- Osmosis shows as blisters under the gelcoat on the underwater hull.
- A moisture meter reading alone is not a diagnosis; an experienced surveyor interprets it.
- Many minor cases are cosmetic and can be monitored.
- Prevention is mainly an epoxy barrier coat and a hull that can dry out from time to time.
What is osmosis?
Water slowly passes through gelcoat into the laminate underneath. There it can react with soluble materials in the resin and form pockets of liquid that push the gelcoat out as blisters. Warm water speeds the process up, which is why boats kept afloat all year in warm seas are more exposed.
Signs to look for
- Blisters of various sizes on the underwater hull, visible after the boat is washed.
- Blisters that release an acidic-smelling liquid when opened.
- In advanced cases, larger blisters within the laminate rather than just the gelcoat.
Not every blister is osmosis. Paint blisters in the antifouling or primer layers are common and much less serious.
How a survey works
The boat is hauled out and allowed to drain. A surveyor inspects the hull, opens sample blisters and takes moisture readings at several points, comparing them with areas above the waterline. A single reading out of context means little, so ask for the full results and photos.
If you are buying, make the survey a condition of the sale and ask for readings to be taken after the boat has been out of the water for a reasonable time.
Treatment options
- Monitor: small, stable gelcoat blisters on an older hull may be left and checked each year.
- Local repair: open, dry, fill and seal isolated blisters.
- Full treatment: remove the gelcoat below the waterline, dry the hull thoroughly (which can take weeks or months), then rebuild with epoxy and barrier coats.
Prevention
An epoxy barrier coat under the antifouling greatly reduces water absorption. Periods ashore let the hull dry. Careful sanding during antifouling work avoids damaging the gelcoat.
Krautic coordinates hull inspections, osmosis repairs and barrier coats with vetted boatyards and painters, with written quotes before any work starts.