This article is no more than an introduction to a topic that is fairly complex, involves quite a bit of chemistry, and shouldn't really even be defined under the the simple heading 'Osmosis'...
What is 'Osmosis'?
Used in relation to glass fibre reinforced plastic (GRP/FRP) boats the word ‘osmosis’ is usually used to describe blisters in the hull regardless of whether or not they are caused by osmosis. Different plastics have differing absorption characteristics and polyester resins (the plastic that most fibreglass boats are built from) are not impermeable. It gradually absorbs water over the years and this can, and often does, lead to blistering.. Vinylester and isopthalic polyeter resins are less permeable than the less expensive orthopthalic polyester resins usually used for boat building and solvent free epoxy resin, correctly cured can be considered virtually impermeable.
Most superficial gelcoat blistering is fairly benign and can be successfully treated locally as and when necessary. Occasionally it's a symptom of a more serious problem which can lead eventually to loss of structural integrity and eventual disintegration of the vessel. Especially if the vessel was built using low quality resins and/or poor resin saturation of the reinforcing fibres.
The glass fibres should not absorb water but if they were not completely saturated with resin during the layup of the hull any water that permeates the resin will be drawn deeper into the laminate by capillary attraction. This may then accumulates in any voids or micro-cracks in the layup and react with various components of the resin to create, through hydrolysis, acetic and hydrochloric acids, glycol and other compounds. These are then trapped and will not be released simply by removing the vessel from the water and leaving it to dry. As the levels of chemicals in the absorbed water increase they become a more and more concentrated solution than the external salt water and accelerate absorption by attracting more water into the matrix due to 'osmotic attraction'. This is real 'osmosis' and it becomes a serious problem when internal hydrostatic pressure develops causing localised delamination of the fibreglass and this can eventually lead to significant structural deterioration.
To release this 'osmotic fluid' the gel coat has to be removed as it is impermeable to these large molecule chemical solutions. They then have to be diluted and extracted. Simply drying the surface out simply leaves an even more concentrated solution in the laminate ready to 'suck' water back in when the vessel is relaunched.
One factor that seems to accelerate osmosis is poor ventilation or moisture on the inside of a vessel. Fresh water in the bilge or in poorly lined integral tanks should be avoided as the salt water attracts the fresh water through the plastic from the inside (osmotic attraction).
Most ‘osmosis’ blistering is fairly superficial and is confined to gel coat blistering with only very localised deeper absorption and negligibly affects the overall structure of the vessel. Blistering of the gel coat is common and, in itself, is not serious but it's a clear indication that there is a probability of deeper osmosis developing and should not be ignored. Not all GRP boats suffer from the problem and I've inspected boats more than 30 year old that show no signs of ever having had 'osmosis'. Some only ever develop gel coat blisters and the GRP laminate remains completely healthy. Even a relatively high moisture content in the GRP laminate doesn't in itself appreciably weaken the hull as long as deterioration of the resin or delamination take place.
Radio Frequency moisture testing below the waterline is usually useful only on boats that have been out of the water for at least a few days. Most GRP boats give high readings for the some time after slipping unless they are epoxy coated or were built with an exceptionally impermeable (eg isopthalic, vinylester or epoxy) resin. RF testing is useful for checking how dry the laminate is before re coating it with epoxy resin when treating a boat for osmosis and can be useful for testing decks, topsides and structural elements such as sheathed timber stringers and the like. There are many factors that can influence moisture meter readings and they don't, in themselves, provide a diagnoses.
The best preventative treatment (apart from keeping your boat out of the water) is to apply a generous barrier coating of epoxy resin to the abraded gel coat before there are any signs of blistering.
