One of the most frequently asked questions that a marine surveyor gets is, "Should I buy a boat that has blisters?" This is a question that I've wrangled with for many years, and after a great deal of research involving thousands of boats. This essay will answer your questions a bit more directly than some of our more detailed blister essays, which many of you found hard to understand. That's understandable because this is a very complex subject. But be prepared that the answer is populated with a lot of ifs, ands, and buts. 

Let's face it, an awful lot of boats have blisters, so that finding one that doesn't (or won't get them) can be a difficult proposition. The short answer is that if at all possible, you should try to avoid that, if for no other reason than the potential expense you may face in the future. That expense may not result from the absolute necessity to repair the blisters, but the position you may find yourself in when it comes time to sell the boat. Particularly with newer model boats, say 1 - 3 years old, it is not unusual for buyers demand a reduction in price, or that the blisters are repaired. 
For older boats, its usually much less of a problem, for the fact is that moderate blistering on an older boat rarely impedes the sale. Unfortunately, another fact of boating life is that there is a great deal of misinformation on this much talked-about subject. One common misconception is that blisters seriously weaken and/or damage boat hulls. In 30 years of surveying and examining around 4000 hulls, I have seen less than 10 cases where blisters have resulted in serious structural degradation of a hull where it was weakened to a point where some type of failure was immanent. 

What is a blister? First, let's understand that all fiberglass hulls absorb water to some degree because both the gel coat finish on the exterior, and the fiberglass reinforced plastic is porous. Since water is a solvent, it will react with the plastic resulting in the water and solvents in the plastic mixing to create a weak solvent solution, usually with styrene. This then softens the gel coat somewhat and, combined with a bit of gas or fluid pressure, results in the blister. 

Moisture Meters Here's a subject I get a lot of questions about, and one that I want to address it upfront. Since I have already stated that all fiberglass boats absorb water to some degree, and often without causing blistering, it follows that the use of a moisture meter is useless. Moisture meters measure only the surface moisture, and since gel coat and paint is very porous, the moisture meter is only going to tell you what you already know; it’s wet. It cannot tell you anything about the propensity of a hull to blister. While these instruments have their uses, predicting whether a hull is prone to blistering is not one of them

Are blisters harmful? Yes, but. This is a question of how much harm. Blisters form at the interface between the gel coat and what is called the skin out mat, which is a layer of chopped, short-strand fiberglass that is used to prevent the coarser weave pattern of heavier fiberglass cloth from telegraphing through to the finish surface. You've probably seen boats with a checkerboard pattern showing on the surface, and this is the reason why. Now, fiberglass fabric, being made of bundles of very fine glass fibers, is very porous also, most especially the outer layer of mat. Once the gel coat absorbs water, the fibers in the mat that are unsaturated with resin then spread the water around via the capillary effect. 

Blistering involves only the gel coat and surface mat in 99% of the cases. This is due to the fact that the structural fabrics, such as roving, get saturated better. Its also because the water is less likely to penetrate beyond the mat and, even if it does, woven fabrics do not have the weak gel coat factor and are much too strong to allow whatever pressure may develop within a void to cause a separation. The incidence of blisters occurring within structural laminates is extremely small. 

If the resin used to make the hull is of a lower quality that will react with water, a process known as hydrolysis, which means nothing more than becoming saturated with water and dissolving, then the hull is poised to develop blisters. Many other factors also come into play here, such as how well the mat layer is bonded to the gel coat. 

Since the vast majority of blisters occur between the mat and gel coat (depicted in illustration above), this bond has to be fairly weak for the blistering process to occur. If the bond is strong, then blisters will not occur, even though there is a lot of water absorption. This is a very general, even generic, description of the blistering process. There are frequently numerous other factors involved which I will not address here. 

Aside from the damage it causes to the surface, most of the damage done by the blister is to the gel coat and the skin out mat, which is not a structural part of the hull laminate. Remember, the mat is only there to prevent the fabric pattern from showing through to the surface. The obvious question is now, "But isn't the water dissolving the rest of the plastic resin in the laminate?" The answer to that is "No, its not." At least not to any considerable degree. 

You may have noticed that I have not used the word "osmosis" that we hear so much these days. Technically, water passing through the porous gel coat is not osmosis; its just water passing through a porous material. However, the blistering process may involve osmosis, a process which concentrates solvents within the space formed by the blister void. This concentration of solvents does indeed dissolve the plastic, but fortunately the amount of fluid involved is so small that it does not seriously threaten the laminate. 

Of course, the larger the blister, the more concentrated solvent is present, the more damage it will cause. Therefore the amount of damage, and therefore structural weakening caused by blistering, is directly proportional to size and number of blisters. This explains why only boats with very large blisters can end up with serious structural weakness problems. 

by David Pascoe
 
 

