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HSH Yacht Articles

Oh no! My boat's got blisters!


Jeremy R. Hood

It was a cold blustery March day as I drove east along the south coast of England toward Lymington to see several boats one of which I was hoping would be the right one for my plans of long distance cruising. By 10.00am I was sitting in the brokers office hearing a little about two boats, both Rival 32's that he had for sale. And soon after that we were on our way around Southampton Water to look at them. The first was out of the water in a field behind a derelict house. Like a thoroughbred put out to pasture, the lines immediately appealed to me but the age and condition were not in its favor. After a few minutes aboard I was ready to move on. The next boat required a trip out into the Hamble river where the boat was floating at the yacht club dock, tugging at its dock lines. It was love! The huge cleats, samson post, even mast steps. And below was a well installed 25 HP diesel engine. Back at the brokers office I took the first steps in the process of buying Melos.

Some weeks later after we had settled on a price, the survey took place though I was not able to be present. But when I received the written report I wished I had been there. Besides numerous minor items that are nearly always present in a meticulous survey, there was mention of BLISTERS below the waterline. Within minutes I was on the phone to the surveyor and we arranged to meet at the boat yard the next saturday with the owners of the vessel to discuss the problems. They were as upset as me with the findings, having only the year before paid to have a protective barrier coat applied below the water line.

When we all met at the boat the problem was obvious. Several patches of small, dime sized blisters which were easily broken and shown to be wet inside. My dream boat had blisters. I nearly pulled out of the deal at that point especially when I learned how much it would cost to repeat the barrier coating but after I had time to think about it and talk again to the surveyor I decided to proceed. The surveyor suggested to me that all I needed to do for the foreseeable future was sand off the small blisters which were in the barrier coating.

That was nearly 9 years ago. Melos has hardly been out of the water since and now has many thousands more miles under her keel. Each time I haul her out there have been a few small blisters but the problem has never gotten any worse and I have never had to do more than sand the bottom and apply bottom paint. I think I have been lucky but as I have learned recently, there is no cause for complacency because, blisters or not, all fiberglass boats gradually deteriorate if left in the water...

What are blisters?

Blisters on a fiberglass boat occur nearly always below the waterline. They appear as bumps in the otherwise smooth surface of the hull. They range in number from just a few blisters to thousands; from pinhead size to 6" (or more) in diameter. They can occur all over the bottom of a boat, just at the waterline or just on the keel. They can be hard bumps that have to be ground open or soft squishy ones that can be pricked with a finger nail and that ooze fluid. In short, their size and appearance can vary immensely.

How common are they?

They are common; very common. If your boat has blisters then so have those of your neighbors. In talking with several marine surveyors, they all confirmed that around 90% of the boats that they see have some form of blister problem. And as you may expect, most are not major problems. They may have a few largish silver dollar size blisters or a covering of very small, pinhead size ones. Relatively few boats have serious blister problems that require immediate treatment.

What causes blisters?

The answer is water. It is the water that the boat sits in that causes them. Though fiberglass was once considered the ultimate material from which to build boats being light, strong and waterproof, it is now recognized that it is not the perfect, utopian material that it was once thought. Steel rusts, wood rots, aluminum is prone to electrolysis, and now we have to accept that fiberglass absorbs water and gradually deteriorates.

Although you have probably only heard of blisters in relation to boats, the problem occurs in other areas as well. Chemical manufacturers have known of the problem for years, using fiberglass containers for many corrosive compounds but not for water. And the pool and hot tub industry have had their fair share of blisters too.

Though boats are built with an outer layer of gellcoat which is intended to stop water getting past it to the layers of fiberglass matt and cloth below, it just does not do this. Gradually water will pass through the gellcoat and through the fiberglass as well. If you don't believe this, build a fiberglass box, fill it with water and seal the top with fiberglass. Leave it several months and open it up. You'll find it dry inside. The water will have passed through the fiberglass and evaporated on the outside.

So, fiberglass is a little porous. So what. Well it is the effect that water has on the chemical constituents of the resin used to hold the glass strands together that causes blisters. As water enters the hull it begins to chemically react with the resin in a process called hydrolysis. Though this happens throughout the whole underwater surface of the boat, where there are areas of resin that have not been mixed properly, the hydrolysis becomes worse. The water dissolves some of the resin constituents to form a concentrated solution and it is in these areas that blisters can begin.

The words blisters and osmosis are often used interchangeably in the marine industry or are sometimes combined as in a reference to osmotic blistering. But though they are connected, they are not the same thing. Blisters are the outward appearance, the symptom of a process called osmosis, and so to understand how blisters occur it is necessary to understand osmosis. Osmosis occurs where two water solutions of different concentrations are separated by a semi-permeable membrane. A semi-permeable membrane has a molecular structure with holes in it just large enough to allow small water molecules through but small enough to trap other, larger ones. When a semi-permeable membrane has two different concentrations of water on either side of it, the large molecules of water-plus-dissolved-solid are trapped in place but lone water molecules can pass from one solution to the other. The process of osmosis means that water will travel from the weaker solution to the stronger solution until they are both of the same concentration. In the fiberglass of your boat there are strong concentrations of the hydrolysed constituents of resin so that through osmosis, water will tend to pass from the outside in toward these pockets of concentrated solution. As the water builds up inside, the area begins to push out the hull to make room for the water and hence a blister begins to form.

