 on a heavily blistered bottom, I dried the boat out for eight months in the Sonoran desert (San Carlos, Mexico) and then did the above. Nine or ten coats of Interprotect, six inches above the waterline. I covered the top part of the epoxy with a new bootsripe. I barrier coated the bilge and used dripless stuffing for a dry bilge. Result: never another blister.
My other boat, a 1999 Leopard 45
cat, was not barrier coated at the factory (inexcuasable) but was vacuum bagged and cored. Leopards have a good history
regarding blisters and they are all built this way. Over the seven years I have owned her, there have been maybe ten blisters that have appeared in various places during various haulouts. She is in the tropical water all year. I have just ground out those few blisters (none deep at all), filled and epoxied. It is not the classical remedy, but it has worked pretty well, for this boat. Someday, I will strip the bottom paint (it is ablative so not too thick). When that day comes, if I do it mechanically, rather than chemically, I will apply a thick barrier coat, just as I did on my other boat.
Good luck with your decision...not an easy decision, and no guarantees on the result.
By the way, I know more than a few F/P boats with blisters, considerably more than the manufacturer acknowledges.