WHAT IS OSMOSIS?  Osmosis is the name given to the degradation of polyester resin in the hulls of GRP boats. Water that has diffused into the hull causes the ester molecular links in the polyester resin to hydrolyse and degrade, to form a hygroscopic residue which attracts more moisture into the laminate. Over time, the moisture pockets become pressurised, enlarge and form blisters on the surface of the laminate. Unchecked, osmosis can work deep into a polyester laminate causing extensive damage and weakening the structure.

Osmosis will eventually occur in any polyester laminate exposed to moisture unless it is treated from new to prevent water penetration. Modern gel coats are very moisture resistant, however being polyester based they will never resist moisture completely. The only effective long-term solution for preventing osmosis is to apply a moisture barrier coating.

There are a number of factors that will contribute to the rate at which osmosis will occur. The quality and type of gel-coat and resin, workshop conditions at the time of lay up and the quality of the hull lay up are important. Additionally, the length of time the boat remains in the water, its salinity and temperature will impact the rate of osmosis. (e.g. is she dry sailed or hauled out during the winter months?)

A point to remember is that water can penetrate from inside the boat as well as out, therefore keeping a dry boat down below is important.

HOW DO I KNOW IF MY BOAT HAS OSMOSIS?  A good DIY check for determining if your boat is beginning to be damaged by osmosis is to carefully examine the bottom of the boat looking for the existence of blisters. This is best done in the half-light of early morning or evening after the hull has been thoroughly cleaned and allowed to dry. Any large blisters will be readily apparent. In the half-light, any slight dimpling of the hull indicating the beginning of blistering can be detected. Mark any suspect areas and then remove any bottom paint to ensure that the hull is affected and that the blistering is not merely due to poor adhesion of the bottom paint. If in doubt get a surveyor to inspect the hull. The surveyor will generally remove some sections of the bottom paint and check the hull laminate with a moisture meter. This will enable him to detect the presence of moisture in the laminate, indicating the presence of osmotic activity, before the osmosis has caused any visible damage. When detected at this stage, prevention against further damage can be done at minimal cost.