The 15-minute Blister options trading system

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Practical Boat Owner

DIY Osmosis Repair

If you’ve found a few blisters on your gel coat, don’t despair. You may not need to do a full gel coat strip – which on cheaper boats may not be worth it anyway. Jake Kavanagh explores the DIY repair options

Finding that your boat has osmosis is about as much fun as discovering your house has subsidence. While your house may eventually collapse, the good news is that as far as we know, osmosis has yet to sink a yacht. The bad news is that in both cases you’re looking at an expensive repair and devaluation of your asset until it’s done.

But what happens if your boat is not worth very much, or you simply don’t have the funds to fix her? Is it all doom and gloom? In a word, no. But what is this awful affliction, osmosis?

Essentially, the shiny outer layer of your glassfibre boat known as the gel coat isn’t entirely waterproof. Over the years small amounts of water find their way through its ever-so-slightly porous surface and react with uncured chemicals in the glassfibre mat beneath. The reactions create by-products which, usually over a period of several years, create enough pressure to force up the blisters.

That’s when the surveyor sucks in through his teeth, and the value of your boat falls by the equivalent amount of a full repair. As both the materials and labour are expensive, you’re always talking a four-figure sum.

The blisters themselves are a symptom of advanced osmosis, but your hull may still be working up to them if it has a high moisture content. Osmosis is more prevalent in warm climates, and the blisters can reach the size of dinner plates. If you’re buying a boat that has spent a lot of time afloat in the Med, for example, get a surveyor to take a close look at the underwater hull.
Hulls can be protected against osmosis by adding a few layers of epoxy, such as International’s Gelshield 200. But before they can be applied, the hull has to be very dry, or the problem will simply be sealed in. As blistering is a sign of advanced osmosis, applying a barrier coat once they’ve appeared will only make things worse.

The professional cure for osmosis is straightforward, but costly. The first stage is to remove the underwater gel coat completely, along with any delaminated substrate. Then the exposed glassfibre mat has to be dried out until the moisture content is reading as low as five on a Sovereign moisture meter. Once the hull is dry the old gel coat, and often some of the underlying mat, can now be replaced with more robust materials. These gel coat replacements are made from epoxy resin, which form an almost impermeable barrier, and last much longer than the old polyester gel coat. So, although a repair is expensive and your boat will be out of commission for at least six weeks, it should last for another 20 years.

The DIY Osmosis Repair
But what do you do if your boat is only worth a couple of thousand pounds, or if you simply can’t afford to pay for the job to be done?
We asked expert Richard Blake of the Hayling Yacht Company Ltd, one of the busiest osmosis treatment centres in the UK, to take us through the options. It’s a question he’s very familiar with.
‘If someone rings up with a very small boat, we tend to advise them that it may not be worth the full treatment,’ he told us. ‘Spending thousands on a full professional cure won’t add very much to the boat’s value – you’ll just make it a bit easier to sell.

However, a DIY temporary repair is actually possible, if you have somewhere to dry the boat out, and you don’t mind a bit of hard work.’

So, what are your options if you’ve found some blisters in your gel coat?

What boatowners fear to find – typical blistering from advanced osmosis

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Option 1 – Leave it
The gel coat is simply a protective outer layer and has no real strength. It takes a long time for the blisters to grow, and keeping the boat ashore for longer periods can slow their progress down. Thousands of boats are sailing happily throughout the world with high moisture readings, and an underwater hull resembling bubble wrap. Just keep an eye on the situation. If the blisters start to join up, or get too big, then there is a small risk that they could fracture open and allow a lot of water into the hull. If delamination gets too bad then water could find its way inboard, but in 20 years of osmosis treatment, Richard has only seen one case that bad.
‘If you want to sell the boat, be prepared to sell it at a knock-down price. If it’s an almost hopeless case, it may prove cheaper to sell all the fittings and simply chop the hull up.’

Option 2 – Full repair
This will be a labour of love, but can be done by a competent DIY enthusiast.

