Boat hull blisters.

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This page updated: April 2007



David Pascoe's "To Buy or Not to Buy ... A Blistered Boat, That Is."
David Pascoe's "Hull Blisters"
David Pascoe's "The Wonderful World of Hull Blistering And Other Interesting Scams"
David Pascoe's "Failed Blister Repairs"
Chris Caswell's "Fiberglass Blisters"
Rockett and Rose's "The Causes of Boat Hull Blisters" (PDF)
Sailnet thread on blisters"
Craig Bumgarner's "Blisters and Laminate Hydrolysis"
[Fiberglass] "Repairs You Shouldn't Ignore" article by Nick Bailey in issue 2000 #3 of DIY Boat Owner magazine.
"The Good, The Bad, The Ugly" (delamination) article by Nick Bailey in issue 2000 #3 of DIY Boat Owner magazine
Article by Steve D'Antonio in May/June 2004 issue of Ocean Navigator magazine
Article in 7/15/2001 issue of Practical Sailor
WEST System

Book: "Sailboat Hull and Deck Repair" by Don Casey.


From Charles M on Cruising World message board:
Re: moisture meter:

Very little moisture or a lot seem to give very similar readings. Dry is okay. Even there some boats have a bottom paint or something behind the hull that gives a moist reading. I tried experimenting with layers of paper with one moist and it doesn't take much to get a wet reading. The meters don't adjust for thickness. Really had to open it up to tell. Who is going to let you drill holes to find out? Many lookers would be a lot of holes ... Sounding by an experienced person doesn't always work either. Had dry reinforced areas labeled wet and wet areas missed by surveyors.
From Larry / CT on Cruising World message board:
I own a moisture meter, but as Charles M pointed out -- you can't get a reading on the core unless you can drill small pinholes. 1/16" holes are sufficient. You can get a reading on the surface but what will that tell you?

If you're looking at wooden boats, no problem. You'd still have to get some penetration on the needles to get a good reliable reading. If you're looking at FRP boats, I would be surprised if the owner would allow invasive testing without a contract and deposit down. You'd have to repair any holes regardless.

"Vagaries of the Moisture Meter" article by Dave Gerr in 6/2001 issue of Sail magazine

From blister article by Tom Pawlak in Epoxyworks #17 Spring 2001:
  • Can get Epoxyworks guide 002-650 "Gelcoat Blisters: Diagnosis, Repair and Prevention".

  • It may take a couple of years of monitoring and repairing your small blisters to see if they are just a one-time occurrence, a continuing low-level irritation, or the first sign of something much more serious.

  • Look for blisters immediately after boat is hauled out; they may deflate quickly and be hard to see after an hour or so.

  • An overlooked cause of blisters is moisture inside the hull, trapped by poor ventilation of compartments. Add vents, increase air-flow. "Warm/moist air will aggressively permeate fiberglass laminate."


"No boat ever sank from blisters."

From John Owen on the Morgan mailing list:
A more conservative view (opinion) on blisters:

MOST (not all) yard recommendations to sand blast (be very careful, an inexperienced operator can go deeply into the hull), walnut shell blast, peeling machines, etc are made to generate work orders! Our experience here in FL is that most "epoxy jobs" do not solve the problem: i.e. peel, dry, dehumidify for six months, re-coat with X number coats of epoxy -- and in many case blisters reappear in a year or two! (Not all of course, but not all boats of a given age, manufacturer, water, dry/wet stored blister, either). It is just not worth the time and expense to strip a hull and not stop an inevitable process. This is not saying that it is not the appropriate process in some cases - it may be. Large (2"), soft (push them with a finger-squishy) blisters should be ground out individually, dried, filled, faired and painted. If there is widespread true delamination deep in the hull, stripping and coating may be the right thing to do.

