Osmosis is a condition that affects GRP boats, cars and water tanks and manifests itself by the appearance of blisters between the different layers of the layup.
Polyester resin is slightly porous. Moisture does after a time travel through it and will collect in any voids in the GRP construction. These blisters can be very small, about the size of a half of a hundred and thousand cake decoration, about the size of a split pea or up to the size of half a cherry. Any bigger than this is described by surveyors as being serious delamination.
The problem is caused by many different factors or a combination of a number of factors. Osmosis is more common on vessels used in warm waters or fresh water lakes or rivers. I have seen terrible osmosis in yachts in yards around Lake Geneva. They keep on using them and apart from being a little heavy due to water ingress not much else happens and they are fine to carry on using. Boats used in the Lake District tend to suffer from Osmosis more than vessels found in the cold salt water of England. My own Hurley has bad osmosis in its water tank and also under any areas of Treadmaster decking. This is where warm rainwater has collected. I carry on using the water tank and don't notice a lot of smell. I use plastic containers of water from Tesco to drink or brush my teeth.
Reasons for osmosis.
So the moisture travels through the one or two layers of gelcoat (resin and pigment) towards the layers of matting or woven rovings which are wetted in with resin. The moisture collects in pockets and creates areas of high pressure, i.e. the chemicals in the dissolved resin mix with water to form a mixture that has a higher pressure than that of the water outside the hull. Osmosis is often most evident when a vessel is first taken out of the water. If the sun is strong the warmth will increase the volume of the water in the blister pockets and make it expand. The blisters will become larger. If you burst a proper osmotic blister you might be able to smell acetic acid. You should taste it too by rubbing your finger into the blister. The acetic acid is a product of the styrene and acetone used as solvents in the resin.
Osmosis is commonly found on boats from the mid-seventies, which was around the time of the oil crisis. Boat builders were experimenting with alternative types of synthetic catalysts to save money. Not all worked. Lucky for us Hurley used standard isophthalic resins which were of good quality and the boats were well made.
Modern boats are built in workshops where humidity is controlled. Boat builders work throughout the clock to ensure that curing and layering times are kept precise. Resins are now advanced. However, USA and Canadian boats can suffer as their governments have forced boat builders to reduce styrene levels by using alternative resins.
If you go Emsworth Harbour near Chichester you will see lots of almost new Trader motor yachts being treated for osmosis under warrantee. Emsworth is shallow, the water is warm and brackish. Same thing happens to boats in Christchurch. There are loads of Westerly Centaurs in Christchurch. Many suffer from osmosis but don't come to come to any real harm. The owners just antifouled over the blisters.
Alternatives. What to do.
You will need to remove all of the gelcoat and allow the hull to dry for a very long time until the moisture levels fall to about the same below the waterline compared to what they are above the waterline. This will take up to nine months but depends on climate. Drying times can be speeded up by using infra red lamps or de-humidifiers but this is expensive. It is not a good idea to paint epoxy resin onto a damp hull. This will just trap moisture in and cause the lay-up to delaminate. If this happens the structure will be scrap. Epoxy resin should be applied to a dry hull stripped of gelcoat or you will be wasting your money.
This is a simplistic explanation of osmosis. There will be lots of folk who will disagree as it is a very contentious issue.More about why boats aren't just built of epoxy resin later. That is a different story.
treatment of Hurley 24 'Circe'
Thanks to Stuart Shaw for supplying the pictures