SailNet Community banner
1 - 6 of 6 Posts

35 Posts
Reaction score
Discussion Starter · #1 ·
Might have read this before. Interesting article. Comments?


I'm going to say this right here, right now: "Gelcoat blisters don't matter!" There, I said it, at the risk of being called an opinionated, stubborn Dutchman!
Probably, I inherited those terrible qualities from my Grandfather who built two fiberglass boats in his shipyard in Holland in the early fifties, This he did, despite predictions of doom and gloom, forecasted by numerous experts. Until that time, the yard had been using respectable materials such as wood and steel.

I remember those first two plastic boats because they created quite an upheaval amongst the workers in the yard. "The old man has gone off his rocker", they said. The boats were named after my two aunts, Cornelia and Josephine. They were (the boats, not my aunts) 37 foot, full keel sailing yachts, designed by Sparkman and Stephens.

But, I do not want to blame my Grandfather, rest his soul, for my blister problem. My problem is that gelcoat blisters do not appear to be a problem. Yet, everyone says, "they are!" Many articles have been written by experts about their destructive qualities. They are referred to as the pox, cancer and all sorts of other horrible diseases which will cause certain death when inflicted upon a human being. "Don't buy a boat with the pox", they say, "it will delaminate" - another scary term. Some say blisters should be repaired by removing layers of laminates from the hull and then to be laid over with new layers of glass. Peeling machines are now being marketed to strip the layers from the hull. As a result, thousands of dollars are being spent on blister repair. I have seen figures as high as twenty thousand dollars for a thirty foot boat.

Why are gelcoat blisters considered to be such a disastrous problem? Based on what research and by whose authority?

To answer that question we have to examine the business of pleasure boating as a whole. In North America, pleasure boating and almost everything that is related to it, is pretty much a wide open, non regulated and uncontrolled industry. As opposed to the building of a house, you don't need building permits, there is no building code, there is no building inspector. Anyone can build, use any type of design, any kind of material. Cardboard? Why not? No one will stop you, as long as you have the appropriate amount of life jackets, fire extinguishers, navigation lights and baling devices, you will probably pass the voluntary Coast Guard inspection, providing the boat doesnt sink before it reaches the Coast Guard dock. Anyone can sell boats, no license required, hang up a shingle and you are a boat surveyor - again no license required. And then, anyone, no age limit, no experience required, can take that eighty foot cardboard boat, powered by twin super charged one thousand horse power engines, out for a spin on the beckoning wild blue yonder. You think Star Trek is the last frontier? No way, the boating business is. Isn't it wonderful? Yes, most definitely, it is. Pleasure boating is probably the only ungoverned pastime left on this polluted earth. No helmets, no license, no seat belts, and you're allowed to smoke on your boat too! You can even have a sip of brew while driving, although, that is pushing it a little bit. But, you can get away with it as long as you don't hit anybody. I'm all for it! However, because of this, the boating industry attracts a lot of self appointed experts. The government doesn't appoint, so we do it ourselves. So, the experts are where the boats are. The number of experts closely equal the number of boats. Ask one question and you're bound to receive a multitude of answers and opinions. Most of the experts cast spells of doom and gloom and readily relate some tale of woe that has happened to so and so to add credit to their advice. Bear in mind that a gloomy view of something demands more attention and sounds more impressive. There is always a knowledgeable answer, let's face it, a reply such as "Gosh I don't know", or, "I have to read up on that", just doesn't cut it!

