Service

Boat Osmosis Treatment

Osmosis treated properly, from the laminate up

Osmosis is the blistering you find under the waterline of older fibreglass boats, when water works its way through the gelcoat into the laminate. Left alone it spreads. We check the hull, explain how far it has gone, and treat it properly: open up, dry out, and seal the hull with an epoxy barrier coat.

What's included

01
Survey & moisture readings
We inspect the underwater hull and take moisture readings, so you know whether you are looking at a few cosmetic blisters or osmosis that needs full treatment.
02
Gelcoat removal & drying
The affected gelcoat is removed and the laminate is left to dry out properly. Drying is the step that can't be rushed.
03
Epoxy barrier coat
Damaged laminate is repaired, the hull is faired and built up with several coats of epoxy barrier coat, then primed and antifouled.

Found blisters on your hull?

Get in touch with our experienced team. Send us photos from the haul-out and we'll tell you what they mean.

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Why choose MMR

Honest diagnosis
Not every blister is osmosis. We tell you what you actually have before recommending treatment.
Dried before sealing
Sealing a wet hull just traps the problem. We don't apply the barrier coat until moisture readings say the laminate is dry.
Built to last
The barrier coat is built up to the thickness the manufacturer specifies, so it keeps water out for years.

Brands we work with

What is boat osmosis?

Gelcoat isn't completely waterproof. Over years in the water, moisture passes through it and reacts with chemicals in the laminate, forming an acidic liquid that pushes the gelcoat up into blisters. It mostly affects older GRP boats, and it develops faster in warm water like Malta's. Read the full explanation: what is osmosis boat treatment?

Signs of osmosis

Blisters under the waterline, from pinhead size up to coin size
Blisters that weep a vinegar-smelling liquid when punctured
High moisture readings in the underwater hull

How osmosis is treated

Survey. Inspection and moisture readings to see how far it has gone.
Remove the gelcoat. The affected gelcoat is taken off to open up the laminate.
Wash and dry. The hull is washed to remove the acidic residue and left to dry, with moisture readings until it is dry enough to seal.
Repair the laminate. Any damaged laminate is cut back and rebuilt.
Epoxy barrier coat. Several coats of epoxy are built up as a water-resistant barrier – with WEST SYSTEM 422 Barrier Coat Additive or an epoxy primer such as Seajet 117.
Prime and antifoul. Ready to go back in the water.

Can osmosis be prevented?

A new or newly treated hull benefits from an epoxy barrier coat before the first antifouling, and from time out of the water at haul-outs. Keep an eye on blisters at every haul-out – the earlier osmosis is caught, the smaller the job.

How long does treatment take, and what affects the cost?

Most of the time is drying, which depends on how wet the laminate is and the weather – from weeks to months. The cost depends on the size of the boat, how much of the hull is affected and how much laminate needs repairing. The survey tells us both, and we'll give you a clear quote before work starts.