I hauled my boat out for the winter and found a scattering of small blisters under the antifoul, below the waterline. Some weep a bit of fluid when I pop them and it smells slightly vinegary. Is this osmosis, and what should my next move be?
That combination of below-waterline blisters and an acidic, vinegary-smelling fluid is the classic signature of osmosis in a polyester laminate, where water has migrated through the gelcoat and reacted with soluble material in the laminate. Before committing to anything drastic, get the hull properly assessed rather than reacting to a few blisters. The usual first step is to haul out, strip the antifoul, and let the hull dry, then take moisture-meter readings over several weeks to see whether the laminate is genuinely wet. A proper osmosis repair means grinding out the affected gelcoat, washing and drying the laminate fully, then rebuilding with a water-resistant system, so knowing the true moisture state first saves a lot of wasted work. If you want the fluid or laminate assessed objectively, our materials testing service can help you make that call.
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