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Many owners of old GRP boats live in fear of osmosis, but what exactly is osmosis and what can be done about it?

Osmosis comes about when water molecules make their way into the laminate of a GRP hull. Water molecules can pass through gelcoat and into the laminate, condensing into liquid water when they reach voids or small air pockets. The water then reacts with uncured chemicals in the polyester resin, known as hydrolysis, causing it to decompose. This forms a water absorbing solution that then attracts more water into the laminate. This process then continues and the chemical reactions cause a build up of pressure which results in blisters being formed. These become visible in the gelcoat.

The build up of water in the hull can take many years to become a problem and it can be challenging for surveyors, let along boat owners, to determine exactly where the water is. The question is whether water is permeating the gelcoat, the CSM layers or whether it is in the structural laminates which are made of marine ply, or whether it has penetrated a balsa core.

In severe cases, large areas of the hull can become rotten which can lead to structural failure if the situation is not dealt with. In less advanced cases, the hull can be sand or pellet blasted in the affected area, then allowed to dry out and a gelcoat filler applied in the small cavities left where the blisters had been. Surveyors will also advise that the affected boat is stored ashore for several months over the winter each year to help the hull to dry out.

Moisture meters

The first visual signs of osmosis are blisters and by then the osmosis is already advanced. Early warnings of osmosis developing in a hull can be diagnosed by taking moisture readings using moisture meters. While high moisture readings of the hull produced by moisture meters can indicate the presence of osmosis, surveyors usually warn that meters do not always produce infallible results. Readings will be affected by the atmospheric conditions on the day and how long the boat has been hauled out for.

In ideal circumstances, therefore, the most reliable method to determine if hull cores are wet is to take the moisture readings from the inside of the hull, on a dry day, after the boat has been ashore for at least a few weeks in the summer months – clearly this is not always possible.

Treating osmosis – professional cures

Professional cures of osmosis can be expensive because of the labour and materials involved. The process involves removing the underwater gelcoat and any delaminated lay-up beneath. The exposed hull is then dried out thoroughly, often using infra red heaters to speed up the drying process. Once the hull is completely dry, new epoxy resin and an epoxy gelcoat are applied to form a long lasting impermeable layer.

The end result usually produces a stronger hull than the boat had to begin with. It should never need treating again, provided the repair is done to professional standards.

Treating – the DIY solution

While a professional job will be just that, there are ways for a resourceful DIY boat owner on a tight budget to deal with osmosis themselves, depending on the severity of the case and the extent of the blistering. This will entail plenty of hard work and it will help to have a sheltered spot ashore where the boat can be properly dried out under cover.

Knowing where you might access the various bits of kit mentioned in the following step-by-steps would be useful before launching ahead.

How to treat mild cases with localised blistering:

  1. Remove the antifouling from the affected area to exposes the gelcoat. This can be done by hand using scrapers or if the area warrants it, consider hiring a professional to blast the hull for you.
  2. Grind out the blisters and wash the area thoroughly with fresh water to remove all the impurities. Steam cleaners are sometimes used to do a thorough job.
  3. Check there is no de-lamination taking place and grind back further if necessary to expose sound lay-up.
  4. Leave to dry inside and out as thoroughly as possible, using infra red heaters and fans if necessary. One or two weeks drying in warm weather should be sufficient.
  5. Once the area is dry, mix up epoxy filler and fill the craters where the blisters were.
  6. Sand back the filler ensuring the hull surface is smooth.
  7. Finish off with primer before applying antifouling.

How to treat more advanced cases with widespread blistering:

  1. Remove the gelcoat from the affected areas. This is best done using an electric gel-plane – it is worth considering having this done professionally as it is a tricky job.
  2. Grind back the blisters and areas of de-lamination to sound lay-up.
  3. If there has been serious de-lamination, consider having a new lay-up done professionally. Taking professional advice on this would be a sound plan.
  4. Steam clean all the ground back areas to remove all residues of impurities.
  5. Leave the hull to dry inside and out. Fans and infra red heaters can be used. Drying can take a few weeks, depending on conditions – remember that moving air evaporates water more quickly than anything else.
  6. Use a moisture meter to check the old lay-up is dry. Compare readings with those taken in an area of the hull that is always dry, such as the topsides.
  7. Prime the area with two coats of epoxy barrier paint, carefully following the manufacturer’s instructions.
  8. Fill all holes with epoxy filler and fair back when dry.
  9. Apply more coats of epoxy paint according to instructions prior to a coat of antifouling primer before finishing off with antifouling.

Further steps can be taken to slow down the osmosis process. Given that water moisture can permeate a GRP hull from the inside as well as the outside, it is advisable to keep the bilges dry and the interior well ventilated. In addition, surveyors also advise that a GRP hull with high moisture readings should be dried out ashore for several months each year as a preventative measure.

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