OSMOSIS TREATMENT

Definition

A dictionary definition of Osmosis is "the passage of a solvent through a semi-permeable membrane into a more concentrated solution". When GRP yachts were first produced people thought they were practically impermeable to all forms of rot; this theory has been proved wrong with the occurrence of osmotic style degradation.

When two liquids with different concentrations are separated by a semi-permeable membrane one liquid will penetrate the membane in an effort to equalise the difference in concentration. During this process the volume of the most concentrated solution will increase and a pressure can develop.

In this case, the membrane is the gelcoat of the glassfibre boat and the solution is water. in time water will be absorbed into the GRP; the majority if this moisture passes directly throught the hull into the bilges with little damage. However, a certain amount of the moisture will eventually start to break down some of the materials used on the laminate itself which will lead to a build-up of "osmotic" fluids containing, for instance, acetic acid and glycol.

The osmotic fluid has a higher molecular weight to water and is therefore unable to pass back through the gelcoat or laminate. As more moisture is absorbed through the GRP, an "hydraulic" effect occurs which eventually leads to blisters being formed in the gelcoat.

Most unprotected boats at some stage during their life will develop osmotic symptoms to some degree. the timescale and extent of the occurence depends upon various factors including: the temperature of water, the type of water, the periods afloat versus ashore and, most importantly, the quality of the original laminate.

Identification

The first visual signs of osmosis will be the formation of blisters in the gelcoat. if you poierce an osmotic blister the liquid contents will have a sour smell similar to vinegar. the problem may be discovered prior to its visual symptoms appearing by checking the moisture content of the hull with a moisture meter

At this stage professional advice should be sought to determine the degree of the problem and the future action. ABC Powermarine are a Blakes Approved Osmosis Treatment Centre and are perfectly qualified to assist you.

Prevention

This is usually done at an early stage of the boat building, so the customer usually doesn't have to worry about it. It is also prudent to apply a preventative expoxy coating to used boats and to measure the moisture levels at the time. If there is a potential problem treatment can be used at an early stage, otherwise a preventative layer can be applied to avoid potential problems in future. As a general rule, whenever buying a used GRP boat, always have it checked for any potential osmosis problem.

Treatment

Where moisture readings are high or there is apparent blistering of the gelcoat a full Osmosis Treatment will be required. Full treatment involved the total removal of the gelcoat which, under the correct conditions, allows the hull to dry out to the recommended levels, prior to coating the bare laminate with SFE200 to restore the hull's resistance to moisture ingress. For further details or prices, please contact us.

Note: this description has been adapted from the Blakes Paint Manual and is the copyright of Blakes Paints, copies of which are available from ABC Powermarine on request or from Blakes Paints themselves on 023 80 63 63 73.