Shopping Cart

How To Cure Osmosis Blistering in GRP (Glass Reinforced Polyester)

To cure osmosis blistering in a GRP hull involves a series of operations. These are broken down into preparation and restoration, and are discussed in detail below.

If blisters develop and are ignored, your hull will develop major structural damage. I believe blistering of fibreglass hulls can be prevented for decades, depending on hull condition and repair workmanship. Steve Smith developed this Barrier Coat Technology in 1975 and hulls correctly done since that time have shown no further damage.

There is a sister article describing the causes of osmotic blistering, which fully details the mechanisms behind the formation of osmotic blisters.

We do sell a kit comprising all the things you need to repair osmosis blistering on a grp hull.

cure osmosis blistering

Principle of the Cure for Osmosis Blistering

The essence of the problem is that water permeates the resin system and then causes what damage it can because polyester resin and its additives inherently decompose in water.

The essence of the solution is this: If one can put a totally impermeable barrier between the water and the hull, then the damage could not occur, regardless of how susceptible the resin of the laminate might be.

There is no such thing as a totally impermeable moisture barrier, but the U.S. Navy has, over the last fifty years developed a great deal of coating systems technology. Their remarkable moisture resistance comes partly from the blend of mineral fillers used in the system, which increase the diffusion path length to several times the coating thickness, and partly from the unusually dense and water-repellent molecular structure of the resin system. This reduces the concentration of water in the hull.

The rate of the decomposition chemical reactions depends on water concentration in the laminate. The probability of blistering occurring depends on the rate of the chemical decomposition reactions.

This is the fundamental principle: If water evaporates off the inner hull surface of a fibreglass hull boat faster than it can diffuse through the barrier paint coating, this results in a very low moisture content in the laminate. Theoretically, if it leaves faster than it comes in, the resulting water concentration in the laminate should approach zero, EVEN WITH THE BOAT SITTING IN THE WATER! With the water concentration so low, the probability of blisters developing moves out in time perhaps ten times farther, to many decades or even hundreds of years, making it no longer a concern. The factors that will prevent blistering in all but the most egregious cases are, first, the barrier on the outside, and second, some positive means of ventilation throughout the bilge, and any part of the hull that is below the waterline.

Keep the Bilge Dry

Sometimes it is impossible to keep the bilge dry due to inevitable water leakage around the shaft. In that case use that same moisture-diffusion barrier coating to coat the inside ONLY WHERE THE WATER RUNS down into a sump where the bilge pump sits. That way the part of the bilge that is inevitably wet does not feed water into the laminate, and water vapour is free to diffuse out of the laminate on the inside, where the ventilation can carry it out and away.

Ventilate the Bilge

Ensure that there is adequate ventilation in the bilge so that the water that does migrate into the fibreglass laminate can indeed evaporate from the inside of the hull.

Background of the Cure of Osmosis Blistering

Water decomposes polyester resin

The barrier coat technology for stopping osmotic blisters on GRP (Glass Reinforced Polyester) hulls was invented by Steve Smith in the middle 1970s. Much of the technology was developed at that time with little or no practical experience, but simply as a series of logical deductions from an initial premise “If we reduce the concentration of water in the polyester resin then the chemical reaction (the decomposition of the polyester resin in the presence of water) whose rate depends on water concentration should proceed more slowly” which follows from the most fundamental scientific principles. The reaction that creates polyester resin being reversible in the presence of water:

polyester decomposition chemistry to understand cure osmosis blistering in grp


The resultant organic base is a small water soluble molecule that readily dissolves in seawater. This leaves the acids behind to osmotically draw in more water, and of course to generate the unpleasant odours you may have noticed.

Reduce water concentration around polyester resin to help cure osmosis blistering

In order to reduce this abstract idea to something workable it was necessary to take the first step of “Assume something that ought to work, try it and see if it does the expected.” This first step was in the design of the barrier. Even though liquid organic resins (epoxy, polyester, etc.) are readily available, can be easily spread over a surface and can cure quickly, they are not the ideal material from which to construct a barrier to water molecules.

Minerals, particularly those which do not chemically react with or dissolve in water should be the ideal barrier. There are two reasons for this. First, if the barrier can dissolve in water then a water solution of those mineral salts can obviously migrate through the barrier. Second, water molecules are very small. Minerals of certain types have much smaller spaces between their atoms than those through which water molecules can pass, and have many fewer “polar groups of atoms” which can attract and hold water molecules compared to organic resins, including polyester, vinyl ester, acrylic, epoxy or others.

