Osmosis is the word that empties the colour from a hull surveyor's face, and the one that makes buyers walk away from an otherwise sound boat. It is also widely misunderstood, over-diagnosed by the nervous and ignored too long by the optimistic. If you keep or buy a fibreglass boat in France, you will eventually have the conversation, and it pays to know what the treatment really involves and what it costs here before a yard quotes you a frightening number.
What osmosis actually is
Osmosis is water finding its way through the gelcoat into the laminate of a fibreglass hull, where it reacts with soluble compounds left over from the build and forms an acidic fluid. The pressure of that fluid pushes the gelcoat outward into blisters. Pop one and a sharp, vinegary smell and a sticky liquid confirm it. A few small blisters on an older hull are common and rarely urgent. Widespread blistering across the underwater area, weeping when pressed, is the case that needs real treatment.
The key thing to grasp is that osmosis is slow. A hull does not fail overnight. This buys you time to plan the treatment properly rather than panic into the first quote, and time matters here because the cure itself is measured in months, not days.
Spotting it at the lift
You can only assess osmosis with the boat out of the water and the antifoul off, so the lift is the moment. While the hull is up for its annual paint, run your hand and eye over the gelcoat below the waterline looking for the raised, dome-shaped blisters that distinguish osmosis from simple paint flaking. A damp meter reading taken by a surveyor confirms how wet the laminate is. The lift is also when you fix everything else below the waterline, so combine the inspection with the antifouling in a French yard and its rules and costs rather than paying for a second hoist.
If you are looking at osmosis on a boat you are thinking of buying, treat it as a price negotiation, not a dealbreaker, and read the wider checklist in 10 hull inspection tips when buying a used sailboat. A boat with treatable osmosis at the right price can be a good buy. A boat at full price with hidden osmosis is the trap.
The prevention worth doing first
Before you talk treatment, it is worth knowing that a sound hull can be protected against osmosis ever taking hold, and on a newer or recently dried boat that is the cheaper path. A preventative epoxy barrier coat, applied over a clean dry hull, seals the gelcoat against water ingress and is a fraction of the cost of a cure. On a YBW owner's costing the preventative route on a mid-size hull came in well under the four-figure curative job, because there is no peeling and no long drying stage, just cleaning, a few coats of epoxy primer and antifoul. If you buy a boat with a clean hull and no blistering, a barrier coat at your first antifoul lift is money well spent, and it folds neatly into the work you are doing anyway.
The treatment, stage by stage
A proper osmosis cure is one of the bigger jobs you can throw at a hull, and it runs in stages:
- the gelcoat is peeled back to expose the laminate, using a hull peeler, sandblasting or hydrogumming
- the bare laminate is left to dry, the slowest part of the whole job
- the dried hull is recoated with a solvent-free epoxy system to seal it
- fairing, priming and a fresh antifoul finish the job
The drying is the part foreigners underestimate. The laminate must dry out before it is sealed, and that takes around three months in summer and three to six months in winter. The active curative work is perhaps three weeks once the hull is dry, but the boat is ashore for the whole drying period, so the storage clock keeps running. That alone shapes when you start: peel in autumn, dry over winter, seal and launch in spring is the natural rhythm in France, and it lines up with a winter ashore.
What it costs in France
The headline figure most French yards quote works out at around 400 euros per square metre of hull surface treated. For a full professional treatment, expect to pay between roughly 1,500 and 4,000 euros depending on the yard and the extent of the damage, including labour, materials and usually a guarantee on the work. For a large boat over 15 metres the figure climbs easily past 6,000 to 8,000 euros.
On top of the treatment itself, remember the costs that run alongside it. The boat sits ashore for the full drying period, so the storage at around 3.70 euros per square metre per month on open-air hardstanding adds up over a winter, and you pay for the lift in and the relaunch out. The whole anatomy of a winter ashore is in the guide to wintering ashore in France and its yard costs, and it is worth budgeting the storage as part of the osmosis job rather than a separate line.
DIY or hand it over
Osmosis treatment is at the limit of what a competent owner can do themselves, and most of the cost is labour, so doing it yourself saves real money. Peeling, drying and epoxy coating are within reach if you have the time, a yard that allows the work, and the patience to let the hull dry properly. The risk is the same as the reward: rush the drying, seal in moisture, and you have spent the money for nothing.
If you hand it over, the yard's reputation matters more than its price. A botched osmosis treatment is worse than none, because sealing a wet hull traps the problem and a future buyer will see fresh epoxy as a red flag. Choose a yard with a track record on osmosis specifically, not just a general chantier, and the principles of vetting one are in the guide to finding a good boatyard in France. Ask to see hulls they have treated and get the guarantee in writing.
Timing it around a French winter
Osmosis treatment forces a rhythm on your year that suits a winter ashore. Because the laminate needs three months to dry in summer and up to six over a French winter, you cannot peel and seal in the same haul-out. The natural sequence is to lift and peel in early autumn, leave the hull to dry on the hardstanding through the cold months, then fair, seal and antifoul in spring before you launch for the season. That ties the job to the winter ashore you might already be planning, and it means the storage cost runs the whole time rather than for a fortnight. Booking that long slot early matters even more than for a routine lift, because a treatment hull occupies hard-standing for half a year and the good yards fill those slots through the summer, the same way they do for the antifoul lift covered in booking a lift-out and hard-standing in France.
Is it worth treating at all?
Not every blistered hull needs a full peel. A small number of isolated blisters on an older boat you intend to keep can be ground out, dried, filled and faired locally for a fraction of the cost. The full treatment is for hulls with widespread, weeping blistering or for a boat you want to sell with a clean bill of health. The honest yard will tell you which you have, and a good surveyor will distinguish cosmetic blistering from the structural kind before you commit four figures.
Weigh the cost against the boat's value too. Spending 4,000 euros on osmosis treatment for a boat worth 15,000 makes little sense unless you love it. On a boat worth 60,000 it is straightforward maintenance that protects the value. As with most things below the waterline, the lift is when you find out, the winter is when you fix it, and France is no more expensive than anywhere else for the work, provided you let the hull dry on its own schedule rather than yours.