Personal experience: We bought INTERLUDE (1978 Morgan Out Island 415) in 1983 (5 years old)- she had made the annual migration from NY to FL both ways each year for five years - so spent half of her time in NY (northern) waters, half in FL (hot water). When we hauled for survey, she was covered with hundreds of 1/4 to 1/2" blisters, all were hard. Knowing they were not structural (soft and deep into the laminate), but only in the gel coat and cosmetic, we bought the boat and ignored them. In the past 17 years, we have hauled every year or two. Sometimes, we could not find a blister!, other times, some, others many. Any time we find ones that persist and seem to grow (see large-soft above), we cut them out, fix and relaunch. Many times gel coat blisters very evident at haul out will "dry out" and disappear in a day(?). We have continued to watch, repair as warranted, and stop worrying about blisters.

A few years ago a major marine paint company offered a 5 year warranty on blisters if you used their process. They withdrew the program and most yards offering it stopped recommending it, because they could not stand behind it - re-blistering well before the 5 year point! It is still a good "re-coat" process, but no guarantees!!

My point is, that while blisters MAY develop into a serious problem, in most cases they are just another normal cosmetic maintenance job. For those who have never had to deal with them, congratulations, you lucked out.

From Debby Lloyd on the Morgan mailing list:
"Pimple blisters" sounds like it might be what we called "diaper rash" on our previous boat, a 1969. We sanded the hull well (tough, tough, job) but didn't gouge it, then applied West epoxy with none of their additives, in six coats, going round and round as each coat tacked off so we wouldn't have to sand between coats. We did add a bit of gel coat pigment to each coat, alternating between yellow and blue, so we could see where we'd been. Then sanded, again tough job.

The next year, there were lots of little pimples, where air had gotten in the epoxy and popped off. But none went thru all the layers, so didn't worry about them, except to Dremel and patch 'em. Next year a few more, next year none.

Did this about 10 years ago, and boat is still in good shape.

Our '73 Morgan has a similar look, though not as bad, and planning on doing the same process next year, unless we hear of a better process. We really considered all that West has to say about their water-barrier plate additives, vacuum bagging, and decided it was better on a lab bench than in the real world. Any do-it-yourself coating process is going to incorporate some air. I doubt we will live longer than the half-life of a halfway decent epoxy job.

Not impressed w/ the Interlux 1000/2000; like real West epoxy better.
From Don Desmond on the Morgan mailing list:
My experience with the Interlux products for osmotic blistering has been very positive. As I've previously stated, I did the job once the right way over 15 years ago and have never had a single blister since. Unlike your own experience, it was not necessary in the following couple of years to touch up the previous work. I have no financial or personal interest in Interlux whatsoever, so my opinion is based solely upon the criteria that when I followed the directions given for the products I achieved very good results. I have nothing against West products either, they are a quality manufacturer and I also use their products for my maintenance needs. As a matter of fact, I'm doing a project at this very moment using their epoxy resin.

My point is that in order to get the best results, pick a good manufacturer and then take their advice. When it comes to knowing how best to utilize a given material, who should know better than the people who have created it? They want you to succeed and get good results so that you continue to be a customer and recommend them to others. You would be well advised to use their water-barrier plate additives. In case of point, they are the equivalent of the 2000/2001 and 3000/3001 coatings marketed by Interlux. With either West or Interlux, the epoxy coating is meant to seal the surface and give it strength while the barrier coating is meant to waterproof it. As for vacuum bagging, I concur that it is not practical for the do-it-yourself boater.

From someone on The Live-Aboard List:
I went with the Hot-Vac system. It uses these big heating pads that pull a vacuum on the hull and heat up to about 230 degrees F. They do three sections at a time until it's all dry. If your hull isn't really dry, and they laminate over it, it'll just blister again.

From article in 7/15/2001 issue of Practical Sailor:
  • Acidity of blister fluid is best clue to severity of degradation of the laminate; higher acidity means worse degradation.

  • Passive drying removes moisture only to a certain depth, about 20/1000". Addition drying time beyond a certain limit is ineffective.

  • Drying with heat and vacuum works quicker and reaches further into laminate, but only to about 1/10" deep.

  • Begin repair soon after heat and/or vacuum are removed.

