Barrier Coats

Keeping water away from the hull helps prevent blistering
Since the unwelcome discovery that fiberglass gelcoat can be permeated by water vapor, barrier coats for the underwater portions of hulls have assumed greater importance. The theory is that keeping water away from the surface of the fiberglass will prevent osmosis, the formation of high-pressure blisters between the layers of glass mat or cloth.Some boatbuilders now use expensive vinylester resin in place of polyester resin because it is more water-resistant, but there is no real consensus concerning the best way to waterproof the underwater hull.Epoxy manufacturers recommend stripping off all old paint and coating the underwater hull with their special epoxy formulations. Ordinary epoxy resin, however, is not waterproof, merely water-resistant. So perhaps it is significant that they recommend as many as seven barrier coats of special epoxy. In fact, osmosis studies conducted by the University of Rhode Island suggest that several coats of marine-grade enamel paint might work just as well.There is still much to be learned about the formation of blisters in fiberglass, but it is generally agreed that the inside of the hull—the bilges, the walls of lockers, and so on—should not be barrier-coated because that would only trap any water present in the laminate, making matters worse. If the inside of the hull is simply kept dry and well ventilated, it is likely that unwanted water will evaporate faster than it can penetrate the gelcoat.See also Blisters in GRP; Epoxy.