Where do they occur most often?

Because the resin constituents can be hydrolysed anywhere within the thickness of the hull, blisters can form at any depth. However partly because of the way gellcoats are formed and also because the penetration of water through the hull is a relatively slow process, the gellcoat and outer layers of fiberglass are more prone to show blistering than are the deeper layers.

What makes blistering worse?

Knowing how blisters form, it is easy to see that there can be a number of different factors that could make them worse. Fiberglass resin is made of different chemicals which have to be mixed together to create the resin and this has to be done immediately before it is used. If the two (or more) constituents are not mixed together thoroughly, or are mixed together in slightly wrong proportions then there will be parts of the resin more susceptible to hydrolysis than others. If water gets into the resin when it is being mixed (as it can just from a humid atmosphere) then the hydrolysis can begin immediately.

When your boat is put in the water, the process of permeation begins as the water molecules begin to enter the hull. Where they find resin compounds, hydrolysis starts and then osmosis will begin. The speed with which this proceeds will depend on a number of factors such as the permeability of the gellcoat and fiberglass, the temperature and the relative concentrations of the liquids outside the hull and within the hull. If the gellcoat is highly permeable, if the temperature is high or the water concentration outside the hull very dilute then the process will proceed faster. And so this explains to some extent why boats in warm water are more prone to blistering than those in colder waters; why fresh water is worse than salt water; and why some manufacturers are more prone to the problem than others.

What to do if your boat has blisters

Before writing this article I had to undertake some research and so I spoke with a number of marine surveyors and boat yards as well as undertaking some pretty intensive reading on the subject. The question of what to do if your boat has blisters has many answers depending on which book you read or person you speak to. But the general consensus of opinion is that you should do nothing if the blisters are small unless the sight of the bumps offends you (or perhaps affects the smooth profile of a racing sailboat). Everyone that I spoke with could recall cases where blisters had formed on a boat but, once formed, had not worsened over the years but just remained the same. This can perhaps be understood in reference to the process of osmosis, for when the two solutions reach equal concentrations, the process stops. If the blister sites had only small quantities of chemicals that could be hydrolysed then the osmosis could lead to equal concentrations more quickly. So, if your boat has blisters, one option is to do nothing but monitor them.

But if the problem appears to be getting progressively worse and the blisters get larger or more numerous then you may need to undertake some repairs. Large blisters that penetrate deep into the laminate layers of the fiberglass will begin to affect the structural strength of the hull.

Treatment plans

It's a little like going to the doctor, discussing what to do with osmotic blisters. For a few isolated blisters, each can be treated individually whereas a more extensive covering will require major surgery below the waterline in the removal of all gellcoat and possibly some of the fiberglass laminate.

In repairing blisters, there is a need to open up the affected area, to dry it out and then repair it. Though individual treatment versus the more extensive complete bottom repair may appear different, they are essentially the same and the only reason to choose one over the other is usually cost. Beyond a certain number of individual blisters it is more economical for an owner to treat the whole of the underwater section of the hull rather than the specific blisters.

Individual blister treatment

Where there are only a few large blisters on a hull that need to be treated it is sensible to deal with then individually. All of the boat yards that I spoke with do this by grinding out the area of the blister until all soft and wet fiberglass has been removed and the affected area faired out beyond the original blister border.

Total bottom repair

Where the number of blisters is large or the area covered by them is extensive, it is cheaper to treat the whole of the bottom rather than individual sites. Though in the past this was sometimes done by preparing the gellcoat and then applying a barrier coat on the top of this, now all boat yards and surveyors agree that at a minimum all existing gellcoat has to be removed.

Where the blistering problem is in the gellcoat or just below it, this is usually done by using a power grinder to sand off the gellcoat but where the blisters occur deeper in the laminate a power plane is often used to remove gellcoat and laminate to a preset depth. Though the planer quickly removes material from the hull it is not able to effectively deal with hard chine areas (particularly the V-hull forms on many power boats) and in all cases the planer leaves ridges which then have to be sanded smooth. All yards indicated that the choice between grinding off the outer layers and using a planer is made on a cost basis. Where the problem area is near the surface it is cheaper for the customer to have the yard grind the surface; where the problems are deeper using a plane first becomes economical.

All three yards used both techniques. Two of the yards employ an outside contractor specializing in hull planing when this is necessary; the other yard owns their own equipment.

Whether a grinder or planer is used, it is necessary for sufficient material to be removed so that solid unaffected fiberglass remains.

Drying out the hull

Before replacement of material is commenced the affected area is allowed to dry out thoroughly.

Once the outer layers have been removed all yards wash the affected areas using either a pressure washer or steam cleaner in order to flush out the offending concentrated solutions. This process is repeated by all yards for a period of time while the area is drying. One yard cleans the affected areas every other day for the first two weeks; another yard washes it weekly for the whole drying period. The reason these washes are necessary is that as water evaporates from the hull, glycol (a chemical similar to the coolant used in your car) which is a constituent of the resin rises to the surface and effectively seals it from further evaporation. By repeated washing the glycol is removed thus aiding the drying process.