Now let's get back to blisters. I have also formed an opinion about blisters, just like all the other experts. I will back up that opinion by reasoning. Here is my case:

The first time I had to deal with a gelcoat blister problem was in the late sixties. Until then, I had not encountered it before, most certainly not to the extent as in that particular case. As a result I did a lot of research and talked to a lot of people in the fiberglass trade. I did not get any deeply researched scientific answers other than the fact that the gelcoat was blistering, as in "paint blistering from wood or steel". In other words, it's coming off! Perhaps it was because the surface underneath it was not prepped properly, or it was not dry when the gelcoat was applied or, it is just plain tired and worn and needs to be done again. Since that time I have seen a lot of boat bottoms. These busy days, through my Company, I see or hear of and deal with as many as close to a thousand per year. Therefore, I think I can safely say that I see more "bottoms" than most and that includes boat yards. It is therefor, I think, safe to say that it is likely that I have seen, tapped, probed, dug into and caressed more blisters than most. Yet, after having had a working part in the boating industry since the days well before fiberglass, I have never encountered a fiberglass boat that was in any way structurally damaged or unsafe to operate as a result of gelcoat blisters. To silence any critics of this statement I suggest they produce or point out a boat of normal solid fiberglass construction which has become structurally unsound and unsafe to be operated for it's intended service as a result of gelcoat blisters. If they can, I'll eat it!

Gelcoat blister problems can occur due to three basic reasons:

1. The use of type of resins, such as fire retardant resins. In this case blisters are generally also apparent above the waterline of the hull and on the superstructure. It is not caused by water. It can not be fixed because the problem is within the composition of the hull laminates.
2. Poor building procedures. The fiberglass cloth is not stored properly and has become damp prior to layup. The result is intermittent and generally poor bonding of the resins with the cloth and the gelcoat. The blisters may appear above as well as below the waterline. As a rule the vessel will have internal delamination of the fiberglass layers and may or may not have any gelcoat blisters. The delaminated areas may show up as very large blisters and the outer layer of the blister will be thick since it may contain several laminates. This is a structural problem and is difficult if not impossible to repair.
3. Water migrates through the gelcoat. Blisters are located under the water line and sometimes located along the water line only. When punctured, the blisters contain a colorless liquid which is acidic.

Number 1. Is mainly encountered in a certain series and make of production boat. The manufacturer corrected the problem several years ago.

Number 2. Is rare and not typical of production type boats. I have encountered it sporadically. In each instance the boats were manufactured by companies who normally produce a very good product.

Number 3. This is the one occurring frequently and which is the one most of us have seen evidence of and which is the topic of this article.

Let's examine the procedure. Water migrates through the gelcoat. This takes time. The speed depends on the thickness and or the porosity of the gelcoat. In fresh water, the effect can be quicker because fresh water is thinner and lighter than salt water.

Lessons to be learned from this are: Do not sand or scrape the gelcoat prior to painting with anti fouling paint because it makes the gelcoat porous and thinner. (I have been preaching against sanding and scraping for years. Now, some manufacturers, such as Hunter Marine, do not allow sanding of the gelcoat. It will void the warranty against blistering of the gelcoat. Perhaps my cries of agony have been heard!)

Older boats are less likely to get blisters because their gelcoat is thicker, as a matter of fact, so is their entire lay-up, thicker that is. Some older boats will never get blisters because they were built differently.

The laminates of most glass boats consist of mat and woven roving impregnated with resin. Roving are strands of glass fibers which are woven together as in a piece of textile. Mat, on the other hand is made up of chopped up strands of glass fibers which are held together by a starch like substance. In each case, the whole thing becomes a solid piece when saturated with resin.

Woven roving has tear strength. It is difficult to pull apart. Mat has no tear strength and can easily be pulled apart. It acts as a filler, making the hull thicker and stiffer. It has some impact strength, but that is all.

Early fiberglass boats were built over male molds, up side down. The inner layer of the glass was the first layer, the outer layer the last. As a rule, the last layer was woven roving, not mat. Gelcoat was applied last, as a finish. (The very early boats had no gelcoat at all. The hull was faired, sanded, and then painted) In some cases, especially with hard chine hulls, the hull was gelcoated and tightly wrapped with plastic. When the gelcoat was cured, the plastic was removed, and bingo, a smooth finish!

