Osmosis Scare in the Med
October haul-out in any Mediterranean yard, and you'll watch a procession of dismayed boat owners staring at pinhead-sized white spots that have appeared on the hull below the antifouling. The yard manager calls it osmosis. The marketing brochures call it serious. Sometimes it is. Often it isn't.
What you're actually looking at
Small surface blisters that pop with a fingernail and contain only water (not vinegar-smelling fluid) are typically hydration of the gelcoat — not true osmosis. They're cosmetic. The hull is fine. You can wait years before doing anything, and many boats live their whole lives with these never progressing.
True osmosis is different: blisters that grow over time, contain a vinegar-smelling fluid, and indicate water has actually penetrated the underlying laminate. That's serious and needs treatment.
How to tell which is which
- Size: True osmosis blisters typically 5mm+ across; cosmetic ones smaller.
- Depth: Osmosis blisters are under the gelcoat (need scraping to see). Cosmetic ones are at the surface.
- Smell: Vinegar smell = true osmosis. No smell = probably hydration.
- Progression: Photograph and re-check at next haul. True osmosis grows.
If treatment is needed, where
Mediterranean yards do excellent osmosis work at significantly lower cost than US or UK yards. Marmaris (Turkey), Preveza (Greece), and the Spanish yards all have teams that have done hundreds of treatments. Cost for full osmosis treatment on a 40-footer: USD 8,000–12,000 in Turkey/Greece, vs USD 15,000–25,000 in the US.
The decision
Cruisers who discover cosmetic surface blisters can usually defer treatment indefinitely while doing routine antifouling. Cruisers discovering true osmosis face a choice: treat now (expensive but stops progression) or sell the boat. The trend over decades is unfavourable; deferred treatment costs more later.
Frequently asked questions
Are all blisters osmosis?
No. Many are surface hydration — cosmetic only.
How to tell?
Size, depth, smell, progression over time.
Treatment cost?
USD 8,000–12,000 Turkey/Greece. USD 15,000–25,000 US/UK.
Can I defer?
Cosmetic yes. True osmosis — defer carries cost.
Where to treat?
Marmaris, Preveza, Lagos all do good work at competitive cost.