Osmosis is often seen as the scourge of the fibreglass boat owner, an evil to be feared as much as terodo and gribble were to wooden boat owners, and which can wipe a serious amount off resale values. Having spent more time than is healthy studying the subject, I’d like to share a few points that come from  discussions with surveyors and from the literature on the subject.

The first thing to appreciate is that every fibreglass boat will absorb some moisture unless it is kept ashore  which, although an option for some, rather defeats the purpose for most of us. The  amount of moisture absorbed mostly depends on how well the hull was made. What I’d like to cover here is the extent to which it might be a problem and what can be done about it.


In this context osmosis is the process by which water molecules move through the gel coat and, over time, combine with fibreglass chemicals within existing voids or air bubbles to form pressurised blisters. Build quality is everything; poorly made hulls will have numerous voids in the laminate or bubbles in the gel coat which allow the osmosis to begin. The blisters will eventually burst, exuding an acidic fluid or solute and exposing the underlying laminate. The process does not cause the hull to  absorb any significant quantity of water or go ‘spongy’ and it can take years, even up to the useful life of the boat, to advance to a stage at which the hull is significantly weakened.  In short, for most boats osmosis is a cosmetic problem that makes them difficult to sell rather than a danger to sail.

Before you cast a suspicious eye over your boat, it’s worth considering when and where she was built. Some early fibreglass boats were moulded to far greater thicknesses than those used today. Because designers and builders didn’t understand the material, they tended to play safe and laminate hulls to at least 10mm thickness. They also used far higher proportions of resin to glass mat, as much as 50-50 compared to the 30-70 used nowadays. These early boats are the proverbial brick outhouses and will probably be excavated and studied by archaeologists one day. For examples, look at early Westerlys or Drascombes. They might absorb some moisture, but given their age, construction and likely resale value, it really isn’t necessary to do more than grind out and filling the odd blister as and when it appears.

Builders Cutting Costs

As oil prices increased, builders looked for ways to decrease costs by making greater use of balsa or foam-cored sandwich construction in order to get the same strength for less material. Internal mouldings became an essential part of the structure, compared to earlier hulls which were effectively monocoque shells.  The trouble with this was hulls were still mostly laid up by hand, sometimes by low paid laminators in poorly controlled conditions. This was rarely an issue when massive amounts of material were used but became increasingly relevant once strength margins were reduced and the integrity of the fibreglass laminate became critical. This led to many of the problems with GRP hulls, often lumped together under the heading of ‘osmosis’.

From the mid 90’s the boatbuilding industry went through further upheavals which saw the demise of many small volume boat builders and their replacement by production line manufacturers employing vacuum bagging or resin transfer moulding techniques to reduce the likelihood of voids in the structure.  Resins also improved with the introduction of more  impervious isophthalic instead of orthophalic types from around 1990.  This isn’t to say everything from between 1980 and 1990 is potentially suspect,  there are plenty of good and bad examples either way. But a boat from around this period is more likely to have problems.

What should you do about it?

So how do you know you’ve got a problem and what, if anything, should you do about it? The earliest indications of potential problems can only be checked with a moisture meter. Surveyors will have a dual reading type or two instruments to check shallow and deep seated moisture levels which allows them to determine the extent of the problem. To use the meter effectively, ideally the boat needs to have been out of the water for several days. Lunch time haul out surveys are common for yachts sold through the marina brokerages but they have limitations for older boats.

It is only at these very early, non blistering stages that it is worth allowing a boat to dry  and then coating it with an epoxy resin such as Blakes Gelprotect or International Gelshield. You can tell if this has been done already by the presence of a series of thin grey and green or maroon and cream coating between the antifouling and the gelcoat. A recent article in a national yachting magazine suggests it is possible to use epoxy coatings over hulls that are already showing signs of blistering, after first grinding off and filling the blisters. This simply is not true,  the blisters will reappear because the solutes that created the problems in the first place will still be within the laminate and the epoxy can not completely prevent the ingress of further moisture.

More advanced stages will show as blisters varying from 2-3 mm up to about 25mm diameter which will readily show up if the antifouling is removed with a sharp scraper. They will usually be filled with an acidic, vinegary smelling fluid which is under pressure. If there are few of these and they are at the lower end of the size range, there is rarely any point in paying for expensive gelcoat stripping treatments  grinding out the worst ones, checking that the laminate within has remained sound, and filling with epoxy is fine and will keep the boat in good order for years. A surveyor will also thoroughly go over the hull with a light hammer to check for voids or delamination, tapping to detect any variations from a clear resonant sound to a duller, more deadened noise. This isn’t difficult – the difference is very noticeable and it is a check you can readily do for yourself.

The point at which it is worth paying to get the hull stripped back to the laminate is subject to much debate. Everything that I’ve heard suggests that it’s best to leave this to as late as possible and not to bother at all with older, heavily laid up hulls. Bear in  mind the cost of treatment is about £10k for a 35ft / 10m yacht which for many vessels may not be recouped through higher resale values.

If having reviewed all the evidence, you do decide that stripping and replacing the gel coat is worthwhile, it’s important to have the work done properly,  there are examples of boats having their original gel coat stripped, dried for a few weeks in front of heat lamps and then recoated with a far thinner epoxy resin system which isn’t even as good as the original gel coat. This kind of fix is useless. A proper job involves removing the gelcoat with a specialised planer, then lightly slurry blasting the underlying laminate and washing it thoroughly to remove solutes, and then either allowing the hull to dry under cover for several weeks or applying what’s known as the ‘hotvac’  system, which is a kind of suction mat with electric blanket elements and which both heats and draws out any residual moisture. After this, the hull is over laminated with epoxy resin and mat and the gelcoat replaced with epoxy fillers and numerous top coats. On an old, otherwise sound high quality boat, this will give all the benefits of a brand new hull using the latest resin technology and can be a good investment.

So to summarise, most boats will survive their useful working lives without any need for osmosis treatments. Preventative epoxy coatings are only worth doing if there are very few or no signs of blistering and if the hull laminates are completely dried. If you do have an otherwise sound and good quality boat with extensive blistering, then it may be worth having the gel coat stripped and replaced, but only get it done by a well established and professional company.

This is a very brief overview of the subject – there’s an excellent and free pamphlet at Nigel Clegg’s website, or for further more detailed reading see the book ‘Fibreglass Boats’ by Hugo de Plessis.

Aidan Tuckett, Marine Surveyor
17 November 2008

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