Fiberglass hull with blisters

Blisters have been the bane of many a boatowner. There are literally hundreds of causes, many of which have to do with the quality of a fiberglass boat’s construction. The primary cause is the presence of water-soluble molecules in a laminate. These impurities come from innumerable sources, including glycols in improperly cured resin, any dust, sweat, or snot that falls into a mold during lamination, trace chemicals left over from a catalyst or mat binder, as well as certain agents used to treat fiberglass fabrics.

It’s impossible to ensure that none of these pollutants will ever find their way into a laminate, but the likelihood is greatly decreased if a builder is truly fastidious in his practices. The other major factor is the presence of voids in a laminate. These can never be absolutely eliminated–there is always some air in a laminate somewhere–but as we discussed in our last discourse on boat construction, steps can be taken to keep voids to a minimum.

What precipitates blistering is the migration of water through a laminate. Though once upon a time we all believed the myth that fiberglass is absolutely waterproof, now we know better. Compared to wood, for example, fiberglass absorbs little water (no more than about 3 percent of its total weight in a healthy laminate), but there is no way to prevent some water from coming in. As it moves through a laminate, water dissolves any water-soluble molecules it finds, creating an acid solution that can pool up in voids. These acidic pools are eventually pressurized, either through osmosis, wherein the laminate around the pool continues to allow water to pass through but traps the accumulating acids inside; or when the pools are heated, as may happen when a boat is hauled out and left to bake in the sun. At a certain point the pressure is sufficient to raise a visible blister in the laminate’s surface.

Blisters on a fiberglass boat hull
An array of blisters prior to treatment

Most blisters are merely cosmetic. These occur directly under the exterior gelcoat and are relatively easy to repair as they crop up. If they are prevalent, however, the gelcoat must be removed and replaced. The most serious blisters occur deep within a laminate and form larger bulges on its surface. The pressure in these blisters is sufficient to cause delamination, and repairs will involve relaminating affected portions of the hull. Within these two basic categories of blisters, there are many sub-variations exhibiting different characteristics and surface patterns. To ensure the most effective and least intrusive repair, it is best to study these closely and figure out if possible what caused them.

Boat blisters being repaired
Blisters under repair

As mentioned, the best way to deter blistering is to maintain scrupulous construction standards and minimize laminate voids and impurities. Most builders, however, focus more on improving water resistance in a laminate’s surface layers and brag in their literature about such things as epoxy barrier coats, special gelcoats, and vinylester resin. These certainly do help retard blister formation. Bear in mind, however, that builders are most concerned about keeping a boat blister free until its warranty expires. Even an epoxy barrier coat is somewhat permeable and in the end, no matter what is on the outside, water will enter the laminate, in which case only the quality of the laminate will save it.

Geography and how a boat is used are also significant factors. Warm water and fresh water migrate through a laminate more easily, thus boats kept in southern and/or inland waters are generally more blister-prone than saltwater craft kept in colder climates. Continual immersion also accelerates water absorption. If you’re looking for a boat to buy, the ones with the fewest blister problems are usually found in northern coastal areas where boats spend much of the year sitting on shore. Boats from the south that are in the water year-round, especially those kept in fresh water, have many more problems.

If you own a boat that already has blisters, I urge you to learn all you can about the subject so you don’t waste time or money on unnecessary or ineffective repairs and treatments. There are numerous so-called experts (many equipped with gelcoat-peeling machines) who will be happy to fill your ears with self-serving advice.

The best references I know of are Fiberglass Boats by Hugo du Plessis; Osmosis & Glassfibre Yacht Construction by Tony Staton-Bevan; and Blisters, a collection of articles published by Professional Boatbuilder magazine, most of them written by Bruce Pfund.


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