Bad cases of Osmosis can lead to delamination, which is time consuming and costly to repair

The gel coat needs to be stripped away with a special stripping tool – which resembles an electric plane – and then the exposed substrate is lightly grit blasted. Professionals can be hired in to do each part of the operation for you, with prices starting at around £15-£20 per foot LOA.
The exposed laminate then has to be washed and thoroughly dried, and may need some lamination work before the hull can be faired smooth again. This requires patience, elbow grease, and a good eye for fairing to preserve the yacht’s underwater shape. It’s not really for the faint-hearted or workshy (we’ll be looking at the full process in next month’s PBO).

Option 3 – Partial repair
It is possible to treat the symptoms rather than the cause by popping open the blisters, washing them out, and then filling them with an epoxy filler.
‘Some people are tempted to put on an epoxy barrier treatment anyway, but this is inadvisable,’ Richard explained. ‘All this does is accelerate the blistering.’ Instead, he suggests simply coating the hull with a flexible primer such as Primocon, and then antifouling in the usual way.
‘If the hull remains slightly porous, it will allow the moisture to leach out during time ashore,’ he says.

As treating the symptoms will keep the water (largely) out, and allow you to keep sailing, let’s look in more detail at this third and cheapest short-term remedy:

Cleaning the hull

Scrape off the antifouling over a suspected blister to ensure that they really are in the gel coat, and not just in the paint. Once confirmed, then it’s time to strip off all the antifouling so you can examine the entire underwater hull. This is a good job for the winter lay-up.
We covered the various methods of antifoul stripping in another article, but it’s worth a quick recap here because you can kill two birds with one stone. Some of the stripping methods will actually take out the blisters.

Slurry blasting
You’ll need to hire a professional, who will arrive with a blasting machine and protective sheets so he doesn’t affect neighbouring boats. The slurry is composed of water and grit which is blasted out at high pressure. It’s noisy and messy, but strips away all the layers of antifouling in short order while knocking the tops off any osmotic blisters.
A skilled operator can adjust his jet to accomplish anything from a gentle abrasion of the gel coat, to blasting it completely away.

Dry-ice blasting

Dry ice will freeze the fluid in the blisters and explode them

Fairly new to the marine industry, dry-ice blasting uses pellets of frozen carbon dioxide rather than grit to remove the antifouling. The pellets ‘frack’ on impact (turn straight from solid to gas) which blasts the antifouling away and drops it as a frozen mush.
Although the gel coat isn’t abraded, the moisture in the blisters is frozen and explodes outwards. The operator may need to play the jet on the affected area for some time to pop all of the blisters, especially the tiny ones. The process is noisy, but ‘clean’ – although not so good for climate change.

Hand removal

Using a sharp edged implement such as a ground-down chisel, the antifoul can be scraped off by hand. Tiring and messy, the process is effective so long as you keep the blade sharp. Any exposed blisters can be drilled out with a counter-sinking drill bit, or taken out (carefully) with an angle-grinder or chisel edge. The hull will then need to be sanded to provide a key for treatments.
Hand scraping can be used with antifoul removers, some of which, such as Remove-all, can be washed off with a pressure washer rather than scraping.


With the blisters hunted down and popped open, usually weeping a fluid that will smell of vinegar, the hull will have to be washed and then thoroughly dried out. Washing is easy enough, but drying could be a challenge.

An initial wash with soap and water will remove most of the impurities, but Richard advises using a steam cleaner to purge any remnants of styrene.

Once the hull appears to have dried off from its wash, the use of a moisture meter will determine how ‘wet’ it still is. If you want to apply an epoxy barrier, such as Gelshield 200, then you’re looking for a reading of less than 10, and preferably around five before you can start the repairs. Anything above this will require some vigorous drying, and it’s very difficult to properly dry out a substrate through the gel coat.

If you can get the hull at least a bit dryer, then the problem will be slowed (the only way to get it dry enough for a Gelshield treatment will probably be a full gel-coat peel and force-drying of the exposed laminate).

