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boatcare services

Malta Yacht Marina Services

Osmosis Treatments

GRP (glass reinforced plastic of fiberglass) is a very good boatbuilding material. It is strong, relatively easy to work and mould, and needs a lot less maintenance than most other materials like wood, steel or even ferro-concrete. In fact, GRP has become the material of choice for most yachtbuilders for the past 40 or 50 years.

It is, however, prone to problems of its own. GRP hulls can develop blisters filled with a foul liquid: a nightmare for the owner. This is what has been known as osmosis since the 1970s.

How the hull is made

To understand what is happening and how to keep a hull solid and safe, it helps to understand how a GRP hull is made. It is essentially plastic, usually a polyester resin with a reinforcement of glass fibre embedded inside it, sometimes with a light wood core. The glass fiber, woven into a cloth or mat, is laid first. Then the resin is applied, impregnating the cloth and building up the shape. This is then allowed to cure to a solid, then coated with an epoxy gel coat to seal it off.

Ideally, the resin should be completely homogenous, with no spaces or empty bubbles. This is, however, practically impossible to achieve in the real world, however well built the hull even with the vacuum infusion systems now used by the top yachtyards.


With time, water does manage to diffuse through the gel coat and into the resin. This is a slow process, but a GRP hull can absorb up to 2% of its weight in water. Initially, the water is present as single molecules within the structure of the resin but it can collect as drops of liquid in any small cavities or flaws in the resin.

Once liquid water is present, the real damage begins. Water soluble components in the resin – solvents and parts of the resin that were not transformed during the curing process – dissolve, creating an unpleasant, corrosive mixture of acetic and hydrochloric acids and glycol. This, in turn, enlarges the voids.

Glycol is hygroscopic - it attracts water. It increases the rate at which water is absorbed into the hull, accelerating the decay and in the process generating the characteristic blisters filled with the “osmotic fluid” which smells of vinegar and feels soapy to the touch. This fluid cannot get through the gel coat, unlike water, so it has nowhere to go, and the problem cannot be solved simply by drying the boat out.

Osmosis Treatment

But it must be solved. If not, the very structure of the hull will fall apart, with the osmotic fluid corroding increasing volumes of the resin. The glass on its own does not provide the support needed for a solid, rigid hull. The only solution is professional treatment done as early as possible.

Should your boat be unfortunate enough to be affected by osmosis, or if you would like have it treated to nip the condition in the bud, contact us at Boatcare Trading Ltd. We have extensive experience in this work, which we carry supervise closely to ensure that the treatment is thorough and complete.

We also strongly suggest that the boat’s hull should be stripped of all paint and properly treated with anti-osmosis solutions to prevent any further occurrence of osmosis. If treatment is not done correctly on a properly prepared hull, it may cause more damage than the original problem, and water may be trapped between the laminates setting off the chemical reactions and decay caused by osmosis.

Our service

Boatcare provides a complete service, starting from a diagnosis of the hull’s condition. First, we examine the gel coat visually for signs of blistering, or wicking. If we do find blisters, we analyse the chemical composition of any liquid they contain. Finally we determine the moisture content of the hull using a moisture meter. We do not base our diagnosis solely on the results of the meter test: this tool, while powerful, has its limitations. A high moisture content reading on an old hull, for example, does not necessarily indicate osmosis or poor laminate condition. Equally, a simple visual examination can often be enough in advanced cases with extensive blistering.

Treatment begins with the removal of the gel coat. If necessary, the underlying laminate will need to be repaired before a new, protective epoxy coating is applied. We usually use five coats of a solvent-free epoxy system, applied by roller to give a final thickness of 1mm, but the specifics depend upon the manufacturer’s recommendations for the product chosen.

Osmosis can damage your boat irretrievably, however well you keep her. Call us at Boatcare for a diagnosis and treatment.