As building techniques progressed, hulls were built in female molds. It was discovered that the gelcoat could be applied first to the inside of the mold and then the layers of roving and mat were laid over that. It was soon discovered that, upon close examination, the pattern of the woven roving, which was adjacent to the gelcoat, would show, especially if the gelcoat was thin.. This is called "print through". This caused the manufacturers to reverse the procedure and put the mat next to the gelcoat instead of woven roving. ( Mat has no pattern and a much finer texture) There is only one problem - the fibers in mat are held together with a starch like substance. I believe this to be the culprit of the blisters to some extent and the reason of the damage caused by the blisters to a great extent. The general consensus amongst chemists is that when water migrates through the gelcoat and remains there for some time without circulation, (REMEMBER THIS) that a chemical reaction takes place which produces acid. It appears that the acid dissolves the starch like substance which in turn dislodges the resin and possibly evacuates this through the process of osmosis. In other words, it slowly disperses back out through the gelcoat to set up a balance with the surrounding water. When the blister becomes the approximate size of a quarter, as a rule, when opened up, a small cavity can be seen in the outer laminate, which is the layer of mat. What one sees are the loose glass fibers, the resin is absent. Providing the boat was built properly in the first place, meaning same is not kept afloat on this layer of mat, in which case it would have broken up well before the blisters appear, these cavities do not weaken the structure. I have never seen the woven roving affected by this process, which again proofs the fact that it is the acid combined with the binding agent in the mat, and not the resin, which is the culprit.

Remember that in order for the chemical reaction to take place, the water has to have been trapped under the gelcoat for some time without circulation. It therefore follows that if the blisters are punctured at an early stage, before they have been able to cause any damage, there will be circulation, thus, the acid will not form or get to a stage where it can be harmful. In any event, as I said, the damage, if any, is restricted to the outer layer of mat, which is not a structural component of the hull laminates anyhow and is there strictly for cosmetic purposes. In fact, many fiberglass boats have been built and repair jobs have been done, without a coating of gelcoat. Obviously, there is no need to spend a fortune by peeling layers of glass from the hull. At most, to achieve a cosmetically pleasing appearance, the blister can be removed by sand blasting or grinding to remove the gelcoat, or by peeling of the gelcoat only, faired with filler and then sealed with any of the products readily available and suitable for that purpose. Or, otherwise, just grind the blisters off and paint!

Whatever you do, do not strip layers of laminates from the hull. This will structurally harm the vessel. There is no guarantee that the new layers applied will stick properly. In many cases where I have seen this done, the new layers are blistering, then you do have a problem because the hull laminates are now weakened!

A fiberglass hull does not absorb water. Core material, yes, but solid glass, no! Glass and resins are inert. There is no space for water. That plastic bucket that has contained water for many years, is as dry as a bone when the surface is wiped and dried. I put a humidity meter on a water bucket which my wife had used for many years in the paddock of her horse. I emptied it, gave it a good wipe and left in the sunshine for twenty minutes. The humidity reading was 5 percent, exactly the same as a brand new one.

It is often claimed that a hull has absorbed water and has therefore become heavier. These findings are based on readings with a humidity meter. Both statements are inaccurate. The humidity meter in the wrong hands is a dangerous instrument. The thing works by means of conductivity. The electrical current takes the shortest route through material with the least amount of resistance. A humidity meter cannot be used over anti fouling paint because it is more conductive than fiberglass. Fire retardant resins cannot be metered for the same reason. Damp cloth, interior liners, paint, steel fuel tanks, water tanks, wooden frames and floors, if installed against or near the fiberglass measured, will throw the meter off scale. I started using a humidity meter in 1981 and I am still learning. Further more, what is often misunderstood is that the meter gives a reading of RELATIVE humidity. Not solid water content. Listen to the weather report, the announcer may state that the relative humidity is 65 percent. That does not mean that we are standing up to our necks in water. In fact, I was told by an employee of Sovereign Moisture Meters that when the meter's needle is at 100 percent, the actual water content is equivalent to one drop of water per cubic foot of fiberglass.

