Does My Boat Have Osmosis? 5 Signs and a Simple Check

Osmosis advances quietly; caught early it's a small job, left late the whole hull needs work. Here are the signs that help you tell whether your boat has osmosis.

1. Blisters below the waterline

The classic sign is small blisters on the underwater surface when the boat is ashore. Pressed with a finger they may feel soft or fluid-filled.

2. A sour smell

If the fluid from a pierced blister smells of vinegar, that's a strong sign of osmosis — the product of the chemical reaction in the laminate.

3. Moisture and dullness

Patchy moisture marks, dullness and colour variation in the gelcoat can mean water is moving beneath the surface.

4. Surface unevenness

Bumps you feel running a hand over the underwater surface can herald blisters not yet visible to the eye.

5. Moisture-meter reading

For a definitive answer professionals use a moisture meter. High moisture means risk even without visible blisters.

What to do

One or two small blisters aren't cause for panic but should be monitored. Widespread blistering and high moisture mean it's time to treat. We explain how osmosis is treated in our osmosis treatment guide. To be sure, we offer a free survey and moisture reading under our fibreglass repair service.