Osmosis: serious or superficial?
Osmosis blisters form when water enters a polyester hull. A few small ones are cheap to treat. Deep, widespread blistering is a major repair.
Updated 2 min read
Key points
- Warm water speeds osmosis, which makes it relevant in the Med.
- Small, shallow blisters in the gelcoat are often treated cheaply. Deep, widespread blisters need a full treatment.
- Get a surveyor to map and measure the blisters, and ask for a treatment quote.
What it is
Osmosis happens when water passes through the gelcoat of a polyester hull and reacts with soluble chemicals in the laminate. The result is liquid trapped under the gelcoat that pushes it up into blisters. The liquid is often acidic and smells of vinegar. Warmer water speeds the process, which is why Mediterranean boats can blister earlier than boats in cold water.
Who gets it
Hulls made of polyester resin, especially boats built in the 1970s and 1980s, boats with a thin gelcoat or poor lamination, and boats that stay in the water all year. Modern builders use better resins and barrier coats, and the problem is less common but not absent.
Superficial or serious
- Superficial. A scattering of small blisters, each a few millimetres across, confined to the gelcoat, with dry laminate underneath. Often handled by opening them, drying, filling and applying a barrier coat.
- Serious. Large or numerous blisters, blisters that reach into the laminate, laminate that is soft or wet, or blisters over much of the underwater hull. These need the gelcoat peeled, the hull dried over weeks or months, and a full epoxy barrier system applied.
What to ask the surveyor
- Where are the blisters, how many and how big?
- Do they go through the gelcoat?
- What do the moisture readings say about the laminate? See Hull moisture readings and delamination.
- Has the hull been treated before, and are there records?
Costs and decisions
A localised treatment is a small job. A full peel and barrier coat is a major job that takes the boat out of use for weeks to months. Get at least two written quotes from yards with osmosis experience, and compare the total with the price reduction you can ask for. See Post-survey renegotiation: reduce, repair or walk away.
Prevention
A barrier coat applied in a dry state, a boat that spends part of the year ashore, and regular inspection at each haul-out reduce the risk. See Gelcoat and hull care: UV damage and osmosis prevention.
Related guides
General information, not legal, tax or financial advice. Rules change; check your own case with a professional or ask us.