What is Osmosis and how is it treated?
||Peeling blister getcoat with
The following is a brief explanation of the condition
usually refered to as Osmosis or Osmotic Blistering
and a general outline on my position on the subject.
The term osmosis was originally used to describe the biological process
where by a liquid (usually water) will pass through a semipermeable
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.
The term osmosis was coined in the early 70s to describe
the blistering found on many GRP boat hulls and is now in common usage.
The osmotic process probably does occur within the blisters 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 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 even within a well built
GRP hull there will be small voids, air pockets and microcracks within
the resin matrix and at the interface between the resin and the glass
Water can diffuse into, and through the gel coat and the laminate (the
polyester not the glass fibres) as water molecules, not a liquid. A boat
hull can absorb a maximum of approx. 2% water in this way. Water may pass
slowly through a GRP hull 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 she is afloat, the moisture content will similarly
reduce slowly when she is out of the water.
As mentioned earlier there are various small voids within the laminate.
The water molecules can collect and condense with in these. Within the
GRP laminate and the micro-voids are various water soluble components.
These are solvents, by products and unreacted constituents from the manufacturing
The water within the micro-voids is able to dissolve and
chemically react with these components. This process is known as hydrolysis.
Hydrolysis will continue with the voids enlarging, a dissolved
solution is formed, the main ingredients being, acetic and hydrochloric
acid and glycol. These products give osmotic fluid its characteristic
vinegary small and greasy texture.
The glycol in particular is hydroscopic (water absorbing).
Once this is released in the voids it will accelerate the rate of water
absorption into the laminate. This process will now continue and will
not be 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 various hydrolysed products cannot pass through the
polyester gel coat / laminate but the water molecules can.
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
As this continues the voids are increased in size by Hydrolysis
and the pressure within is increased by Osmosis. At some point
the pressure may become too high for the surrounding material to support
and a blister is formed.
This all sounds fairly alarming, however it is important not to over react.
Many boats are used for years in this condition, at this stage the processes
are chemical with very little loss of mechanical strength of the laminate
/ hull. Many boat hulls may take 10 - 20 years or longer to reach this
As this process continues, moisture continues to be absorbed, the laminate
break down accelerates and more blisters are formed. In time some 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 (See later).
Diagnosis of the osmotic condition and the decision as to
what level of treatment if any is required and when, is made by considering
a number of factors.
The hull gel coat surface is visually examined for signs of blisters or
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 one tool and have their
limitations. A diagnosis based on meter reading alone is flawed. Relatively
high readings on older hulls in particular is not necessarily an indication
of osmosis or poor laminate condition.
At the other extreme, a visual examination revealing extensive gel coat
and deeper seated blisters may be all that is necessary to produce a diagnosis
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 requird prior to epoxy coating the laminate (see
As noted earlier there are a number of factors associated with osmosis.
The treatment process has to deal with all these factors. Simply drying
the hull and covering with an epoxy paint system will not work.
The laminate does have to be dry, however the removal of just water from
the laminate will not result in a long lasting treatment, the contaminants
and components dissolved in the water also have to be removed. As noted
earlier some of these are hydroscopic (water attracting) and in general
are large molecules. These have to be removed from the laminate when still
in solution with in the water.
||Drying using infrared lamp.
Simply drying or forced drying using dehumidifiers and or heat lamps
will tend to remove the water but leave many of the contaminants behind.
For this reason steam cleaning and washing the hull surface 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 this reason the gel coat has to be removed. 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
The surface left by the Gel Peeler however has several problems,
firstly the very smooth surface does not promote drying well and doesn't
provide a good abraded surface for a good mechanical bond for the epoxy
coating. In addition the Gel Peeler will not remove softer
material within the blisters deeper than this smooth surface. For these
reasons the peeled surface should be grit blasted after peeling.
As the aim of the treatment is to remove the contaminant from the laminate
along with the water it is best to initiate treatment, peeling and drying
shortly after the vessel has been lifted ashore after a sailing season.
If left ashore for some time (a winter say) before initiating treatment
some of the water may have been removed and it will be more difficult
to wash out the contaminants.
||Steam cleaning hull
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,
over coating, application procedures etc 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 a summary of what is required.
Most solvent free epoxy systems require to be applied and cured in controlled
temperature and humidity conditions. For this reason generally applications
should be carried out inside a workshop