Blisters and Osmosis

"Should I buy a boat with blisters?"

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 has been discussed for years, and a great deal of research has been carried out and expenses have been incurred involving hundreds of boats. Hopefully this article will answer some of your questions.

Here in the Greek Islands an awful lot of boats have blisters. Whether this is due to the build quality, warm saltwalt water conditions, or the fact that many of the boats are left afloat all year round, its difficult to tell. Finding a boat that doesn't have some form of blistering below the waterline, can be very difficult. If you can find one, great, if for no other reason than the potential expense you may face in the future. That expense may result from the position you may find yourself in when the time comes to sell the boat, particularly with newer model boats, say 1 - 3 years old, it is not unusual for buyers to demand a reduction in price, or that the blisters be repaired.

However, in my experience, for older boats it's usually less of a problem, moderate blistering on an older boat rarely impedes the sale. Unfortunately, another fact of boating life, especially on the inland waterways, is that there is a great deal of misinformation and confusion on this much talked-about subject. One common misconception is that blisters seriously weaken and/or damage boat hulls. In years of surveying and examining various hulls with differing types of blistering I have seen very few cases that have resulted in serious structural degradation of a hull where it has weakened to a point where some type of failure was imminent.

What is a blister?

Although often argued, all fibreglass hulls absorb water to some degree because the gel coat finish on the exterior, and the internal fibreglass reinforced plastic is porous. Even epoxies, the most waterproof resinous coatings available, are permeable. Since water is a solvent, it tends to react with the fibreglass, resulting in the water and solvents in the plastic mixing to create a weak solvent solution. Moisture continues to be absorbed through the gel coat and if resulting solutions are not able to escape by condensation on the interior, hydraulic pressure will be generated within the laminate, eventually leading to gel coat blistering.

Moisture Meters

Here's a subject I get a lot of questions about, and one that I want to address upfront. Since I have already stated that all fibreglass boats absorb water to some degree, and often without causing blistering, it follows that the use of a moisture meter is useless. It shows a hull that has been completely water saturated for 10 years, but has not developed blisters. 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 skinout mat, which is a layer of chopped, short-strand fibreglass that is used to prevent the coarser weave pattern of heavier fibreglass 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, fibreglass fabric, being made of bundles of very fine glass fibres, is very porous also, most especially the outer layer of mat. Once the gel coat absorbs water, the fibres 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 cases. This is due to the fact that the structural fabrics, such as roving, get saturated better. It's 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. 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.

Is the Repair of Blistered Bottoms Mandatory?

Based on the foregoing chapter, the obvious conclusion in most cases is no. If the blisters cannot be shown to be causing significant damage, then repair is certainly not mandatory, despite the many horror stories you may hear from people trying to sell you a costly repair job.

My experience has shown that early treatment of blistering tends to be less successful than treatment of vessels with more advanced problems. While this may seem to contradict normal precautionary practice, it has been found that breakdown reactions in GRP laminates take some time to reach their conclusion; and if treatment is carried out prematurely it is much more difficult to remove reactive or hygroscopic compounds from the laminate, with the result that a recurrence of the blistering is much more likely to occur. The overriding factor must always be the integrity and safety of the hull. Osmosis in its early stages is very much a chemical condition, which has very little effect on hull strength.

This photo (right) represents a typical case of blisters, small enough to be called pimples. They are two penny sized and smaller, but no matter how many of them there are, they are very unlikely to threaten the structural integrity. These examples were found on a hull only three years after full osmosis treatment had been carried out. Remedial works were expensive; approximately £200.00 a foot (30ft vessel). The works included a full gel peel and an extensive washing and drying out period! More information at this link:

This is one of the few examples I have seen where large blisters threaten the integrity of the hull. (below right). These larger blisters (a number the size of side plates) were on a Catamaran, and after close examination, the blistering was found to be noticeably deeper, widely spread and containing high volumes of liquid glycol. In other words, too much of the structure was invested in a weak material. It should be noted that the Catamaran hull was over 30 years old and was considered one of the unlucky few.

If the blisters are large and numerous, it would be wise to seek unbiased, professional advice before you proceed. Bear in mind that blister repair jobs are now big business for boat yards, so taking advice from yard managers may not be a good idea.

Yes, blisters are unsightly and may cause a resale problem. These are all factors you must weigh, in addition to the very high cost, when deciding to repair or not. Further, you should also be aware that the number of failed blister repair jobs that I find is very high. No one's ever going to know why blister repairs fail because no one is going to spend the money to find out. That there are so many should also play a role in your decision to repair or not. And even though the repairer may give a warranty on the repairs, you should get it in writing and read the fine print. Then make sure the repairer is likely to be around years later to honour that warranty if it becomes necessary.

Having said this, I have recently been to a CPD seminar in London on the subject 'Hot Vac Hull Cure'. This really does appear to be the most successfull treatment for osmosis to date, with a high success rate. Please read more about this great invention at

Fibreglass Stress Cracks.

The causes for stress cracks are much the same as for blistering. Stress cracks occur because of voids under the surface of the gel coat, and or a lack of strength in the underlying fibreglass or compounds used in the sub structure of the boat. When these voids are present, or when the fibreglass is lacking in strength because it was not mixed properly, or because enough fibreglass was not applied it paves the way for stress cracking. Your boat body will flex some even during normal use. Fibreglass is strong, and if mixed properly, and if enough is applied, the fibreglass will take up much of that flexing. But gel coat is very brittle and has no strength to it. When the boat flexes and twists, the fibreglass flexes and twists. When that happens, the gel coat can not flex with the fibreglass, it just cracks. Hence you get these little hairline cracks. See example photo (right) taken last season on a Nauticat cruiser.

As in this case the condition has no significant effect on the strength of the hull laminate. The defect, though unsightly, is not structurally significant at this time and repairs may be safely deferred until a suitable time.

How to Prevent Blistering in the first place.

Over the years GRP hulls have been coated in a variety of paint systems hoping for protection from blistering. Some manufacturers have progressed through many coatings over the years. Some have been successful at times, but on other boats, they have had no noticeable effect. The underlying laminate is usually the reason for success with top quality resins also contributing. If boats are badly laid up, lacking in resin and riddled with air voids, moisture will get to the bad laminate despite the coatings.

Many in the building business thought that epoxy would solve all permeability and blister problems, but it's not quite that simple. Epoxy is not recommend over gel coat on second hand boats as boats that have been in the water six months or more will have absorbed too much moisture into the gel coat and two years or so later, off it all comes again. Epoxy is too waterproof a coating to put on moisture-contaminated, aerated coatings like gel coat. Success has been proven over gel coats on new hulls, as long as they are sanded down with the right grade of paper. If using epoxy it is also imperative that all the coats are applied wet on wet to eliminate chances of the epoxy delaminating between coatings.

Getting back to the original question, "Should someone buy a boat with blisters?" can be answered from several viewpoints. If you insist on a boat without blisters, fine, then go and try to find one. If it's an older boat you want, you may have little choice, since blistering is common and also tends to run in certain local builder's lines. You may have to look at quite a few before you find one without blisters.

All things being equal, you would certainly want to choose a boat without blisters. Unfortunately, unless the seller is kind enough to tell you, you can't find out until the boat is hauled out for survey, at which point you've already invested some money in it. It's a fact that most blistered boats are sold without regard to the blistering, and this is one of the reasons why, in my experience, the number of cases where blisters cause the boat to be rejected, or give rise to price renegotiations is considerably less than 5%.

Important: Please note that whilst every effort has been taken to ensure accuracy of information contained in this subject, no responsibility can be accepted for any errors or omissions that it may contain.