Find blisters on a GRP hull at haul out and the word osmosis follows immediately, usually with an alarming price attached. The condition is real but widely misunderstood, and not every blister demands a full treatment.
What is actually happening
Gelcoat is slightly water permeable. Over years, water migrating into the laminate dissolves residual chemicals from the original build, and the resulting solution draws in more water by osmosis. Pressure builds, and blisters form between gelcoat and laminate. Warm water accelerates the process, which is why boats kept permanently in Gulf marinas show it earlier than the textbooks suggest.
Diagnosis before panic
Small scattered blisters in the antifouling layer are often just paint blisters. True osmotic blisters sit in or under the gelcoat, weep an acidic fluid when popped, and read wet on a moisture meter after weeks ashore. A surveyor with a moisture meter separates cosmetic from real in an hour.
The treatment sequence
Established cases follow a fixed path: strip the antifouling, peel or grind the gelcoat in affected areas, wash and dry the laminate until moisture readings stabilise, fill with epoxy compounds, then rebuild the barrier with 4 to 6 coats of solvent free epoxy before antifouling. Drying is the step everyone shortcuts and the reason treatments fail; in the UAE summer, under cover, laminates dry faster than European guidance assumes, but weeks are still normal.
Prevention on newer boats
An epoxy barrier system applied while the hull is still dry is cheap insurance, especially for boats that never leave the water. Ask us for the scheme on our marine coatings page.