In order to check the dryness of these areas a moisture meter is normally used and all three yards that I spoke with regularly took readings using the Sovereign Moisture Meter which is said to be one of the best available. Typically the yard is looking for moisture levels of lower than 5% though all yards reported times when they could not get a hull to dry out to this level. One yard owner uses the difference in moisture content between topsides and underwater sections as a guide as to when the hull is dry enough. Other yards waited until the moisture levels reached an equilibrium and have stopped falling.


One boat yard that I spoke with repairs small individual surface blisters with an epoxy putty applied to the area which is then faired smooth though larger and deeper blisters are repaired using a build-up of fiberglass cloth and resin. Other yards scorned the use of epoxy putty for all repairs saying they had found that it did not adhere well and so for all individual repairs they used a build up of glass cloth and resin. Once the blister sights have been faired then these individual repairs are usually treated with an epoxy protective coatings in a similar way to that used for a complete bottom repair.

Where the whole underwater section is being repaired there was a remarkable uniformity in the process used by all yards. Once the hull is dry enough fiberglass cloth and resin are replaced to build up the hull to the original thickness. Then a protective barrier coat is applied to the hull in an attempt to prevent further water penetration. All yards that I spoke with used the Interlux? Interprotect? products though there are others available on the market.

The Interprotect system requires two different two-part epoxy coatings to be used in the process of sealing the hull. Once the hull has been cleaned, dried thoroughly, and cleaned again, a coat of Interprotect 1000 is applied to seal all bare laminate. Once this has cured several coatings of either Interprotect 2000 or 3000 are applied until a minimum thickness of 10 mil (0.010") has been obtained. In applying these coatings, fairly strict timings have to be adhered to or the surfaces re-prepared before additional coats can be applied.

The effectiveness of repairs

As the problems with blistering have become more understood, the treatment for them has developed accordingly. What the epoxy barrier coats (such as the Interprotect system) seek to do is stop the migration of water into the hull by providing a more waterproof barrier. The Interprotect 2000 /3000 products incorporate microscopic mica platelets which overlap forming a physical barrier to the water but even this is not sufficient to stop water penetration entirely and eventually problems can reoccur after such treatments. How long this takes to happen is dependant, like the formation of blisters, on many things, not least how the products are applied. And this is a real problem. Manufacturers have difficulty in controlling the circumstances in which their boats are built such as temperature, humidity and product usage; boat yards have even greater problems because they are trying to rebuild the boat from the outside applying many thin coatings, any of which could be the site of later problems.

How new boats are better protected

With an increased understanding of the causes of water permeation and blistering, boat manufacturers have begun to use new products in an attempt to prevent or minimize the chances of blistering. Much research has been concentrated (for good reason) on the resins used to bond the fiberglass and new vinyl ester resins have shown a greater resistance to water penetration than the more frequently used polyester resins and so these are now being used by some manufacturers especially for the layers of fiberglass matt immediately below the gellcoat. But even these are not the whole answer as one local boat builder found out. He used vinyl ester resins for all of the fiberglass on his own boat yet he subsequently got blisters in the hull.

Can you protect a new boat from blistering?

If blistering occurs as an indirect result of water permeating into the hull and if the new epoxy barrier coats can stop or delay this process maybe you should consider applying such a preventative barrier coat on a new boat before it is ever launched. This is a recommendation made by one local boat yard and the idea seems to have considerable merit though if this work is undertaken it may jeopardize any warranty offered by the manufacturer and so this should be checked and the consequences considered before you rush out and get this done.

Where this leaves us as boat owners

Whatever treatment our boat has had or will have in the future it seems that, as yet, there is no certain way to stop the migration of water into our boats. This water will gradually cause the fiberglass resins to break down resulting in a slow but progressive weakening of the hull over a long period of time even when blisters do not occur. When blisters do occur they can be monitored to see if they progressively grow and if they do the hull can be repaired at the sight of each blister or by a complete treatment of the underwater sections of the hull. But even when this is done you may have problems later as it is extremely hard, if not impossible to apply these coating in perfect conditions to a perfectly dried out hull. And even if this were done you will eventually get some water migration into the hull.

Those of us who keep our boats here in the warm waters of the gulf coast and who do not regularly haul our boats out in the winter months have a greater chance of blisters than those who keep their boats up north (though even in England which is further north than most of the U.S. the problem is extensive). In the end we probably just have to accept that fiberglass is not the perfect boat building material after all. Though it has many advantages it will absorb water, is likely to blister and won't after all last forever. It's just part of the cost of owning a boat. Perhaps we can get the I.R.S. to amortise a fiberglass boat over a few less years!

Special thanks to:

The staff at Youngs Boat Yard, Seabrook Shipyard Repair Yard and South Texas Yacht Service for the time they took in explaining to me how they treat blister repairs.

And to the marine surveyors that gave me the benefit of their experience , advice and information about blisters.

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