Osmosis
Contents
Osmosis and delamination on Westerly Yachts by Nick Vass
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. Osmotic blistering occurs because free water-soluble chemicals such as glycol found inside the laminate cause a pull on the water outside of the hull. Some water molecules penetrate through the gelcoat and as water is drawn into the pockets of hydroscopic liquid the pressure inside the pocket rises creating a raised blister that can be seen on the outside of the hull. The water drawn the hydroscopic liquid combines with the liquid forming a new solution or chemical with a larger molecular structure that can't be pushed back through the gelcoat as its molecules are larger than that of water. These new solutions can be acidic or alkaline and can contain glycol.
Glycol is a chemical used in boat building that helps pigment to mix properly with gelcoat resin to stop it becoming streaky and can help with the mixing of polyester resin used in conjunction with glass fibre matting and woven roving glass fibre cloth to create the laminate. The glycol can be separated from the laminate by water ingress as this chemical is hydroscopic (it attracts water). The glycol that is liberated from the laminate by water often collects into pockets. As these pockets collect more water due to osmosis the liquid changes into a sickly-sweet smelling sticky brown solution.
The PVA binding solution that is used to glue the strands of glass fibres together to form matting can react with seawater to create an acid. The resultant solution will be thicker than water and will create a blister.
A strongly acidic, vinegary smelling liquid detected inside of the sample blisters. I test this liquid with PH paper acidic indicating that they were osmotic blisters as seawater is slightly alkaline. The liquid within the blisters are at great pressure which is another indicator of osmotic blistering as apposed to dry void non-osmotic blistering. Westerly blisters are very often between the two layers of gelcoat or gelcoat and the laminate. This is much less serious than if the blisters were within the laminate itself.
Gelcoat is polyester or epoxy resin coating that is painted or sprayed into a female mould as the first operation of the moulding process. The gelcoat forms the outer shiny, waterproof surface of the hull and is clear in its natural form. Pigment is added to the gelcoat to colour it. The pigment is an impurity, which makes the gelcoat slightly porous.
When the blisters are opened up for examination I often notice that the underlying laminate contains dry fibres of glass strands that had not been properly wetted in or soaked with resin during manufacture. This manufacturing defect could well be the cause of the osmotic blistering as the PVA binding compound that surrounds the strands of glass and which is normally safely absorbed into the resin has been allowed to come in contact with seawater and created an acidic solution.
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 pee or up to the size of half a cherry. Any bigger than this is described as being 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. Ice often does not form within the laminate as the glycol or acid within the blister does not freeze thankfully! If it did the laminate would delaminate. My own yacht 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
1. Failure to mix the resin and catalyst together properly during manufacture which creates pockets of uncured resin and glycol;
2. Leaving the layers to cure for too long between coats of resin. For example over night or over a weekend;
3. Poor attention to humidity levels in the workshop where the boat was layed up. If condensation is allowed to form between the layers the gelcoat or resin will not adhere properly;
4. Poor levels of cleanliness in the workshop. Contamination by dust etc.
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. It is attracted by the pull of the
hydroscopic solutions of glycol. Glycol is used in boat building to aid mixing of the pigment in gelcoat and prevent streaking. It is hydroscopic and similar to brake fluid.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.
Symptoms
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 fingerer into the blister.
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 Westerly used 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 owner's just antifouled over the blisters.
Alternatives. What to do.
Ask a surveyor who is passing how bad the blisters are and ask him or her to check moisture content. If the blisters are small or the vessel is of low value then just carry on using the boat as if nothing had happened. Monitor the blisters each year. Take photographs and record moisture levels. However, the moisture levels will change depending on atmospheric conditions and should be taken as a relative reading against the reading taken of the topsides above the waterline. Typically three or four times higher below that waterline compared to above is normal. Typically a Sovereign meter the reading above would average 4 and below would be between 14 and 19. This would be fine for a Westerly but wet for a newer boat. A condition called wicking might occur where moisture `wicks' up through the glass fibres to the topsides and creates small blisters around the waterline. This is not a great problem.
Cures. 1. Do nothing and carry on using the boat knowing that the value of the boat might be affected; 2. Remove the gelcoat by pealing it off by a hired plane type device, by having it slurry blasted of or scraping it off with a scraper and hot air paint stripper. This last way is messy, might damage the laminate, might poison you with fumes or might set fire to the boat but is cheap.
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 hot vac heated pads heated pads. see article by Roger Ball here
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 striped 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 Before inspecting the vessel I would gather the following tools:
`Scarsdale type scraper to remove `coupon' sized areas of anti fouling paint; Sharp wood chisels to remove sample areas of any epoxy coating should they be found to be painted to the hull; Cloths to dry areas to be tested for moisture; Moisture meters. I own a Tramex Skipper and a Sovereign Moisture Master; PH test papers; PPE including rubber gauntlets, coveralls, facemask, hat, goggles and visor. Antifouling dust can be very irritating; Magnifying glass; Digital camera; Hammers to check for delamination; Paint brush, disposable gloves and anti-fouling paint should the vessel be returned to the water after the inspection.
Some surveyors carry a small bottle of acetone to evaporate surface moisture from the sample scraped areas but others believe acetone to be a strong solvent, which can soften the gelcoat and damage it. The gelcoat should be removed by peeling machine and soda or slurry blasting. Voids and pockets of liquid should be opened up. The laminate should be cleaned with fresh hot water several times to wash away any acidic or undesirable solutions and the hull should be either allowed to dry naturally or dried out with infra-red lamps or heated vacuum pads. The hull should be dried to the extent that the moisture meter readings are the same or as close as possible below and above the waterline. This is known as the target level. All skin fittings should be removed and later replaced and all internal locker lids and portable upholstery should be removed to aid the drying out process. De-humidifiers could be fitted to the interior to dry out the hull from the inside. The bilges should be kept dry. The poorly wetted in strands of loose glass fibre should be painted with a thin penetrating solvent free epoxy resin to seal the surface. Voids or imperfections could be filled and faired off with solvent free epoxy resin filler. The hull should then be painted with at least five coats of standard solvent free epoxy resin and finished with antifouling primer that is suitable for use on epoxy resin.
Nick Vass
Epoxy Treatment of Centaur Hull
View presentation by Winston Waller to East Coast Group Westerly Winter Workshop 2013 about how he epoxied the hull of his Centaur Epoxy Treatment of Centaur Hull
Fulmar Blisters
Osmosis is always a worry for any old fibreglass boat. Many Westerly's from the late 1970's and early 1980's have areas of small bubbles in the underwater gel coat. The bubbles are usually 1 to 4mm across. This is not osmosis, but air bubbles trapped within the original 2 layers of gel coat.
See photo
.
It was caused by the rush to produce boats. During mixing, the gel coat was whisked and air was allowed to enter the mixture, then not leaving the gel to stand to allow the bubbles to escape before applying. The best solution is to have the moisture content checked by a surveyor, and provided it is within acceptable limits, fill the bubbles with a gel filler like Plastic Padding. It is best to remove all antifouling and sand the gel coat. Once filled, then give the hull 3 coats of epoxy paint before re-antifouling. The first two coats of antifouling must be a hard, non eroding paint in any colour. Then apply your final coats of self polishing antifouling in your colour choice. Make sure there is a marked difference between the colours of the hard and self polishing coats as then you know when a different colour appears, it is time to apply a fresh coat.
