No word in yacht broking causes more panic, or more bad decisions, than osmosis. Plenty of sound boats get rejected over a handful of harmless blisters, and plenty of genuinely wet hulls change hands because someone waved a meter about and pronounced it fine.
What osmosis actually is
Polyester laminate is not fully waterproof. Over years afloat, water permeates the gelcoat by diffusion and reaches small pockets of water-soluble material inside the laminate — unreacted resin components, binder from the glass matting, contaminants trapped during layup.
Those pockets form a solution. Because that solution is more concentrated than the water outside, osmotic pressure draws in more water through the gelcoat, which acts as a semi-permeable membrane. Pressure builds. Eventually the gelcoat lifts locally and you get a blister containing acidic fluid with a characteristic vinegary smell.
Two consequences follow that are worth separating. The blisters themselves are a cosmetic and gelcoat problem. The real concern, in advanced cases, is hydrolysis — chemical breakdown of the resin and of the resin-to-glass bond within the laminate, which does affect structure. Most boats described as having osmosis are nowhere near that point.
Osmosis is far more common in hulls built roughly between the mid-1960s and the mid-1980s, before isophthalic and vinylester barrier practice became normal. It is not a sign of neglect and it is not contagious.
Why a moisture reading on its own means little
Ask any surveyor what number means trouble and you should get a careful answer, because moisture meters do not measure moisture. They measure electrical impedance or capacitance, and that varies with a great deal besides water content.
Things that shift readings independently of any osmosis problem:
- How long she has been ashore. A hull lifted yesterday reads high everywhere. The same hull after six weeks in a breeze reads far lower. Comparing two boats hauled at different times tells you almost nothing.
- Antifoul type and thickness. Copper-bearing coatings will happily produce alarming readings on a bone-dry laminate.
- Air temperature and humidity on the day, and the meter's calibration.
- Laminate thickness and layup, which differ across one hull, let alone between models.
- Whether the surface is clean and dry when the pad is applied.
What is genuinely diagnostic is the pattern. Readings taken in a grid across the whole hull, with elevated zones mapped, is meaningful: uniformly moderate readings on a recently lifted hull are unremarkable, whereas a distinct band of high readings along the turn of the bilge with normal readings elsewhere is a finding. Readings that fall steadily over weeks ashore indicate free water leaving. Readings that stay stubbornly high after a long dry-out suggest something more established.
This is why a hull inspection is worth more than a number, and why any surveyor quoting a single "moisture percentage" for a whole boat should be treated with caution.
Blisters: when they matter and when they do not
Not every blister is osmosis, and not all osmosis needs treating.
Often not a structural concern:
- Small blisters within the antifoul or primer layers only, not in the gelcoat.
- A scattering of tiny gelcoat blisters, dry when opened, on an older boat that has been afloat for decades and shows no other symptoms.
- Isolated blisters around a specific repair or fitting, where the cause is local rather than general.
Worth taking seriously:
- Widespread blistering over large areas of the underwater body.
- Blisters that weep acidic, vinegary fluid when opened.
- Large or deep blisters, particularly any reaching below the gelcoat into the laminate.
- Blistering combined with sustained high moisture readings after a genuine dry-out.
- Any sign of a soft, fibrous or resin-starved laminate under a blister — that is hydrolysis and the picture changes.
- Blistering with delamination on sounding.
The honest answer on many older boats is: monitor it. A hull with modest blistering, stable readings and no laminate softening can be antifouled and sailed for many more years. Grinding a sound boat back to laminate because of cosmetic blistering is an expensive mistake.
Judging severity
A useful rough framework, though every hull needs judging individually:
| Picture | What it usually means | Typical action |
|---|---|---|
| No blistering, moderate readings shortly after lifting | Normal for an older hull | Antifoul and monitor |
| Scattered small dry blisters, sound laminate | Cosmetic | Local repair or leave; monitor |
| Moderate blistering, weeping fluid, sound laminate | Active but not structural | Consider dry-out and coating; get quotes |
| Widespread blistering, high readings persisting after long dry-out | Established | Peel, dry, re-laminate as needed, barrier coat |
| Soft or fibrous laminate, delamination on sounding | Hydrolysis — structural | Specialist assessment before any purchase |
What treatment costs
Full osmosis treatment means grinding or peeling the gelcoat off the underwater body, washing the laminate, drying it under cover for weeks or months until readings stabilise, making good any degraded laminate, then building a new epoxy barrier coating system.
It is slow, weather-dependent and labour-intensive, and it is priced accordingly. As an indicative UK range in 2026:
| Work | Indicative UK cost |
|---|---|
| Local blister repairs and epoxy barrier coat, small yacht | £1,000–£2,500 |
| Full peel, dry and epoxy scheme, 28–32 ft | £7,000–£12,000 |
| Full peel, dry and epoxy scheme, 36–42 ft | £11,000–£20,000 |
| Additional laminate repair where hydrolysis found | Highly variable, often several thousand |
| Storage ashore under cover during dry-out | £100–£400 per month |
Two points that reliably catch people out. First, the storage bill is part of the job — a hull may need three to six months ashore to dry properly, and on some boats considerably longer. Second, a rushed dry-out is worse than none: coat over a wet laminate and the blisters return, with the barrier coat now trapping the moisture in.
On a boat worth £25,000, a £14,000 treatment plus storage plainly does not add up. That arithmetic, not the presence of blisters, is usually what should drive the decision.
If you are buying a boat with osmosis
- Establish where on the spectrum it sits. Cosmetic blistering and hydrolysis are entirely different purchases at the same asking price.
- Ask how long she has been ashore before reading anything into the meter figures.
- Get the hull sounded as well as metered. Delamination matters more than any reading.
- Ask about previous treatment. A boat treated fifteen years ago that is blistering again needs a careful look at how the first job was done — and whether it was dried properly.
- Get real quotes, not estimates, from yards that do this work regularly, before you fix a price with the vendor.
- Factor in a season out of the water. The cost is not only money.
Plenty of good boats have some osmosis. Knowing precisely which kind you are buying is the whole point of the exercise.
Can it be prevented?
On a new or dry hull, an epoxy barrier coat system properly applied to a clean, dry substrate substantially slows water ingress, and most quality builders have used isophthalic or vinylester skin coats for decades now — which is why boats built since the early 1990s suffer far less.
On an existing boat, the sensible measures are modest but real: winter ashore rather than afloat where practical, so the hull gets a drying period each year; keep the antifoul system sound so water is not sitting against bare gelcoat; and fix leaks and damaged gelcoat promptly rather than antifouling over them.
What does not work is a barrier coat applied over a wet hull as a shortcut. It is the most common way of turning a manageable situation into an expensive one.
Concerned about a hull you are buying?
A hull inspection with full moisture mapping will tell you what you are dealing with before you commit. Send the vessel's details for a guide price.
Related reading
- Pre-purchase survey: what to expect on the day
- GRP, wood, steel or aluminium: how each hull fails
- How much does a boat survey cost in the UK?
- Frequently asked questions
This article is general guidance, not advice on a specific vessel, and does not replace a survey. Cost figures are indicative UK market ranges as at August 2026 and are not quotations.