This  brief explanation of the condition usually referred to as “Osmosis”  “Boat Pox” or “Osmotic Blistering” is a general outline of the subject as it affects you, the GRP boat-owner. Generally early GRP boats moulded prior to the early 1990’s are more likely to get “Boat Pox” than those moulded from more recent types of resin.

The scientific term Osmosis was originally used to describe the biological process where by a liquid (usually water) will pass through a semi-permeable membrane. The membrane is permeable to the water molecules but not the various compounds dissolved within the water. The overall flow of the water molecules will be from the solution of lowest concentration towards the solution of higher concentration. This flow can be reversed by the application of an external pressure as used in “reverse osmosis” water treatment systems.  In the case of a boat, the GRP hull is in essence a semi-permeable membrane

The term “osmosis” referring to boat hulls was coined in the early 70’s to describe the blistering found on many GRP boats and is now in common usage. The osmotic process probably does occur within the blisters mentioned above, but is not the only process involved and is not the full story.

A GRP (glass reinforced plastic) boat hull is a matrix of (usually) polyester resin reinforced with fine glass fibres, built up in layers. The final laminate will have an approximate ratio of 30% glass to 70% resin. This laminate is not homogeneous, that is to say, there will be small voids, air pockets and micro-cracks within the resin matrix and at the interface between the resin and the glass fibres.

Water can diffuse into through the gel coat and the laminate (the polyester resin not the glass fibres), as separate water molecules, as opposed to a liquid. A GRP hull can absorb around 2-5% water in this way. Water can actually pass slowly through a fibreglass hull below the waterline in this way and disperse in the bilges as water vapour. The moisture content of a new hull will slowly increase during the first few seasons that the boat is afloat, the moisture content will similarly reduce slowly when the vessel is out of the water.

As mentioned earlier there are numerous small voids within the laminate. Water molecules can collect and then condense  inside these voids. Within the GRP laminate and the micro-voids are various water soluble components. These consist of solvents and other by-products and un-cured constituents from the laminating process.

Over time, the water within the micro-voids is able to dissolve and chemically react with the soluble components. This process is known as “hydrolysis”. The hydrolysis process will continue over time, with the voids enlarging. An acid solution is formed, the main ingredients being, acetic and hydrochloric acid and glycol. These products give “osmotic fluid” its characteristic vinegary smell and oily texture.

The glycol in particular is “hygroscopic” (water absorbing). Once this is released in the voids it will accelerate the rate of water absorption into the laminate. This absorption process will continue and will not be fully reversed by simply taking the boat out of the water. Moisture content will drop slowly if left ashore but will rise again fairly rapidly when immersed again. The water absorption process is more rapid in fresh or warm water. The various hydrolysed products cannot pass through the polyester gel coat / laminate but the water molecules can, thus acids build up within the laminate.

As this process continues, at some point, the concentration within the voids will become greater than the concentration of the water the vessel is floating in (sea water). At this point the “osmotic” process occurs and more water is drawn in.

The interface between the glass fibres and the resin matrix can also be broken down. The binder used on the glass fibres (particularly emulsion bound mats with polyvinylacetate binder) are water soluble. This can allow liquid water to pass along the fibre bundles, producing some swelling at the fibre ends and the characteristic “wicking” or “fibre aligned blisters”.

As this continues the internal voids are increased in size and the pressure within these increases; at some point the pressure becomes too high for the surrounding material to support and a blister is formed on the hull surface.

This all sounds a bit dire, however it is important not to over-react. Many boats are used for years in a blistered condition, at this stage the processes are mainly of a chemical nature, with very little loss of mechanical strength. Many boat hulls may take 10 – 20 years or longer to reach the visible blistering stage.

As this osmotic process continues and moisture continues to be absorbed, the laminate break down accelerates. In time larger blisters may develop within the laminate as well as those more commonly occurring between the gel coat and laminate. Eventually at this stage, treatment will be required. Generally at this stage a pungent odour is noticeable and an acid taste will be apparent if a blister is punctured and the fluid tasted. (See later).

 Osmosis (or “Boat-Pox”) Diagnosis

 Diagnosis of the “osmotic” condition and the decision as to what treatment if any is required and when, is made by considering the following factors.

The hull gel coat surface is visually examined for signs of blisters or wicking. The liquid content of any blisters is examined and tested. The moisture content of the hull is gauged using a moisture meter.

With regard to moisture meters, these are only a guise and should be used only as a means of comparing the relative moisture content of topsides and submerged hull.

A visual examination revealing gel coat and visible deeper seated blisters in the sub-strate may be all that is necessary to produce a diagnosis of “osmosis”.

To determine the full extent of the defects and therefore the detail of the complete treatment required it will be necessary to examine the hull laminate after the gel coat has been removed. In some cases additional laminate repair may be necessary,  prior to epoxy coating the laminate (see treatment).

As we saw earlier, there are a number of factors associated with “Boat Pox” as it is sometimes called. The treatment process has to deal with all these factors. Simply drying out the hull laminate and covering with an epoxy paint system will not work.

The laminate does have to be dry, however the removal only of water from the laminate will not result in a long lasting treatment.  The acids and components dissolved in the water also have to be removed. As noted earlier some of these contaminants are hygroscopic (water attracting). These have to be removed from the laminate when still in solution in the water contained in the hull voids.

Forced drying using dehumidifiers and / or heat lamps will tend to remove the water but leave many of the contaminants behind in the laminate. For this reason steam cleaning and washing the hull surface with special detergents is important. The gel coat, although not 100% water proof is still a very effective water barrier and will not allow the passage of larger molecules such as glycols.

For ththe above reason the gel coat has to be removed entirely. This is most effectively achieved by the use of a “Gel Peeler”.  This removes a controlled thickness of gel coat and / or laminate leaving an even smooth surface. Slurry blasting is also used.

Drying using infrared-lamps.

The “Gel Peeler” method results in several problems; firstly, the very smooth surface does not promote rapid and full drying and nor does it provide a good abraded surface to bond to the epoxy coating.  In addition the “Gel Peeler” will not remove softer material within the blisters below the  smooth surface. For these reasons the peeled surface should be slurry blasted as above before drying operations.

As the aim of the treatment is to remove the contaminants from the laminate along with the moisture,  it is generally best if the treatment is carried out immediately after removing the boat from the water at the end of the summer season.

Once peeled and blasted it’s important to wash out the contaminant from within the laminate. This is best achieved by repeated steam cleaning or hot pressure washing. Initially this will probably be on a daily basis and then less frequently for a period of several weeks. It is necessary to monitor this process with both moisture meter readings and using litmus paper to assess the PH of the surface water. When the surface is found to be neutral, a few days after washing the drying process can commence. At first this can simply be air dried but the hull will eventually need to be heated to reduce the surface moisture content to a very low level …. 50 or below (Tramex scale 2) 5 or below (Sovereign scale A)

At this stage coating can commence. The exact specification and application procedures will be determined by the product used and the manufacturers specifications. However in general a solvent free epoxy system, applied by roller, with 4 to 5 coats giving a total application thickness of approx. 1mm, is what is required.

Most solvent free epoxy systems require to be applied and fully cured in controlled temperature and humidity conditions; for this reason generally coating application should be carried out inside a workshop.