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

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.

Washing

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.

Drying

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.

Infrared

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.

Dehumidifer

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.