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Osmosis involves many boats, including those apparently healthy. Our marine expert make the point on what is usually called " the fiberglass cancer", taking down common senses and incorrectness, often used for business interests

Article by
Gino Ciriaci

C. Di Stefano


1. After some years, antifouling should be removed. It is an operation to be done with care because gel-coat damages must be avoided.
It is incredible: laminate blistering, generally known as "osmosis", this small, apparently unimportant fiberglass disease, which only need attention to avoid or cure, is becoming a test to verify the misinformation and dishonesty of users and yards. Day by day, when talking about fiberglass boats and osmosis or tackling an associated problem or assisting in buying a boat, all I heard were either lies, partial truths and falsehoods displayed with total disrespect with the clear object to either minimize or amplify the problem, depending on the interest of who is selling or buying a boat or who is to repair it.

So here, in a nutshell and few sentences, is what you might hear at the waterfront on osmosis:

    2. Big blisters on a rudder. Sometimes osmosis occur on the rudder, without effecting the hull.
  • osmosis do not exist!
  • it is not osmosis but just a small humidity absorption
  • you do not need to cure it, will go away by itself
  • boats of "that" yard never had it
  • this boat can not have osmosis, because the laminate contains Aramat
  • this boat can not have osmosis because, unlike other similar boats, it has not wood encapsulated GRP structures
  • this boat (for sale) has just had an incipient osmosis, but she was cured with a home-made paint
  • our osmosis curing system is classified and cannot be made public
  • in our town osmosis do not exist
  • the osmotic process is a natural phenomenon which can not be entirely avoided
  • osmosis occur only on most recent boats
  • osmosis occur only on older boats
  • generally osmosis do not appear during the first 4-5 years of boat's life
  • that treatment does not work, because osmosis occur right after curing ( usually said about a good treatment wrongly applied, sometimes on purpose).

And here I stop before I loose completely my temper instead of making my point on osmosis, in a clear and practical way so that difficulties and problems of this fiberglass' disease (which is not a cancer but a measles and on which I already wrote some years ago on Nautica) are identified . In reality, it is very expensive measles, which can be definitely cured by applying, more than technology, honesty only as we shall see later. However, let me please give a brief history on fiberglass.

3. Small bubbles easily hide themselves among the antifouling
In the February 1954 issue of "Vie d' Italia" edited by Touring Club Italiano, at page 255, there is an interesting article titled "Plastic material fashion", where it is stated that "the use of synthetic plastic materials to build small boats thus replacing wood, is becoming increasingly popular in Italy as well. TEXCO, a laminate obtained joining several fibers shutters with resincellulose ethers, melted and stratified togheter, is now available. TEXCO is described as extremely strong material, capable of withstand all stresses (shear stresses, impact loads, drilling..), waterproof, unalterable, weights less than wood, lasts longer; but, so far, it also costs more although it allows time saving as it is moulded".

It is clear that this enthusiastic description is, still today, in everybody's mind and fiberglass is regarded as a long lasting, inexpensive and low maintenance material. Small fiberglass crafts production started in Italy at the onset of the sixties. With a LOA of 2.5/5.0 meters these open boats were not easy to sell since wood was still the most popular boat building material.

But within few years things changed for the better for fiberglass which seemed to be a problem free material versus wood, was inherently adaptable to the industry demands and required a relatively limited trained workforce.

4. A frequent case. Blisters are small but well visible. Cured osmosis occurred again thus bubbles are volcano shaped with a swelling in the middle. The spilling liquid from a punctured bubble is to be noticed.
One curious aspect of osmosis is that it was not noticed until the 80's; I myself, while surveying boats of every type, had come across a few rare cases where hulls had liquid filled bubbles. I could not give a name to the phenomenon which, I remember, I found on some American built boats. As I did not know what to do with it, I got in touch with a British colleague of mine who sent me some publications and where the proper name of osmosis was given to the blisters I had noticed. Since the early 80's, things got harder and harder: Italian boats also started showing strange blisters and studies where undertaken in order to identify, cure and eliminate the disease.

The "whys and hows" osmosis appears have nowadays been fully answered and the phenomenon should be a thing of the past; yet I am still here arguing and yelling, facing a ridiculous situation. how it can be that in 1996 there are still people who does not know what osmosis is and how to go about it? This is the unfortunate reality and my only hope rests on the belief that misinformation and not dishonesty belies it. I will then try to illustrate and discuss issues regarding osmosis in a clear and comprehensive way so that users, as well as professionals who, for various reasons have treated the problem as either non existent or, worst, unavoidable and impossible to cure.