From Pierre Julien on SailNet's Gulfstar mailing list:
... on the hard, I would have scraped the gelcoat off the bottom to let it dry as long as possible before re-coating as this is the part of the process that is of unknown length and the key to a good bottom job, the drying that is. ...
From Joey Sowell on SailNet's Gulfstar mailing list:
Dryness is the key here. If your yard has the capability to monitor the moisture levels in the hull you should be able to get a great result when your hull has dried out as much as possible. When our hull was tested and inspected, we found that there were many small blisters and a few medium to large blisters. Further testing revealed what we suspected, a high moisture content. We demasted her, stripped the gel coat off, tented the whole vessel and fashioned an air trap to enter and leave the tent to cut down on the amount of humidity we would let in the tent upon entry and exit. This included a moisture-proof ground cover to prevent ground moisture from entering the tent as well. Initially, we were emptying the two dehumidifiers twice each day, approximately six gallons of water each day, and we continued to run them until she was completely dry. Even with the precautions against moisture, it took about three months of tented, dehumidified, heat lamped, drying before we obtained acceptable levels to re-apply the gel coat on our 1972 GS43MS. Two years and not a single blister later, we are well pleased that we took the extra time to dry her out as much as possible.
From Pierre Julien on SailNet's Gulfstar mailing list:
[Re: wet cement in GS 50 keel:]
If she has been out of the water for over a week, I would run a moisture meter over the bottom and assess how much drying will be required, not that it is a perfect science. But say the meter shows moderate readings, it will probably dry within weeks or months; if it shows off the scale, it will take longer and it is probably due to something inside the hull containing water such as the cement in the keel or water between the laminates, then it needs replacing. I would not know how to access the cement, mine is a fin, lead keel, I had problems with the core material in some areas below the waterline, the core was removed and replaced, yours [GS 50] is a solid fiberglass hull I believe.

From Susan Meckley on The Live-Aboard List:
A friend ground out the blisters on his boat, and then put a paste of baking soda in the craters. This dried them out extremely quickly. Never heard of it before, but it worked for him.

From Mark Mech on The Live-Aboard List:
I peeled the bottom off my 44-foot trimaran last year. It didn't have blisters, it just had a bad gelcoat job that was coming off like swiss cheese, it had been applied in layers with a roller and apparently not prepped between coats. I only encountered 3 legitimate blisters on the hull and the glass was basically unprotected for the last 6 to 8 years.

If you have blisters that are actually in the glass, you have a poor glass job, cheap resin or both. A good Iso-resin and properly wetted out layup will not blister. The worst blistering cases are usually on production boats that were built with cheap general-purpose Ortho-resin. Humidity can contribute too. If the cloth is too moist when the hull is being built, it won't properly wet out and can leave microscopic voids for moisture to wick into the hull.

I peeled my boat to bare glass with a $20 cheap Chinese 4" grinder from Harbor Freight. I used rather expensive ($6) grinding wheels called "Spiro-cut" from "Fiberlay" in Seattle. They cut like a torch through butter, I only used 4 of the 16 grit disks on the entire hull (all three hulls!). I could hold the grinder flat against the hull and use a fast back and forth scrubbing action and clear a 2'x 2' area in about 2 to 3 minutes. It left a fairly smooth surface that didn't require too much fairing, but this was still a big task.

Since I knew I had a decent glass job, I didn't go with epoxy, it is much harder to work with, more expensive and you are stuck using epoxy from that point on. I applied 2 barrier coats of Iso-resin with a little gelcoat mixed in for color, so I could see where I had been. I then faired with vinylester fairing compound. This is the best of both worlds, it is about the same price as cheap poly compounds, but has better adhesion, remains more flexible and has barrier properties like epoxies.

It goes on better if you thin it with a little white gelcoat. You only thin it a little on the first coat, but progressively thin it more with each successive coat. If you do this properly, you never need to sand much. You apply the first coat very thin to fill the voids and irregularities. The second coat fills any low spots from the first and so on. By the time you get to the 4th coat, you are applying a thin layer that has enough gelcoat in it to also make it fairly hard.

I used drywall taping knives since I am more familiar with them and found them to work quite well.

I applied 2 more coats of Iso-resin when finished, then painted the bottom. I could have done an epoxy coat at this time, but with 1/4" of vinylester compound on the bottom it didn't seem necessary.