The question of weight is another mystery. The statement goes, "the glass has absorbed water and therefore has become heavier." In the first place, glass cannot absorb water, it could only absorb water if there was vacant space in the laminates. (poorly built and delaminated) or, if water has displaced those areas previously occupied by fiberglass. In other words due to some mysterious process, water has removed the fiberglass from the hull and taken its place. Let's assume for argument's sake that this is possible. Water is now occupying space that was previously occupied by fiberglass. Well, it now becomes a question of specific gravity. All I know is that if I dump a piece of fiberglass in the ocean, it sinks like a stone. Yet, the water which I pour from a bucket seems to just sort of float around. To me that indicates that water is lighter than fiberglass. In other words, if I substituted portions of fiberglass from my boat with water, the boat should become lighter, never heavier!

Coatings of paint, epoxy and gelcoat will blister. Paint will blister on wooden boats and houses, epoxy coatings blister on steel and ferro cement boats, gelcoat will blister on fiberglass boats. What does all this mean?

When a boat has blisters, I find I have to depreciate its value, not because of her being unsound or unsafe, but due to misinformation. The general public will either not buy that boat, or expect a price reduction. The result? There are a lot of good boats out there which are going cheaply because they have blisters, but will provide the same enjoyment and are as safe as those with the smooth bottoms. For racing sailors, there is actually an advantage to having a boat with blisters. If they loose the race they can blame it on the blisters rather their sailing skill.

How long does fiberglass last? Nobody knows, it hasn't been around long enough. Perhaps my Grandfather can give us some idea. Not long ago I was in Grenada, West Indies. I was looking for an ex crew member of mine who had opened up a bar on Hog Island. I went there to look him up, but he had gone away on some errand. As I stood waiting I scanned the boats in the anchorage. One sail boat looked awfully familiar. I borrowed a dinghy and went in for a closer look. I rowed around the boat and eventually could not stop myself from being rude and climbed on board. I checked her over pretty carefully and concluded she was in pretty good shape and obviously ready and trimmed to take her owners on the next leg of their voyage. As I rowed away, I stopped to give her one last look. The yacht had swung away, showing her transom. On it was her name, it read, Josephine. "Not bad", I thought, "she is now almost fifty years old and she keeps going and going and going..."

) Jan de Groot

Super Moderator
Farr 11.6 (Farr 38)
10,834 Posts
Reaction score
I would say that this article paints a very inaccurate image of the blistering problem and is so misleading as to be dangerous.

To begin with, boats with gelcoat blisters represent a very small percentage of the boats with blisters . Most blister cases involve the laminate which occurs below the gelcoat.

In the rare case that the blisters are simply a failure of the bond between the gelcoat and the laminate, Jan de Groot is basically right that the blisters are pretty inconsequentual; remove the delaminated gelcoat, apply a barrier layer of epoxy or vinylester and the problem is gone. But that is almost never the case.

The article fails to mention the most common cause of blisters that are strictly in the gelcoat. Most cases where the blisters occur in the Gelcoat are caused by the surface of gelcoat 'going off' before the veil coat is laid up or by the surface of the gelcoat getting contaminated by wax or dust before the first lay-up occurs.

One of the key points in which this article is mistaken is the suggestion that no one has actually studied the cause of blisters in depth, when in fact there have been a number of very comprehensive studies that have been widely circulated since the studies were preformed.

These studies showed that there were a series of causes for blistering and that the seriousness of the problem is related to the cause of the problem and the extent to which the problem was left unattended to.

For example, while gelcoat is a poor water barrier, in most cases it is adequate to prevent the sub-gelcoat laminate from blistering. In the case where the gelcoat does separate from the laminate, water can get to laminate more easily and so if left unrepaired, it is more likely that the problem will spread into the laminate.

The basics of osmotic blistering, the most common cause of blistering is that by its very nature Polyester-Fiberglass matrix is porous. In osmotic blistering, water enters the pores in the matrix and mixes with the byproducts of the resin catalyzation and form compounds that attach the lay-up and destroy the bonds between the resin and laminate, which allows additional water to get to previously unexposed portions of the laminate. It is for that reason that blisters spread expodentially rather than linearly once they form.

Articles like this one drive me nuts. They sure sound knowledgeable but regrettably do not match the reality than many of us may some day have to face.


