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Osmosis (GRP yacht hull)
Defined term — osmosis on polyester GRP yacht hulls: gelcoat blisters, moisture migration, survey language and repair scope for used-boat buyers.
Definition
Osmosis on a GRP yacht hull is moisture migration into the polyester laminate, forming osmotic fluid pockets that lift the gelcoat into blisters. Surveyors classify findings as cosmetic, monitor, or treat within a season — not every blister is a deal-breaker.
How it develops
Early GRP production often used resin systems without full osmosis resistance or skipped epoxy barrier coats. Water enters over years; winter storage slows but does not reverse the process.
Survey language
Moisture meters on cleaned, dry hull sections produce a moisture map. Core samples or ultrasound may follow clustered readings. Ask whether measurements were taken after haul-out and wash-down.
Repair scope
| Approach | When |
|---|---|
| Local treatment | Isolated blisters — grind, dry, epoxy barrier |
| Full hull treatment | Widespread active moisture — strip below waterline, tent dry, recoat |
Request yard references and warranty terms on claimed prior jobs.
Buying implications
Documented recent treatment with stable readings can beat unknown history with fair cosmetics. Negotiate on remaining scope, not blister count in photos alone.
FAQ
Q: Should I walk away from any osmosis mention?
A: No — read the survey tier and quote treatment if recommended within your ownership horizon.
Q: Do topical epoxy paints fix osmosis?
A: They manage symptoms; lasting repair addresses moisture in the laminate with proper drying and barriers.
Related
- Osmosis and blistering guide — long-form article
- Fiberglass osmosis repair — repair scope
- Survey checklist — first viewing
- Najad 390 osmosis report — case study