A few traditionally molded fiberglass boats will sail through life without ever breaking out in osmotic blisters. They belong to a minority with exceptional pedigrees, and their perfection comes from careful planning.
A perfect example of this is Currant Bun, a Bowman 46. After 23 years of service, which included two circumnavigations, the hull has no blisters. The boat’s owner, Richard Vaughan, had her built to Lloyd’s rules and under Lloyd’s supervision. Tyler Mouldings, a well-known boatbuilding company in England, strictly controlled the mix of resin and hardener. They laid a limited number of layers of glass a day to avoid the steep increase in heat that curing resin normally produces; very high curing temperatures could lead to laminate separations.
Polyester resin takes a long time to completely cure, and removing the hull prematurely from a mold may put a curve into the sheerline that the boat’s designer never envisioned. There have been cases in which riggers commissioning a hastily finished boat added a permanent upswing to the bow shape just by cranking tension into the backstay and headstay. To avoid hull distortions, Currant Bun remained in the mold for a month, during which time the deck was installed. The builders left out the gel coat so that the hull laminate could be inspected against a floodlight. Voids in the laminate block the path of light, so, ideally, the yacht should end with a translucent hull molding indicating a perfect lay-up.
Very few, if any, production boatbuilders can afford such perfection. Also, pressed by tight schedules, builders routinely speed up construction by using very heavy mat or woven rovings, which saturate with resin less efficiently than does thinner cloth. Dry fibers in such resin-starved laminates will wick liquid by-products of curing resin into empty spacesthe first step to future blisters. Sometimes the shape of the hull presents sharp corners (a knife-thin racing stem, deckhouse corners, etc.) that prevent workers from pressing out air trapped between layers of glass. With so many possible causes, voids are as common in most glass boats as freckles on our faces. Eventually, the pressure difference between the voids and the sea outside the hull triggers an osmotic action that prompts the liquids in the hull to migrate outward and form blisters. Chemical components within blisters will gradually eat into the laminate. However, given enough expert attention, this boat disease can be treated successfully. It involves removing the areas of sick laminate, drying them, and then protecting the hull with a barrier coat.
Scattered blistering
Small, scattered blisters should be opened and cleaned with burr bits in a power tool or a drill. Larger blisters call for using a grinder. The grinder operator should work with the smallest available grinding disks to avoid destroying the surrounding healthy laminate. Let these open sores dry afterwards. How long? Well, weeks sometimes, until on a damp day the cavities no longer turn dark and moist. Post-blister scars larger than a thumbprint should be filled with a mixture of polyester or epoxy resin and fibers. Deep gouges larger than the palm of your hand must be rebuilt with glass cloth and epoxy resin, which has better adhesion than polyester resin.
Severe blistering
Repairing damage from widespread blisters requires removal of the gel coat and possibly the outermost layer of glass down to the healthy lay-up. Grinding or sandblasting often gouges into good laminate, so employ a gel coat peeler machine. A peeler, working like a giant electric shaver, has an adjustable depth of the cut, works fast, and sucks up those tiny glass fibers that cause skin and lung irritation.
Gel-coat removal prepares the hull for applying a barrier so tough that the liquid in the voids that still lurk deep in the laminate will not sense the pressure differential and start migrating toward the sea water. Letting the hull dry before applying the barrier coat aids adhesion and promotes longevity. A moisture meter helps track the drying process, which can be sped up by using dehumidifiers under a plastic tent or encasing the hull in a plastic envelope with vacuum tubes attached. The initial moisture in a blistered hull can read as high as 90% but normally will hover just above 20%. This should come down to between 2% and 5% before the application of a barrier coating. Remember to use the same instrument to get consistent readings, and bear in mind that metals buried in the laminate, like an encapsulated ballast and ground plates, cause saturation level readings around them even in a dry hull.