13,871 Posts
Reaction score
Totally agree with Jeff...From a prior post:
The cause of the problem was well established in the 1987 University of Rhode Island study by Thomas Rocket and Vincent Rose, The Causes of Boat Hull Blisters. In simple terms, what happens is this. Water penetrates the gelkote both as water vapor and as liquid water. Water is particularly good at this due to the small size of the H2O molecule. The gelcoat is a rather poor barrier against water penetration when constantly immersed. The glass fibers assist by acting as capillary tunnels to transport the water molecules into the laminate. Once adjacent to the resin in the gelkote and laminate, the water goes into chemical solution with what are known as "water soluble materials (WSMs)" in the resin in the gelkote and laminate. These WSMs include phthalic acids, glycol, cobolts, mekp and styrene which have not gone to full cure in the hardening process. To varying degrees they are present in all cured polyester resins. Five percent is an excepted norm. In some rare cases the quality of the materials or their application may be inferior causing a higher than normal percentage of water soluble elements."

This from Steve D'Antonio:
So why are blisters and hydrolysis a problem? Well, the blisters themselves slow the boat and are unsightly. The blisters slowly delaminate the fiberglass laminate locally and if there are sufficient number of blisters, may direct affect the structural integrity of the laminates. Nothing seems to scare off a potential buyer faster than blisters though this is changing as the buying public becomes more familiar with the problem and the effectiveness of well done repairs. The affects of hydrolysis on the resin, however, are of more concern than blisters. The hydrolysis process softens, weakens and removes the resin from the laminate, thus reducing the rigidity of the laminate. As rigidity is reduced, the amount of flex experienced in portions of the bottom increases. With increased flexure comes increased risk of fatigue failure. Most yacht hulls have a safety factor of 2-4 to 1, leaving quite a bit of room for deterioration. These safety margins, however, vary widely and are constantly under pressure to be reduced in the name of performance. If a hull is of cored construction, structural damage can occur quite quickly. Large scale core saturation is largely irreparable at a reasonable cost. It should be noted that the presence of water alone in a glass laminate, even when no hydrolysis damage has been done, significantly decreases laminate's resistance to structural fatigue.
In 1991, Zahniser's commissioned Comtex Laboratories to analyze the physical properties of laminate panels removed from the bottom of Gulfstar 50. The laminate was highly hydrolyzed. The bottom was clearly deforming from water pressure due to immersion suggesting low laminate rigidity.. Test results on these panels showed a fifty percent reduction in rigidity from the new condition. Tensile strength, however, was not greatly effected. The loss of rigidity is significant as the bottom will flex ("oil can") more over bulkheads and other hard spots and eventual time to fatigue failure will be shorter. We also tested a "repaired" panel, using the methods discussed in this paper and achieved rigidity approximately 130% of the theoretical new condition.
In real life, we are starting to see failures in hull bottoms we think are directly related to hydrolysis damage to laminate resin. In six separate cases, we have seen serious, though the hull fractures at the keel roots on fin keeled sail boats. In each case, the laminate resin was severely hydrolyzed. We have seen two cases of laminate fracture across bulkhead hard spots in two powerboats which we thought were related to hydrolysis of the laminate resin. The good news is that eight boats is not a lot of boats, but consider that these are only the ones that we have seen. Surely there are more out there and surely there have been boats lost for these reasons as well. Accident investigation on sunk boats is not like aircraft crash investigation. Unless the boat is in the way, it is usually not raised and the cause of sinking investigated.


I hope this 8 boat sample from an out of the way repair yard in the southern Chesapeake is sufficient proof that blisters are more than a cosmetic problem...or that they can be fixed simply or cheaply.

61 Posts
Reaction score
-anyone ever seen a boat sink due to blisters? I don't think blisters should create so much panic.

I live in the worst place for blisters we have warm brakish water and the boats stay in it all year long. Lots of boats end up with blisters, even those high end companies that have said they don't blister.

