Short answer
Osmosis is water penetrating the gelcoat and forming blisters on the underwater hull of a GRP boat. A few small blisters are often no immediate danger. Many blisters, blisters in the laminate or high moisture readings call for full treatment: the gelcoat is removed, the hull is dried for weeks and protected with epoxy. This treatment is usually done by a boatyard.
How osmosis develops
Hempel explains it like this: gelcoat lets a little water through. Most of it passes, but some breaks down substances in the laminate. This forms a liquid that can't get back out. The pressure builds and the gelcoat blisters. If you pierce a blister, the liquid smells sour, like vinegar.
Recognise it
| Sign | What it suggests |
|---|---|
| Round blisters below the waterline, from pinhead size to a few cm | Osmosis in the gelcoat |
| Liquid that smells of vinegar when you pierce a blister | Osmotic fluid, not just paint blisters |
| Blisters only in the antifouling, dry inside | The paint layer is lifting, not osmosis |
| High moisture readings compared with the hull above the waterline | Moisture in the laminate, even without blisters |
| Large blisters, soft laminate, delamination | Advanced osmosis, a boatyard job |
Hempel points out that a moisture meter only gives an indication. Measure several times at the same spots and compare with the hull above the waterline. Measure after the boat has been out of the water and drying for a while. A boat that has just been lifted always gives high readings.
Three options
| Situation | Action |
|---|---|
| Few, small blisters and moisture readings that drop over winter | Wait and check again every autumn. Paint as usual. |
| Occasional larger blisters | Open them, wash out and let dry. Fill with epoxy filler and cover with epoxy primer. See repairing a hole in a GRP boat for the method. |
| Many blisters or high moisture readings across the whole bottom | Full osmosis treatment, usually at a boatyard |
Full treatment
- Remove the gelcoat. According to Hempel, a gelcoat peeler gives the most even result. It requires a professional. The surface is then sanded with 40 grit. Blasting also works.
- Wash. Salts keep migrating out of the laminate. Hempel recommends washing the hull once a day during drying.
- Dry. Practical Boat Owner describes drying until the moisture meter (Sovereign) reads around 5, with the boat standing for at least six weeks. Cover, heat and dehumidification speed things up.
- Protect with epoxy. International requires the hull to be judged dry before Gelshield Plus is applied. It is mixed 2:1 by volume and each coat should be at least 150 microns wet.
- Fill and sand and finish with primer and antifouling.
According to Practical Boat Owner, a well-done treatment lasts around 20 years.
Common mistakes
- Applying epoxy or an osmosis barrier coat to a wet hull. The moisture gets trapped and the blisters come back.
- Assessing moisture on a boat that has just been hauled out.
- Dry sanding old antifouling to see the gelcoat. The dust is toxic, see removing antifouling.
- Paying for full treatment on a boat worth less than the job.
Is it worth it?
Full osmosis treatment means the boat is laid up for at least six weeks, and the work takes a lot of time. On an older, cheap boat it can cost more than the boat is worth. Then it's often better to monitor the blisters and repair individual ones. Before buying: ask for a moisture reading at the pre-purchase survey. Compare quotes from boatyards in your area via Marinas & boatyards.
Frequently asked questions
Can I sail with osmosis?
Yes, small blisters rarely affect safety. Check them every season. Large blisters and soft laminate should be looked at by a professional.
Does applying an osmosis barrier as a preventive measure help?
On a dry hull without blisters, an epoxy barrier can reduce water uptake. On a wet hull it makes the problem worse.
What does a moisture meter show?
A relative figure, not percent water. Compare the same boat above and below the waterline and follow how the readings change over time.
Sources
- Hempel Yacht – How to paint your boat (slippapper, temperatur, osmos) media1.svb-marine.es
- DIY osmosis repair, Practical Boat Owner pbo.co.uk
- Gelshield Plus, produktdatablad, International boatpaint.co.uk
- Fibreglass Boat Repair & Maintenance (manual 002-550), West System eu.westsystem.com