Osmosis is the defect owners fear most and understand least. The fear comes from uncertainty: nobody gives a straight answer to does this finish the boat, what will it cost, how long will it take. This page answers those three, stage by stage, together with the mistakes that are made along the way.
1. How Osmosis Forms
Water passes slowly through the gelcoat. Inside the laminate it meets unreacted residues and forms an acidic fluid denser than the water outside. Osmotic pressure keeps drawing more water in, and when the pressure rises the gelcoat lifts locally, that is a blister.
Three things set the pace: resin type (orthophthalic polyester, pre-1990, is the most vulnerable), water temperature (Mediterranean water accelerates the process compared with northern Europe) and time afloat (a boat that stays in the water year round progresses faster than one lifted each winter).
2. Diagnosis: Three Pieces of Evidence Together
| Evidence | How it is sought | What it says alone |
|---|---|---|
| Blisters | Visual inspection once antifouling is removed; a blister is pierced and the fluid smelled | A vinegar smell is the strongest single indicator. Without blistering, a diagnosis of widespread osmosis is not made. |
| Moisture | Grid scan with a Tramex-type meter, compared against a dry reference area | Not osmosis on its own. Plenty of laminate has taken water without ever blistering. |
| Drying behaviour | Readings repeated days apart | A value that will not fall shows water bound into the laminate, this is the decisive evidence. |
3. The Seven Stages of Repair
| # | Stage | What happens | Critical point |
|---|---|---|---|
| 1 | Survey and mapping | Blister density, moisture distribution and the affected percentage are recorded | Scope is set here. A job that starts without a map is a job that keeps growing. |
| 2 | Gelcoat peeling | Gelcoat and affected outer laminate are removed with a peeling machine | A peeler beats a grinder: thickness comes off under control and the laminate is not weakened. |
| 3 | Washing | Fresh water, with a neutralising wash where required | If acidic residue is left in the pores, the process continues under the new coating. |
| 4 | Drying | Weeks of controlled drying, tracked by meter | This is where the job is decided. Cut the drying short and the repair returns within two seasons. |
| 5 | Filling and fairing | Epoxy filler, fairing, sanding | Polyester filler means closing the job with a material that absorbs water again. |
| 6 | Epoxy barrier coat | Multiple barrier coats to specification | Coat count and overcoating intervals from the data sheet are followed exactly. |
| 7 | Antifouling and launch | Compatible primer and antifouling | An incompatible antifouling can lift the barrier. |
4. Drying: The Heart of the Job
Laminate gives water back slowly. Drying time is set by thickness, ambient temperature, relative humidity and air movement. In practice:
- A small boat with thin laminate, in a warm dry space, can reach target in 4-8 weeks.
- A thick-laminate boat that has been afloat for years may need 3-6 months.
- A boat standing uncovered on a damp coast in winter effectively does not dry, when the air is wetter than the laminate, water does not leave.
Drying is complete when readings approach the dry reference and successive readings stop falling. The decision is made by measurement, not by the calendar. A contractor who says "three weeks is enough" is working to a calendar.
5. What Happens If It Is Done Wrong
| Mistake | Consequence | When it shows |
|---|---|---|
| Drying cut short | Moisture trapped under the barrier; blisters return, this time from beneath it | 1-2 seasons |
| Painted over without peeling | Nothing is solved; the process continues under the gelcoat | 1 season |
| Removed with a grinder | Laminate thickness reduced unevenly; risk of structural weakening | At the next survey, by measurement |
| Polyester filler used | The filler absorbs water; the barrier is defeated | 2-3 seasons |
| Barrier coats short or overcoated too soon | The barrier never reaches its specified thickness or cure | 2-4 seasons |
| Washing skipped | Acidic residue keeps reacting | 1-2 seasons |
| Incompatible antifouling | The barrier lifts, paint will not hold | First season |
6. What Decides the Cost
Osmosis repair is priced per square metre and turns on four variables: hull area, affected percentage, hardstanding cost through the drying period and the barrier system chosen. Yard time is often the larger item, not labour, which is why planning the work across a winter reduces the total.
What to compare between quotations: peeling method (machine or grinder), a stated drying period with verification by measurement, filler and barrier brand with coat count, who pays for the hardstanding, and the warranty. Quotations compared on the headline figure alone may not be describing the same job at all.
7. Should You Buy a Boat With Osmosis?
You can, knowing the terms. The decision rests on three numbers: the market value of the boat without osmosis, the realistic cost of repair (yard time and calendar included) and the reduction the seller will accept. If those three add up, osmosis is a negotiating item. If they do not, the right answer is to walk away.
On the buyer's side our job is to make that arithmetic possible: affected percentage, moisture distribution and drying expectation go into the report as measurements. Because we do not carry out the repair, we have no interest in making the number larger or smaller.
Frequently Asked Questions
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We do not carry out repairs and we sell nothing. That is why we have no interest in making the number larger or smaller.
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