Osmosis is a process of degeneration within a glass fibre laminate. It is caused by a chemical reaction between water and unreacted substances remaining in the manufactured hull. The water enters the hull through the gelcoat and once inside, reacts with the chemical components creating acidic substances. These substances create pressure behind the gelcoat, which causes blisters and eventually cracking.

Once the gelcoat is breached in this manner, the underlying laminate is capable of absorbing water like a sponge.

Glass fibre boats are constructed of polyester resin strengthened by glass fibres. The glass fibre mat is soaked in the resin to form the laminate. A further polyester resin layer, called the gelcoat, provides a water barrier on the outside surface. The final structure is extremely strong but does have its problems as defects in the hull can allow water ingress into the laminate, which can cause osmosis and lead to serious structural problems.

Any unprotected hull is likely to show signs of osmosis eventually like rust on a car The exact length of time before osmosis occurs depends on many factors including: the type of water in which the hull is moored, the temperature of the water and most importantly, the quality of the original hull construction.

Osmosis is not only caused by water on the outside of the hull - bilge water from the inside can also cause the problem. It is therefore worth making efforts to keep your bilges dry.

In some cases, reactive impurities in the gelcoat and laminate will cause osmosis in the earlier life of the boat. This is a structural problem and should be referred back to the boat manufacturer. Approximately 15% of osmosis cases are accounted for in this way and will normally become evident within the first three years of the boat's life.

How to Protect against Osmosis

Prevention is always better than cure and it really does make sense to protect a new boat as well as older craft. To achieve this protection it is necessary to seal the surface. This is done over the existing gelcoat. There is no better time to apply an anti-osmosis system than before the boat has been launched.

However, it must be stressed that protective systems cannot stop osmosis once it has started, or prevent it from occurring in poorly constructed hulls. It is important that a full check is undertaken before starting.

Drying methods

Infra-red heat lamps

Hotvac drying machines

Coating method

Epoxy barrier coats applied in one day to ensure chemical bond between each coat.

Hull Re-lamination

This repair process is sometimes essential due to laminate deterioration. However, more customers are opting for this treatment. It is regarded as the premier treatment for a vessel, reinstating the hulls strength and providing an impregnable barrier against water ingress.

We are one of the few companies in Europe with a laminate impregnator machine and vacuum bagging machine, capable of undertaking this process.

Long-term exposure to water can actually take apart cured resin molecules and break them down into their constituent materials - a process known as resin hydrolysis. Osmotic gelcoat blisters are usually a symptom of resin hydrolysis deeper within the laminate. In the hull laminate, areas with gelcoat remaining appear to have a "Spot" blistering problem. All areas with gelcoat removed however reveal the hull's global skincoat hydrolysis problem. Water-soluble compounds - the products of hydrolysis - vent through the local blister sites.