If superficial localised blistering has occurred it may not be necessary to remove the entire gel coat. Many boats are used for years with minor blistering without it progressing to anything worse. It may just be the superficial layers that are permeable and the main structural laminate is resistant. All 'pimples' should be cleaned out – first using a counter sinking bit in a drill. Watch out when piercing the blisters as the fluid inside can squirt out and get in the eyes. Once all loose material has been removed clean thoroughly with hot water followed by methylated spirits. When dry it's good to stipple the area to be filled with epoxy resin and, as this cures, to fill with a paste using the same resin base and a filler such as colloidal silica. Masking tape can hold the filler in place while it cures. The vessel should then be carefully examined at each slipping to see whether the repairs are sound and whether new blisters are forming. It's not a good idea to leave blisters just to get bigger since they accelerate due to osmosis.
More extensive blistering needs more work. The vessel should have all the antifouling removed, all the blisters thoroughly ground back to “healthy” GRP examining for and removing all “threads” or “veins” of moisture. It should then be rinsed repeatedly with hot water or steam until no acidic compounds form on the surface. Cleaning repeatedly with an alcohol soaked rag is also effective. If there are areas of more than about 50mm in diameter that have been ground more than a few milllimetres deep they should first be coated with epoxy resin and, when this is well on the way to curing, laminated with epoxy resin/glass cloth. Larger and deeper areas must be feathered back over a considerable area and fully glassed to retain hull integrity.
If there is extensive blistering then stripping off the entire gel coat, cleaning, drying, relaminating where/if necessary and applying a thick epoxy barrier coat is probably the best/only option. If not it's likely to continue developing blisters and may eventually start to seriously delaminate and start to lose structural integrity.
If the job is done correctly this can be a "permanent" cure (15 years or longer). If it's 'half done' you are not likely to even get half the time.
First comes removal of the gel coat. Using a special planer type tool (gelcoat peeler) that removes the gelcoat to a preset depth is a relatively efficient an reasonably clean method. Abrading it off with a disc sander or grit blasting are also effective but require more judgement and skill. Some specialists recommend blasting or abrading after gelcoat peeling to roughen the surface slightly to help both in drying and with eventual adhesion of the epoxy barrier coat.
Once the gel coat is removed the hull can be carefully examined. Any areas where the glass fibre appeasr poorly saturated are likely to be the worst affected and should be 'explored' fairly ruthlessly with a chisel and tested with a moisture meter. Chopped strand matt (CSM) laminate is the worst offender as some of the binders used keep the fibres together both inhibit good resin saturation and are water soluble.
Simply leaving the boat to dry out in fresh air will eventually remove much of the water that has been absorbed but not the hydrolysed solution which will stay within the hull and tend to 'draw' water back in through the barrier coat when it is relaunched. Any 'osmotic fluid' left in the matrix it should be 'dissolved out' by steam cleaning and/or vacuum bagging and/or hot fresh water rinsing. When litmus paper testing indicates that the surface is pretty well neutral it's time to start drying at elevated temperature with dehumidified air and/or infra red heaters.
If the vessel can't be treated in a shed then forming a plastic tent around the hull is an option but just recirculating warm humid air doesn't do much for drying. The air has to be dehumidified. Vacuum bagging is another effective option that is not as complicated as it may sound. During the drying process RFE moisture testing should be undertaken fairly frequently to find any areas that may have 'hidden' fluid in them.
I'm convinced that the best barrier coating is solvent free, unpigmented epoxy resin. There are some (glass filled) vinyl ester resin coatings available that seem good but I suspect that they are more difficult to cure. The surface must be suitably prepared and the epoxy should be applied in fairly dry conditions above about 12º C using the temperature approriate hardener to ensure a good rapid cure. Ideally at least 4 generous coatings are applied one after the other as soon as the previous coat is just touch dry. This ensures a good 'chemical' bond and avoids any need to sand back between coats. Roller application is usually the most practical though airless spray can be good. It's a good plan to move the masking tape up the hull about 10mm each coat to avoid a hard edge.
Please get in contact if you have an 'osmosis' issue that this short article doesn't cover or if you would like me to look at a specific problem