I think the blisters I have seen are for the most part cosmetic only. ...except for the effects on speed performance, a lumpy bottom can't be fast. I had a few on my previous boat and I had them fixed -one by one.

684 Posts
Reaction score
There is lots of discussion around about delamination of boat hulls...from blisters or poor layup. While I've seen quite a few people spend lots of money grinding away the fiberglass to get rid of blisters, I have, in 39 years, never actually seen a boat, sail or power, that has had a hull failure due to delamination. It would be interesting if a collection of photos from delaminated hulls could be posted on this website. Keep it simple and post only sailboats, as the typical power boat gets lots more pounding and flexing in its life. While I wouldn't want a boat with a serious blister problem, I think its been over hyped and caused lots of money to be spent that didn't need to be most cases, open up the blisters, let them dry out and fill them with a filler or epoxy, paint, and go sailing. Just the opinion of a non-expert. I'll be watching for the photos.

Dirt Free
3,430 Posts
Reaction score
Two more cents

( a highly simplified explanation )

If I knew what an osmosis test was I could be doing a lot more business as I get asked for it all the time. Dock talkers often use "blisters" and "osmosis" as interchangeable terms however, there are different types, causes and degrees of blisters and using the word "osmosis" just confuses the issue. I am often asked if I can perform an osmosis test to which my response is " What's an osmosis test ?". This response is met with a blank look as the people asking the question really don't know what they are asking.

This is one of the reasons I dislike the word "osmosis" as applied to boats, it has become a generic term to describe all types of blisters and moisture content in fiberglass hulls cored and uncored. Strictly speaking for osmosis to occur you need fluid on both sides of a membrane. If you have this situation on a boat, you have more problems than a few blisters. What you are really encountering is simply water soaking into a porous material. Polyester, epoxy and vinylester resins are not waterproof, they are hygroscopic ie. They can absorb and retain water! (less so with epoxy and vinylester). I have heard many convoluted definitions of osmosis in attemps to justify the use of the word but what we really have is simply a process of absorption.

OSMOSIS: The tendency of fluid substances, if separated by a porous, membrane to filter through it and become equally diffused.

So lets forget about the semantics and get to the issue................

There are many of causes of blistering, To list a few ....... trapped moisture during moulding, undercured resin, overcured resin, aerated resin, incorrect timing of subsequent layers, absorbent fillers, voids, trapped moisture in core materials, stale catalyst, emulsion bound mat, dusty mould, hygroscopic dust, cold mould, inadequately mixed resin, uncontrolled temperature and humidity levels during moulding process, uncontrolled temperature and humidity of raw materials in shipping, orthophtalic (cheap) resins. OK .... enough ! If I really thought about it I am sure I could come up with more but lets just say this, it is an extremely complex issue and "osmosis" just does not cover it. all you need to know is about blisters.

If you insist on calling osmosis then it follows that all fiberglass boats have it !

NO VOIDS = NO BLISTERS (maybe) : There are dozens of reasons for voids in a laminate (some chemical) and they can range from tiny champagne size bubbles to several square feet although most are less than 1/2" in diameter. The average laminate may be 8-15 layers of various types of glass fabric made up of millions of miles of microscopically thin glass strands wetted out with resin. It is unreasonable to assume that all voids will be filled whether the wet-out is accomplished by five guys with rollers or one of the admittedly better (but not perfect) vacuum bag processes. All fiberglass layaups have voids, some more than others. The higher the void content, bigger the voids and the more likely and earlier you will see blisters.

A typical 30' , uncored sailboat hull can absorb about 30lbs. of water or roughly 3% maximum weight of the laminate. The glass fibers do not absorb any water and the resin is chemically incapable of absorbing more than 3% so theoretically 3% water content is sturation point of the material (voids excluded). As all polyester reinforced glass fiber and gelcoats are water permeable to some degree, all fiberglass boats left in water long enough will absorb water and probably develop blisters. This is rarely a structural issue (at least in our lifetime) although it can drive the sailboat racers nuts !