It is a chemical-physical phenomenon, happening during the migration of a solvent, in this case water, through a film separating two liquids with different salinity concentration. This does not really mean a lot, without understanding what really happen in a water immersed fiberglass hull.

5. The same hull of picture n. 7. Once cleaned up hundreds of blisters are visible.
Air bubbles into the laminate, more or less close to the gel-coat surface, are the basic requirement to produce osmosis. First, water migrate through the laminate to fill the blisters; water will then start to solve every material it can: glass fibers, undercured resin, etc. At the end of this reaction, the new solution will attract more water through the gel-coat, so that the pressure inside the bubble will grow and expand.

The phenomenon will go on as long as the hull is in the water; the blister that at the beginning nay measure 3 to 4 millimeters in width will increase, due to the inside osmotic pressure, with time and permanence in the water. In other words, once started, the osmotic process will continue without any possibility to appraise beforehand the speed at which it expands.

It can then be said that osmosis is a degenerative disease which does not stops by itself unless the hull is kept out of the water; even then, and after months of being sheltered, the liquid may be present in the laminate and, due to the fiberglass' permeability, it will move along the fibers. This is why, after a wile, bubbles appear smaller due to reabsorption.

This proceeding characteristic can be "smartly" put to use when selling a boat with osmosis: it is in fact sufficient to keep the boat ashore and sheltered for some time until the blisters are smothered because the liquid inside them has now spread over a larger area and, also, because the antifouling coating has become uneven thus hiding blisters and others defects.

The only way therefore to see if a boat, ashore for some time has osmosis, is to check the hull's humidity with an hygrometer and taking samples of antifouling coats. Care and caution must be exercised during these tests since humidity alone may not mean osmosis: for this to be present, bubbles must be liquid filled, as we shall see later on.

At this point I hope not to have confused the issue too much: all I want to say is that gel-coat osmosis blisters will definitively be liquid filled while humidity absorbed by a hull is not an osmosis generated symptom.

What kind of bubbles will surely indicate osmosis?

    6. Moisture measurement by the hygrometer can be useful when dealing with osmosis
  1. The bubble should be round shaped: this is due to the internal osmotic pressure which is constant in all directions

  2. The bubble must deform gel-coat surface: it may happen that sometimes bubbles are located between different antifouling layers, while the underneath gel-coat is fair after sanding,. In this case it is an antifouling defect only, and there is some solvent or water between plies. This can happen when plies are painted too fast so as not to let the first hand to dry out completely, or when a ply is painted on a wet hull (some humidity is enough) or when antifouling is not properly thinned. My advise is to break some bubbles with your nail: if it brakes easily we are in presence of is an antifouling defect, since it is impossible to break the gel-coat with nails, and a center-punch or a knife is needed. I wanted to be clear on those two situations, because it is commonplace to find bubbles on the hull which are not osmosis generated. Finally, if some bubbles are found just scratch lightly with a knife: if a round bubbles appears on the gel-coat it can be attributed to osmosis.

  3. Another requirement for a bubble to be classified as osmosis, is that it must contain liquid. It may be just a drop on a small bubble, but will be a lot on a large one (say more than 2 centimeters in diameter), to the point that water will spring out when the blister is punctured. Remember that the liquid inside will always have an acetic smell and will feel smeary on touching. This happen because in a bubble filled with osmotic liquid, the polyvinyl clorure which is around glass fibers is converted by the osmotic process into polyvinyl acetate with a characteristical rather strong acetic smell. Another characteristic of blisters' liquid, always smeary, is that it changes color with the passing of time: in a small bubble the liquid has always a bright yellow shade while on the larger ones is darker until it become almost black in the most serious cases.

Is there a relationship between blisters diameter and osmosis' age? Yes it does and it means that a 3-4 millimeters bubble with a bright liquid indicates a very recent osmosis; while a 3-4 centimeters pertain to an old one containing dark, strong smelling liquid and an inside high pressure.

Osmosis age is important, a Marine Surveyor is often called to pass opinions on a second hand boat, either to settle a claim or on other related issues and he must be able to state if found defects are recent and not discovered by the owner or, if it is years old and should have been identified by the same owner. All these issues are of a very delicate nature and usually surface in Court when a boat was bought without a dry hull inspection and osmotic blisters could not be seen: in a situation like this the previous owner will obviously declare that osmosis was not present when he owned the boat; in the same fashion, the buyer will try to demonstrate that osmosis was already some years old and the seller should have been aware of its presence.

