Osmosis repair cannot be rushed.

Osmosis occurs when water seeping beneath the gelcoat of a GRP hull reacts with soluble components inside the laminate, and the resulting fluid forms pressurised blisters. It begins as what looks like a cosmetic flaw; neglected, it becomes a job that concerns the structural integrity of the laminate.

Symptoms: what to look for

• Blisters. Shortly after the boat comes out of the water, bumps of varying size appear on the hull bottom.
• A vinegary smell. The acidic smell of the fluid released from a punctured blister is characteristic.
• Softening and discolouration. In advanced cases the laminate surface goes dull and feels locally soft.

Not every blister is osmosis: loss of adhesion between paint layers looks much the same. The distinction is made by identifying which layer the blister has formed in and by taking moisture readings.

Diagnosis: moisture readings and mapping

A sound diagnosis is made after the hull has been washed and the surface has been left to dry for a while. The hull is scanned area by area with a moisture meter and the readings recorded as a map. The aim is to answer two questions: how far has the moisture spread, and how deep does it go?

This stage must not be rushed. Every hull is wet when it first comes out of the water, and a reading taken on day one is misleading.

The repair, step by step

1. Removing the gelcoat. Over the affected area the gelcoat is peeled off under control, or sanded away.
2. Drying. The longest and most decisive stage. The laminate is dried until it reaches a measurable moisture level; how long that takes depends on the climate, the thickness and how far the moisture has spread.
3. Cleaning and neutralising. Residues on the surface are washed off.
4. Laminate repair. The voids are filled with the appropriate resin and cloth, and the surface is faired back to shape.
5. Protective coating. Epoxy barrier coats are applied.
6. Antifouling. Hull bottom paint over the barrier coat completes the process.

What drives the time and the cost?

Drying dictates the timetable of an osmosis repair, and drying does not negotiate. Epoxy applied over a damp laminate brings the same problem back within a few seasons. Serious osmosis repairs are therefore usually scheduled for the winter: the boat is ashore anyway, and there is time for the hull to dry.

The scope, meanwhile, follows the moisture map. Isolated blisters can be dealt with over a limited area; where moisture is found throughout, the whole hull bottom is taken on.

Prevention: protecting the boat from now on

• An epoxy barrier coat on the hull bottom makes it harder for water to reach the laminate.
• At every haul-out, once the hull has been washed, it is checked by eye and, where necessary, with a moisture meter.
• As the paint build-up thickens, the surface breathes less easily and adhesion suffers; managing the layers matters.
• Ventilating the hull during the lay-up helps moisture leave the surface.