If the hull is cored with balsa or the laminate is all chopped strand (read - very cheap boat) then you may have a more serious and expensive issue to deal with as the balsa rots when wet and in the case of chopped strand fiber, the millions of exposed fiber ends wick water like so many straws.

The more common gelcoats are simply pigmented polyester resin of varying levels of quality and these pigments combined with the aeration caused by spraying the product into the mold can make it more permeable than the resin used in the laminate and therefore most blisters appear in the gelcoat. These blisters are usually small (1/8 - 1/4" dia.) and round in shape. While this does have an effect of the sale value of the vessel it is rarely a cause for concern.

Under the gelcoat is usually a "skinout" mat of chopped strand glass fibers that does not contribute much to the strength of the hull but is used primarily to hide the basket weave pattern of the heavier woven fiber which (hopefully) makes up most of the laminate. When water passes through the gelcoat it may wick up the chopped strand fibers of the "skinout" mat. These blisters are usually small, elongated and again not a major concern.

Water that has passed through the gelcoat and skinout mat into the structural laminate may combine with soluble elements that may occur in the voids. These elements could be uncatalysed resins, silane, glycol or salts (not the table variety) or any of the other chemical soup of ingredients that results from the resin curing (or uncured) process. When water combines with these molecules a new, usually larger molecule forms (hydrolysis), thus preventing escape of the fluid since the molecule is now bigger than the microscopic hole it came in through.

HYDROLYSIS: A chemical process by which the oxygen or hydrogen in water combines with an element or some element of a compound to produce a new compound.

As these newer, larger molecules multiply deep in the laminate they can get big enough to start to pushing apart the various layers of the laminate as the resin dissolves. This can be a serious issue, it is however relatively rare.

Put on a pair of safety goggles and puncture a few blisters. If they are dry or release a clear fluid, you likely do not have the serious type of blister. If an acidic vinegar like fluid appears this could be the more serious "Hydrolysis" type blister. Be careful as some of these blisters contain fluids under tremendous pressure. If the blister is very large and cannot be punctured with an ice-pick, it is likely very deep in the laminate, in this case drilling a hole for closer inspection may be warranted.

THE CURE? Sorry..... regardless of the chemical companies hype there is no cure. You can only delay the inevitable but a very high percentage of blistered boats will still outlast you. I have surveyed one local boat three times over the last 10 years and each time it was getting another $10k "epoxy bottom job". I refused to survey it a fourth time because a well respected shop was about to do it again using the same improper techniques as the previous three jobs done by others. Whoever buys this 42' motor yacht will be doing it again soon (if there is any laminate left).
Small blisters in the gelcoat may be repaired by sanding, drying and applying an epoxy or vinylester bottom coat. This may help the resale value of your boat. Blisters in the skinout mat can be repaired by the same method but with much more aggressive sanding and perhaps some patching. The larger hydrolysis blisters require complete removal of the gelcoat and probably the skinout mat and perhaps a layer of the laminate (perhaps in local areas or over the entire hull in which case new cloth may be needed). The hull must then be dried to the point where the epoxy or vinylester bottom coat will adhere and washed frequently during the drying process to wash off as much of the hydrolytic fluid as possible. This fluid and water may weep from the hull for weeks, months or even years so washing is crucial to providing a clean surface to ensure the adhesion of the new barrier coating.
This can be a bit of a gamble. Many bottom coats fail because the hull was not dried or washed properly. I have seen boats put under heat lamps for six months before bottom coating and new blisters appeared within a few years. The moisture is so deep in the laminate at a molecular level that it is not easily evacuated. A new system of applying heat under vacuum holds promise for drying hulls but for the most part it's still a gamble.

Give this careful consideration before plunking down $5,000.00 - $10,000.00 - $20,000.00 or more for a bottom job and always ask for a written guarantee (unlikely). Consult an Accredited Marine Surveyor® before spending you're hard earned money.

Captain Wallace Gouk AMS®
Port Credit Marine Surveys
  • Like
Reactions: PorFin
1 - 6 of 6 Posts
This is an older thread, you may not receive a response, and could be reviving an old thread. Please consider creating a new thread.