The barrier coat
The ideas about what constitutes an adequate barrier have evolved since 1979 when I first had to consider helping my boat. Back then, Gougeon Brothers, the makers of West System Epoxy, suggested two coats of its resin as the adequate barrier. After a few years the company began recommending four coats, and now Interlux Paints recommends at least four coats of its Interprotect epoxy-based products. Some repair yards rely on vinylester resins instead of epoxies. The hull treatment of my 1961 yawl progressed with the changing recommendations. Several times I had to renew the barrier coat since the blisters, unlike chicken pox, kept reappearing every four years. Each time, I had to grind off the old barrier, dry the hull, and reapply the increased thicknesses of resins and potions.
So it was no surprise to hear from Gougeon Brothers that now it advises six healthy layers of its resin. Having tried other barrier coat products, I favor the West Epoxy resin since it does not introduce solvents that can get trapped and cause a new rash of blisters within the barrier coating itself. However, rolling on six coats of resin presents some difficulties. After the first coat of the shiny, translucent resin it is hard to distribute the following coats evenly, although adding alternating pigments would help. Ideally, all six coats should go on in one day to avoid the curse of sanding the hard, slippery, cured resin between applications. That means finding a yard with a climate-controlled shed since there are not enough dry hours in a day to coat a boat hauled out in the open air. The hull surface must be dry, and it takes a long time for morning dew to evaporate.The ultimate barrier
Considering how difficult it is to maintain the uniform distribution of the barrier coats, I have applied a system that introduces one new layer of glass cloth. The work starts with rolling on two coats of resin, followed by laying on a medium-weight cloth that gets covered by more resin until the cloth weave disappearsa process that takes at least two coats. Another coat follows them, and then a heavy layer of resin mixed with an easily sandable fairing compound (e.g., West #410) gets troweled on. Since the workers must wait for the preceding resin to gel before applying each new coat of resin, it takes two persons about eight hours to do an area six by seven feet. The next day begins with grinding a chamfer into the edge of the now-cured section to assure adhesion at the overlap with the new area.
We did the work during a north Florida summer. By 1000 the dew was gone from the hull, allowing the two of us to work until 1700, when fat clouds would ripen into a threat of imminent showers. Getting the materials prepared was the key to finishing a section on time. The glass cloth came in a 38-inch width, and we precut pieces measuring 19 by 25 inches, a handy size for two people. That second coat of resin, before the cloth application, was thickened somewhat with West #406 additive to discourage the resin from running down the sides of the hull. Even with the slow hardener, the resin gelled fast, so soon after rolling it on we began laying the cloth pieces on the hull. We smoothed the cloth on with our gloved hands and then with a serrated roller. After the cloth pieces were in the right places we rolled a new coat of resin to saturate the cloth until it became transparent. Next, we used putty knives to fill the gaps between the pieces of cloth with a mixture of resin and #406 additive. Then we rolled on more coats of resin to hide the weave of the cloth. We topped that with the fifth coat of resin mixed with #422 Barrier Coat Additive. Finally, working with six-inch-wide putty knives, we covered the day’s section with resin mixed heavily with #410 additive, which sands very easily after the cure.
We progressed one section at a time for several days until the whole boat was coated and covered with the final mixture of resin and #410. Sanding with standard boards covered with velcroed sandpaper was next. In the tight corners of compound curvesthe turn of the bilge, under the stern overhangwe sanded with a block and #40 paper. For precutting glass cloth for such corners we made paper patterns; otherwise the cloth cuts straight with scissors if you first pull out a thread along the line of the weave.
After the final sanding, one more coat of resin should seal the faired hull. Antifouling paint can go on when this last resin coat has almost cured and is only slightly tacky when touched. If this last resin layer is allowed to cure completely before antifouling goes on, somebody will have to sand it thoroughly before painting. It will take a lot of time and effortcured resin acquires a slippery, rock-hard surface.
The introduction of a cloth layer allows better control of the barrier coat application. It will also rebuild any glass material lost while operating a gel coat peeler or a grinder.
Once you have the work station going, you may want to add even more layers of glass in strategic places. Hitting floating debris at sea encouraged me to wrap a few layers of cloth around the underwater part of the stem and along the keel. It meant only a few more hours of fairing work, a small price to pay for improving the hull’s impact resistance.