In short, for a blister to be of osmotic nature, it must have a round shape, it must be located under the gel-coat or similar surface and it has to contain an oily liquid with an acetic smell.


7. A careful survey of the hull is essential, because antifouling plies can hide the "fiberglass measles"
I am often asked who and what is responsible for osmosis and where this defects originates. I will briefly answer that among the primary causes of osmosis development, I can mention the following:
  1. poor gel-coat quality, in the case it is made with a poor moisture permeability resistance resin: nowadays, in order to obtain the best guarantee of low permeability no solvent resins are used.

  2. poor moulding practice so that a void content in the laminate is crated

  3. use of excessive catalyst during the lamination process

  4. poor workshop environmental conditions (too cold or too humid)

  5. presence of impurity or solvents in the glass fibers or in the resin
Further to these factors, mainly depending upon the material suppliers (glass fibers, catalyst, resins...) and upon the yard standards, some more additional parameters based on the boat's life should be considered:
  • if the boat is kept in the water all year round, dry-docking it only for some short maintenance periods, chances of osmosis formation will increase substantially, due to the enhanced gel-coat and laminate water absorption.

  • if antifouling is often removed and the gel-coat bared (sanders and others mechanical tools are used for this operation), this latter surface can be scratched or damaged and locally producing a reduced thickness. As the gel-coat is the outer skin isolating fiberglass from water, such damages can generate laminate water absorption. This could, although not necessarily, cause osmosis.
On this subject, I have to go back to what I said earlier on: water absorption by itself is not enough for osmosis to occur, this happens only when water reacts with some water soluble substances.

There is however a case when osmosis does not happen even if water absorption occur. If the gel-coat surface shows blisters not rounded but elliptical in shape (narrow and long) and pointing in different directions, most likely we are facing only a minor laminate water absorption instead of a n osmotic phenomenon. It usually involves only the first mat layer and the water does not solve any substances. It is also true to say that this is a fearly rare case, mainly noticeable on older boat were lamination has been done in a very accurate way and according the rules, but where with time the gel-coat has weakened and has allowed some water penetration, without generating osmosis.

Here it is why an accurate survey is a must before concluding that blisters are of osmotic nature.

As mentioned before, water absorption by itself creates small oblong bubbles, fairly noticeable as soon as the boat comes out of the water; if on the other side the hull has been ashore from some time it could be that the previously absorbed water pours out of the laminate, which could be found almost dry after the hygrometer test. In this case the blister could result in a dry cavity and this is one more reason to be careful osmosis diagnosis.


Let's state it clear: a hull either has or has not osmosis.

Then if it has it, its age will not effect the approach and curing methods. It does not make any sense to minimize osmosis presence simply because it is recent, and an older phenomenon will not change the curing method. Young osmosis that has not involved a large surface could allow a 2-3 months period before repairing it; if it is spread on a relatively wide surface with wide diameters blisters, it will be convenient to act as soon as possible.

This concept is widely accepted and confirmed by the fact that Italian Classification Society (RINA) Surveyors, when inspecting hull for osmosis, may grant some limited time exemptions instead of requiring immediate repairs.

But what is the meaning of light, young and heavy, older osmosis?

If blisters are small in diameter and concentrate on a definite hull area not completely affected, we can say that this is young osmosis; if on the other hand blisters are spread most of the hull surface are of large diameter with dark pressurized liquid inside, we can say that we are in presence of old and severe advanced stage osmosis.

Let's remember that a neglected young osmosis quickly switch to an advanced, more and more severe stage.

Why is it more severe? What happen inside osmotic blister if not cured?

It is usually said that osmosis is a degenerative process because, if neglected, it becomes worst and worst: in fact, a blister if constantly kept under water will act as a never ending suction pump continuously widening the blister's diameter. The osmotic pressure thus created, will force the inside liquid to expand not only towards the periphery but also towards the inside of the laminate.

If the liquid will find an effective protection on a resin rich ply, the blister will only grow in peripheral dimensions; if on the other hand, the liquid will find voids in the laminate thickness, the latter will absorb more and more water.

In most cases osmosis remain a superficial problem: if caught at its first stage, it will often involve only the first ply after the gel-coat. If neglected, it will assault the deeper plies. It will then be noticed that a large blister not only create a circular delamination but tend to spread inside: something to avoid as curing takes longer time, is more expensive and more difficult.