A skirt around the waterline will keep rainwater away

Cover the boat over, or place an apron around the waterline so dry air can get to it, but the rain can’t. If you have time on your hands (such as with a major refit) then you can simply leave the hull to dry at its own pace. However, if time is short, you may want to speed things up by using infrared lamps or a dehumidifier.


Lamps are placed around the boat (right) and moved to the worse affected areas. Readings are taken at regular intervals (every two days) to see how the hull is drying. Be careful not to let the hull get too hot or it will start to distort.

Some manufacturers of epoxies suggest that a dehumidifier running in a completely sealed tent under the boat will draw out the moisture, but others disagree. Either way, this is
best done in the dry months of the summer, or better still, under cover in a boatshed.

Step 4
Filling and fairing

Step 1 Mix up the filler in the right ratio (International’s Watertite is mixed at a ratio of 1:1). Only mix what you can use in the pot life of the mix (about 20 minutes).

Step 2 Work it into the craters, aiming to smooth it as flat as possible with the scraper. This will help to minimise the amount of sanding required later.

Final steps: with the filler hardened off, all you have to do now is sand it flat, and then roll on a coat of Primocon before antifouling in the usual way

With the hull washed and dried as far as possible, it should be abraded, to knock down any more blisters and give a smooth profile (slurry blasting may already have done this, but a further rub won’t hurt). Give a final wash with soap and water or another steam clean, to remove any dust that may be clogging the exposed blister craters.

Hayling Yacht Company Ltd
■ Hayling Yacht Company are one of the busiest osmosis repair centres in the UK, offering a turnaround of about six weeks (depending on the hull state) with discounts for work done in the summer. Free winter storage is available for winter treatments. The company uses the International Gelshield process with hot-vac hull drying.
The price for a complete job, involving a hull peel, drying and full epoxy treatment on a typical 32-footer was around £6,000 when this article was originally published in 2005, including VAT, free storage, crane in and out, and all materials. For more information, tel: 02392 463592.

Tips to Prevent and Treat Blisters for Hikers, Bikers, and Runners

Anyone who exercises frequently knows foot blisters are an uncomfortable and unfortunate part of being active. If you bike, run, or even hike, you know a blister can literally stop you in your tracks. But thankfully, a blister doesn’t mean the end of your fun. Thinking ahead can help prevent blisters and it’s important to know how to treat them. Start with these tips.

You have a blister, now what?

Stop and adjust

When you get a blister it’s important to stop whatever activity you’re participating in and decide how to treat it. Powering through the pain can make your blister pop or even become infected. Readjusting your footwear can help prevent your blister from becoming larger. Straighten out bunched socks. Change your socks for a dry pair if they’re sweaty or wet, and change your shoes if they’re causing you discomfort.

Use padding

Blister pads, bandages, or moleskin are all great options for preventing blisters. Padding can also protect existing blisters. Keep in mind that not all pads stay in place effectively. You may need to try several options before finding one that works for you.

To pop or not to pop?

The best scenario for treating a blister is to keep it intact. Popping can increase the opportunity for infections to form. Most blisters will heal themselves if you give them a few days. If you have a large blister that’s affecting your walking, it might be better to pop it. Follow these steps to safely pop a blister:

  • Check for signs of infection (pus that’s green or yellow in color and swelling). If the blister is infected you should contact your doctor.
  • Wash your hands with soap and water, then clean the blister with soap and water or rubbing alcohol.
  • Sterilize a needle with rubbing alcohol.
  • Locate the blister’s edge and poke it with the needle in several places. Use clean gauze to soak up the fluid that comes out.
  • Apply antibiotic ointment over the blister and cover with gauze and tape or a bandage.
  • After several days you can cut away the dead skin and apply more ointment, then bandage again until healed.
  • Throughout this process, keep your blister clean. This will prevent infection.

Tips for preventing blisters

If you’re active and tend to get blisters often, here are tips to help prevent new blisters from forming.