The ease with which water can move through mineral versus resin barriers may be likened to the ease with which a motorist may travel a five hundred mile highway through mountains with no rest stops nor gas stations, compared to a highway with not only rest stops and gas stations but restaurants, five-star hotels and entertainment every mile.

It was recognized at the beginning that it would be impossible to mould a sheet of glass or ceramic around the outside of a boat, and so a mixture of glass and ceramic minerals was chosen for their ability to fit together as small particles between the spaces of the larger particles. An epoxy resin system was chosen which did not contain any small unattached molecules (“water-soluble molecules” [WSM], to use a buzzword popular in the U. S.) and which was “water-repellent”, much as oiled cloth repels water. The popular epoxy and polyester resin systems are not made this way because their manufacturers design the products for different market applications and at different raw material costs. Much research and experimental work eventually led to the formulation of the barrier coat and the complete solving of all its performance limitations, and the development of supporting products and a coordinated technology for their use.

When all this was tested in the field it was found after ten years of use by professional boat repair contractors and talented amateurs that the technology did indeed seem to work, arresting deterioration, stopping further blistering and repairing damage done.

Refining the process of applying a barrier coat to cure osmosis blistering

Occasionally it would be found that something would not work right, blisters turning up in the middle layers of a repair being the most common failure of the technology. Knowing the fundamental principles of the technology were correct, it was then only necessary to find the exact departure from the ideal scene of “technology correctly applied” in order to remedy the failure. This was not necessarily the fault of anyone but sometimes a learning process for all.

An example of this was the discovery that if one did not rub down the hull with an abrasive pad before applying the barrier coat, [and again, after a coat or two of the barrier coat], microscopic glass fibres which were sticking straight up as a result of the previous grinding and epoxy impregnation steps would stick through the barrier coat, acting as tunnels or wicks for water to travel down, creating a small blister at the base of each. Other discoveries, such as the possibility of incomplete filler mixing, led to the distribution of a black pigment in a small “salt shaker” to be used to give increased colour contrast in the filler and thus ensure complete mixing.

Eventually it appeared that all the inherent imperfections of the technology had been isolated, identified and all barriers to its application handled with standard, workable and reproducible technology. This is the process described below:

Sometimes you cannot cure osmosis blistering

A very few boats have had many osmosis blister repairs attempted over the years, and will forever have problems associated with them. These boats may have been built with organic fillers in their resin, which will make them extremely susceptible to the recurrence of blisters. The sister article, causes of osmotic blistering in grp, to this one addresses this problem in great detail, and instructs how to test your hull if you think you may own a problem boat.

Preparation to Cure Osmosis Blistering

1 – Grind out or Sandblast all blisters down to sound laminate

Get under all of the loose material. Some blisters may have a coloured liquid in them. Don’t get any of this in your eyes or on your skin – it may be very caustic. Some hulls may have an advanced state of chemical decomposition. This may be recognized by a strong chemical smell such as vinegar or a solvent, which may be noticeable after peeling the gel coat or grinding down the hull. An advanced state of chemical decomposition may also be recognized by liquid, apparently water, oozing out of the laminate where a blister has been ground out, and leaving a visible residue which dries on the surface. Such hulls should have the entire gel coat ground off or peeled to expose the underlying laminate containing chemical decomposition products. This may also require removal of some laminate. Chemical cleaning may then be accomplished in any of several ways: the hull may be hosed down with water every hour for a few days; a garden hose with many small holes such as is used to slowly water a garden may be draped around the hull to slowly and continuously rinse the hull with water; or the hull may be launched and left in the water for a few days to a week and then hauled out again. Some fresh water rinsing and a week or three to dry out (depending on weather and temperature) should then give a hull sufficiently dry to continue.

cure osmosis blisters grind out blisters

2a – Either Sand the Hull to Bare Gel Coat

If the damage is not too bad, this will suffice for the starting point.

If the gel coat is not to be entirely peeled, sand it all down to clean gel-coat. Sand any gel coat that is left. You must get past the gloss surface of the gel coat. It is a wax-rich surface, and it may also be degraded by exposure to the water.

Then let it sit a week outside with good ventilation inside and out, and inspect very closely to see if any small blisters on the surface have now become visible as they dried. If there are no visible signs of blisters, then a good epoxy moisture-diffusion-barrier coat (such as International Interprotect, Interlux 2000 or similar) will offer long-term protection if the boat stays in the water all or most of the time. If there are blisters, then follow the advice here if you intend to use the products that we sell.

2b – Or Remove the Gel Coat?

