Seeing a blister on the underwater surface when the boat is ashore isn't always cause for panic. This problem isn't synonymous with every blister. Correct diagnosis prevents both needless expense and harmful neglect.
What is a blister?
A blister is the general term for a fluid-filled bubble forming under the gelcoat. Its source isn't always osmosis; adhesion failure between paint coats, moisture or application error can also blister.
What is osmosis?
Osmosis is a chemical process where water seeps into the laminate and reacts with the resin. Its telltale sign: the fluid from a pierced blister is sour/vinegar-smelling and sticky. We detail this in our osmosis symptoms article.
How to tell them apart?
- Smell: sour/acidic → osmosis. Odourless → likely a paint/moisture blister.
- Layer: is the blister in the laminate under the gelcoat, or only in the paint coats?
- Moisture meter: high laminate moisture points to osmosis.
What does each need?
A paint blister is usually solved with surface work; osmosis needs gelcoat peeling, drying and an epoxy barrier (see osmosis treatment). A wrong diagnosis means either needless big work or a returning problem.
We determine whether your hull blister is really osmosis with a moisture reading under our fibreglass repair service.
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