Osmotic blistering is water getting into the gelcoat and laminate of a fibreglass hull and reacting with what's left of the resin's ingredients. The fluid that forms draws in more water, the pressure builds, and a blister lifts. It's the defect buyers ask about most, and the one most often over or underplayed in a report.
How a surveyor finds it
Blisters show once the antifouling is clean, but telling osmosis from paint blisters means opening a few. A true osmotic blister sits in or under the gelcoat and usually releases an acidic fluid. A paint blister is dry and sits in the antifouling.
The moisture meter comes next. Readings only mean something on a hull that has been out of the water for some days, on bare gelcoat, and against a reference: the same hull above the waterline. PBO's surveyor uses at least two meters, one reading shallow and one deeper, and warns about false highs from a freshly hauled boat, wet bilges, tanks and damp lockers behind the hull. Practical Sailor adds two limits worth knowing: no meter gives a true moisture percentage in fibreglass, and readings below freezing can't be trusted. The moisture readings log sets each reading against your dry reference.
Sounding with a phenolic hammer tells you whether the laminate under the blisters is still sound. A dull note around opened blisters, or fibres visible in the cavity, moves the finding from gelcoat damage to laminate damage.
What decides the grade
Two things: depth and location. Gelcoat blisters with sound laminate are a B. They need treatment, on the owner's timescale. Laminate damage at the keel root, rudder stock or another load path is an A, because the hull's strength is in question. A handful of dry paint blisters is a C, and calling them osmosis in a report can cost the seller a sale for nothing.