With many large blisters, it will be necessary to strip off the gel coat and treat as discussed here. If the osmotic blistering is particularly bad then it may be required to remove the entire gel coat with a “peeler”.

cure osmosis blisters peel gel coat from laminate

If the damaged laminate is not dry, but rather suppurating strange-smelling liquids, the gel-coat will need to be stripped and the hull re-launched for a week, hauled out again and then rinsed and allowed to dry.

3 – Dry the Hull

Whilst drying the hull will not be necessary if there is no damage, in most cases it will be required to dry the hull. The drying is part of blister prevention. If one does not take effective action, then the need to repair more and more blisters will continue. I personally believe in fixing it once. A person’s labour to fix their boat is usually worth more than the cost of even fancy materials.

it is utterly unnecessary to dry the hull for months with a dehumidifier tent, etc., etc. For mild blistering where removal of the gel coat is not necessary, one to two weeks drying in warm weather (once the hull is sanded to bare gel coat) is sufficient.

If the gel coat were stripped from a seriously damaged hull, a week in warm weather is adequate for the hull to dry sufficiently that the first (moisture-dissolving) coating may be applied.

Drying of a hull needs to be done externally as well as internally. On the outside, a number of those three-foot-square, six-inch thick fans that blow up a storm are what you want, aimed to move the prevailing winds to and past the hull, as well as something to move inside air through the bilges and compartments, and out of the boat. Inside ventilation is even more effective than outside, where the gel coat is largely intact.

Moving air evaporates water faster than anything else, and the removal of excess water in the laminate is the key to a more successful blister repair, whether one uses the fanciest epoxies or the cheapest polyester.

With adequate ventilation most hulls are adequately dry in a few weeks, but there are exceptional situations. A typical turn-around time for the entire job for a franchisee in Europe was three to five weeks, depending on weather and hull condition.

4 – Check the Hull is Dry

Measure the laminate with a non-contact moisture meter (meaning do NOT use the ones with sharp pins that stick into the surface being measured) in an area under the waterline, where blisters are worst, and compare with an area always dry, usually near the topsides. That will give you a clue as to relatively how dry the whole hull is. One such meter is a Wagner L606.

Restoration to Cure Osmosis Blistering

5 – Seal the Voids in the laminate

Apply Smiths Clear Penetrating Epoxy Sealer™ to saturate the laminate. The Cold Weather Formula™ applied late in the day is recommended in all but the warmest climates, where the Warm Weather Formula may be used. This product contains solvents and the choice of which version to use is a solvent evaporation issue. Allow a few days to perhaps a week to dry and cure; then apply a second coat of which little should soak in. In extreme cases a third or even fourth application may be necessary. Allow sufficient time between each application that the hull no longer smells strongly of solvents before continuing; this may take a few days to a week in moderate weather.

6 – Apply the initial barrier coats

Apply two roller coats of the barrier paint, International Interprotect, Interlux 2000 or similar . Use one colour for the first coat and any other colour for the next coat. Use contrasting colours so that no areas are missed when applying successive coats, and so that the depth to which one has sanded is visible and controllable. Allow adequate drying time according to manufacturers instructions between coats, and apply no more that two coats in two days. Allow a day or two for the epoxy paint to fully cure so it can be sanded without dusting. Then rub the entire surface down with an abrasive pad. This is a necessary step. It breaks off any microscopic stubs of glass fibre sticking up through the paint, that would otherwise act as water-wicks and create recurring blisters in the finished job.

7 – Fill all Osmosis Blister Damage Holes

Fill all holes with Smith & Co. Fill-It™ Epoxy Filler. Mix thoroughly according to the application note for thoroughly mixing fillers. Allow to cure overnight, then sand. Stop sanding when you expose the colour of the first layer.

8 – Apply the final barrier coats

Apply three more coats of contrasting colours of High-Build Epoxy Paint. 4 mil wet film thickness per coat is equivalent to 10 sq. m/litre/coat.

9 – Apply Anti-foul paints and finish

Several days further drying time should be allowed to reduce the possibility of solvent blistering. Follow the manufacturer of the barrier paint recommendations for final finishing prior to applying the Antifouling paint. The Antifouling Paint may then be applied below the waterline, and topcoat and boot-stripe above.

What are the Causes of Osmosis Blisters?

The causes of osmotic blistering in boat hulls is described is in a separate article: causes of osmosis blistering

The article is based on an original article written by Steve Smith, a chemist and physicist who has spent his entire life sorting problems on boats. He discovered the cause of gel-coat blisters when the boat manufacturers were denying that there was any such thing. Then he invented the barrier coat technology that is now in common use to prevent and repair blister damage that is described here.

The products required to cure osmotic blistering can be purchased here:

Copyright Steve Smith and Steve Dakin. 1972 – 2017.