Osmosis

What is Osmosis?

The term “Osmosis” was devised in the early 1970s to illustrate the blistering found on many GRP boat hulls. In simple terms Osmosis is a process of degeneration within a glass fibre laminate. Osmosis is similar to rust on a car and any unprotected hull is likely to show signs of osmosis eventually but it 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.

What causes Osmosis?

Osmosis is a chemical reaction between water and un-reacted substances remaining in the manufactured hull. The majority of glass fibre boats are constructed from 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. However, water can diffuse into and through the gel coat and the laminate as there are small voids and air pockets within the resin and at the interface between the resin and the glass fibres. When water enters the hull through the gelcoat it reacts with the chemical components creating acidic substances, these substances then create pressure behind the gelcoat causing blisters and eventually cracking. Once the gelcoat is breached in this manner, the underlying laminate absorbs water like a sponge.

How can Osmosis be avoided?

To protect against osmosis it is necessary to seal the surface of the hull over the existing gelcoat. The best time to apply an anti-osmosis system is before the boat has been launched. However, protective systems cannot stop osmosis once it has started, or prevent it from occurring in poorly constructed hulls. If your vessel is dry you can use Gelshield 2000 as a preventive measure to help reduce the risk of osmosis.

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How is Osmosis treated?

In order to treat boats with osmosis we carry out the following work:

• We strip off the old gelcoat using a state of the art gel plane system
• The hull is then steamed cleaned to remove all impurities built up during the osmosis process
• The hull is then dried using our Hot Vac machine to lower the moisture content to an acceptable level
• The hull is then filled and coated with International Paint’s Gelshield System to build the hull to its original thickness
• Finally the hull is antifouled ready for re-launch

Cardiff Marine Services recommend the Hot Vac system as the most successful treatment for osmosis.