  • Wear better shoes.Shoes are often the culprit when it comes to blisters forming on your feet. Avoid shoes that rub certain areas of your feet or that cramp or squeeze your feet. The right shoe can make all the difference. New shoes may give you a blister the first few times you wear them. Take it slow and easy as you break in a new pair of shoes.
  • Wear better socks. Choose non-cotton socks that wick away moisture. If you know you’ll be doing blister-inducing activities, double up your socks. One layer will soak up moisture and the other layer will give extra padding. Double-layer socks are more expensive but may help you avoid blisters. If you’re in for a long run or hike, change your socks partially through, or whenever they become moist. Good socks can be expensive, but they’re worth the cost if they keeps you from getting blisters.
  • Lubricate your feet before you exercise. Friction causes blisters, so it stands to reason that reducing friction can help reduce blisters. Rub petroleum jelly or other lubricants designed for runners on problem spots on your feet. That way your feet will slide around rather than rub.
  • Keep the calluses. It’s tempting to shave off or pumice down unsightly calluses, but they help protect your feet.
  • Keep your feet dry. Other than changing your socks regularly, you can add corn starch or talcum powder to your shoes and socks to help wick up moisture. If you’re going a long distance, take a minute to add more powder partially through your event.
  • Cover areas that are prone to blister. Just like you’d cover up a blister after it formed, you’ll want to pad areas that are prone to blistering. The extra layer of protection helps to prevent a blister from forming.


Blisters should heal on their own within a week. They can be painful while they heal, but you shouldn’t need to see a GP.

How you can treat a blister yourself

To relieve any pain, use an ice pack (or a bag of frozen vegetables wrapped in a towel) on the blister for up to 30 minutes.

To protect the blister and help prevent infection:

cover blisters that are likely to burst with a soft plaster or dressing

wash your hands before touching a burst blister

allow the fluid in a burst blister to drain before covering it with a plaster or dressing

do not burst a blister yourself

do not peel the skin off a burst blister

do not pick at the edges of the remaining skin

do not wear the shoes or use the equipment that caused your blister until it heals

A pharmacist can help with blisters

To protect your blister from becoming infected, a pharmacist can recommend a plaster or dressing to cover it while it heals.

A hydrocolloid dressing can help reduce pain and speed up healing.

Check if you have a blister





Don’t ignore an infected blister. Without treatment it could lead to a skin or blood infection.

Non-urgent advice: See a GP if:

  • a blister is very painful or keeps coming back
  • the skin looks infected – it’s red, hot and the blister is filled with green or yellow pus
  • a blister is in an unusual place – such as your eyelids, mouth or genitals
  • several blisters have appeared for no reason
  • a blister was caused by a burn or scald, sunburn, or an allergic reaction

Treatment from a GP

Your GP might burst a large or painful blister using a sterilised needle. If your blister is infected, they may prescribe antibiotics.

They can also offer treatment and advice if blisters are caused by a medical condition.

How to prevent blisters

Blisters develop to protect damaged skin and help it heal. They are mostly caused by friction, burns and skin reactions, such as an allergic reaction.

Blood blisters appear when blood vessels in the skin have also been damaged. They are often more painful than a regular blister.

If you regularly get friction blisters:

  • wear comfortable, well-fitting shoes
  • gradually break in new shoes
  • wear thicker wool socks during exercise
  • dust talcum powder in your socks if you get sweaty feet
  • wear protective gloves when you exercise or if you use tools at work

Conditions that can cause blisters

  • Chickenpox – a childhood illness that causes itchy red spots
  • Cold sores – small blisters that develop on the lips or around the mouth, caused by a virus
  • Genital herpes – a sexually transmitted infection (STI) that most commonly affects the groin
  • Bullous impetigo – a contagious bacterial skin infection
  • Pompholyx – a type of eczema
  • Scabies – a skin condition caused by tiny mites
  • Hand, foot and mouth disease – a viral infection that usually affects young children

Page last reviewed: 12 December 2020
Next review due: 12 December 2020

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