I would never barrier-coat over blisters; they need to be ground out. There are also water-displacing epoxies that will fill the divots regardless of moisture content.


The vinylester was applied with a 12" drywall taping knife. I use a 12" and an 8". The 8" is used to pull the material off the mortar board and put it on the hull in fairly even stripes from the keel to where it fairs back into the old fairing.

Then you lose the mortar board and re-trace the steps with the 12" knife. After each pass you clean the blade with the 8" knife and re-apply the material farther down the hull where your first passes have ended. This extends the area considerably. I.E. if you made 2 initial 8" stripes, you will end up with about 6 stripes that are finished as you go over the last one with the 12" knife.

You apply enough twist to the blade as to not leave any ridges behind the pass.

When you get to the end of the hull, you are done with a layer, no sanding if you have the technique down.

If it is a little off, don't sweat it, the next pass will fill the low spots. If you have major lumps, the high spots will need to be sanded.

Get a 50-pound box of drywall mud and practice by using ALL of it in 1/8" layers on a 4' x 8' sheet of plywood.

From blister article by Tom Pawlak in Epoxyworks #17 Spring 2001:
  • Can get Epoxyworks guide 002-650 "Gelcoat Blisters: Diagnosis, Repair and Prevention".

  • A complete barrier coat done by a boatyard may cost up to $200/foot.

  • It may take a long time (months) to dry a hull before a barrier coat can be applied.

  • Fast repair of individual blisters, before bottom-painting:
    1. Use grinder or sander to open blister and remove it, including the edges of the blister dome.
    2. Wipe clean with rubbing alcohol, again and again until laminate is dry to the touch.
    3. Wet out with epoxy.
    4. Before epoxy cures, fill cavity with epoxy thickened with high-density filler, fairing it well (hard to sand).
    5. Let it cure.
    6. Wet sand.
    7. Bottom-paint.

From Jack Kamer on Low-Cost Voyaging mailing list:
I had my boat's bottom sandblasted and it turned into a disaster! Please, try to find a marina or pressure-blasting company that uses some other medium besides sand.

My friend in Ft. Lauderdale had advised me to do so, but no one in the immediate area (western Ohio shore of Lake Erie) had even heard of it so ... I spent two whole weeks under my 33-footer doing a "bottom job". I get tired all over again just thinking about it. In all fairness, I *was* in my early sixties when I did it. :-)

I believe there are at least three other media to use to remove the bottom paint so one can see all the blisters. Baking soda, plastic beads and corn husks. All three will scour the bottom paint off through pressure blasting.

Another thing to consider: If your boat is a 60s to late 70s model, the cost of having the blisters all repaired and a new epoxy bottom job could approach the actual value of the boat. I spent over 2 grand on supplies, materials, equipment rental, etc, and did all the labor myself. If you add in the labor, even just 40 man hours, assuming a younger, much more experienced marina guy could do it twice as quickly as I was able to, you've got at least another $2500, minimum. You may wish to consider grinding the blisters down yourself, allowing the bottom to dry out for a few months, filling with an epoxy filler (which requires days of sanding smooth), then applying five coats of Interprotect 2000E or the VOC compliant and more expensive 3000. Another option is to do nothing at all, after all that bottom is very thick ...

The Boat U.S. 2004 catalog has an excellent treatise on blisters on page 368. I'm pretty sure the West Marine catalog will have much the same thing.

Bottom job veteran, but NEVER again!

From David Doolin on the Morgan mailing list:
> Re: $16K peel-job:
> ... once done right your blister problems go away.

Never say never. I have seen far too many boats that had blisters return after peel jobs. Ask the yard how long they guarantee the new bottom. 4 weeks is no time to dry the boat out. Some boats sit for months and still have too much moisture in the laminate to finish the job but the yard has to turn the spot over so the new barrier coat seals in the moisture to continue its steady silent work.

Three schools of thought. One is keep looking for another boat. Two is to put a lot of money in a boat you won't likely get back. Three is to beat down the asking price - fix the leakers each haulout and don't worry about the others. Funny thing is I owned two 1969 Morgans. One had 1/4" to 2" blisters, up to 10 per square foot, some into the second layer of laminate. The other had no blisters to speak of the 8 years I owned it. I sailed them both hard and sold them the way I bought them.

From Tom Zuzack on the Morgan mailing list:
Re: Peeling to get rid of 500+ blisters:

This is a controversial subject but I will provide my opinions based on experience.

In general, a boat that does not have blisters will sell much easier than one that does regardless of whether the boat without blisters is that way because it never had them or because they were fixed. However, a boat without a fair hull is just as objectionable as one with blisters.

I do not subscribe to the belief that a hull with numerous blisters should be peeled unless you plan to have someone else fix the bottom, money is no object, and the fixer provides a good warranty. Even in the case of hundreds of blisters I believe it is better (i.e. results in a hull that is just as free of blisters and just as fair) to fix the individual blisters. By doing so, you maintain some of the original hull surface upon which to base the fairing of your hull. Also, I know of boats that went through the peel and repair process and ended up with recurring blisters.

[1977 Morgan Out Island 415 Ketch] I wet sand blasted (using a sand blasting wand you attach to a pressure washer) the bottom paint off the entire bottom so I could see the blisters better. This was a hot, dirty, time-consuming job. Then I used a grinder to open the blisters for repair. I ground just enough to remove the top of the blister and any whitish fiberglass surrounding it. This was not difficult but was hot, dirty and required a lot of time and attention to detail. Then I left the hull sit to watch for any seeping. You want the hull to be as dry as possible but if you're like me you can't afford to have the boat sitting in the yard for extensive amounts of time just burning money. Truly drying a hull takes months. I had no seeping so I wiped down the entire bottom with a solvent (I like Interlux products). I then filled each blister attempting to ensure I did not leave a concave surface but not an overly convex one either. Most people like the West System products. I used products from Fiberglass Coatings, a local St. Petersburg company. The product I used was very hard and therefore time-consuming to sand. Sanding was again a hot, dirty, time-consuming job. Once I faired all the blisters I then gave a general sanding to the bottom, a solvent wipe, and bottom paint. I know I did not get all the blisters so when I haul next time I will do more. Once I am sure I have them all I will barrier coat and bottom paint. There is no rush to get them all. They are not a threat to the structural integrity of the hull. Basically I am treating the blisters as a maintenance item. I did the vast majority of them during a single haul out and will do more each haul out until they are all gone. Doing smaller numbers each haul out makes the job manageable.

Obviously I did not go into all the details of the job. It took about three months of part-time work most days of the week. I would have to look through my documentation but I seem to remember a dollar figure of about $2000 not including yard or haul-out fees.

From Stacy Deming:
[Re: Peel-job on a Krogen 42, in Virginia:]
I am not sure how much the peeling cost but I believe that it was between $140 and $160 a foot (for the total job, including painting, yard time, labor, etc). I believe that if a boatyard put their full effort on the job it could be completed in a week or 10 days, maybe less. Moisture was one of the things that delayed my work. There were parts of the hull that never showed that they were dry. Probably dampness in some core material. The hull was structurally ok and we finally decided to go ahead. (The boat was peeled and sat over the winter to dry and it never got to the moisture level they wanted to see.)

I had dime-sized blisters all over below the water line, nothing larger than a quarter or half dollar and none that were deep. Billions but small and not deep; they peeled 1/16 to 1/8 inch and got everything.

There was not much sanding done after the peel, just some smoothing of the edges left by the peeling. On a sailboat I think there might be some filling and fairing to make it smoother. In some cases where you pay a lot more I think they put an additional layer of fiberglass and epoxy on and then sand it well.

Once the sanding was done they put 5 or 6 coats of interlux 2000 epoxy paint on and then 2 coats of trinidad bottom paint.

When I bought the boat the surveyor said that blisters were overrated as a problem. If the hull is not thick and the blisters are deep they should be a concern. If the blisters are not deep it is mostly a cosmetic problem and the boat can be used for years without doing anything. They don't get better and you may get faster buildup of crap on the bottom.

I am glad I did it. I think I will recover all the cost if I sell the boat. Even though blisters may not be a big problem, they are something that will turn most buyers off.