The Facts About Osmosis

The problem of blistering in glass reinforced plastic (GRP) ‘bottoms’ goes back some years now in both New Zealand and overseas. Numerous articles have been written on the subject. There are regular features both here and in overseas magazines. The occasional one is good and informative and up to date, although the majority in my opinion miss the point and are well out of date with what we have learnt and experienced here in New Zealand. Increasingly I see problems on GRP ‘bottoms’, meet very disappointed owners that have put good money into what they thought was the ultimate cure.

For several years now I have been working on perfecting materials and techniques for osmosis repairs that are proving exceptional.

I don’t profess to know all there is to know about GRP and osmosis but my 27 years in the industry have lead me to the position where we can confidently repair by far the majority of osmosis affected GRP craft.

My developments are based on what I believe are logical thinking, practical techniques and of course the best of materials.

Firstly some history:

All gelcoats, resins, coatings, etc, used in GRP boats are permeable, i.e. all let in moisture to some extent. Even epoxies, the most waterproof resinous coatings available, are permeable. It is imperative to have a solid ‘resin rich’ laminate, as free of air voids as possible behind it. Without a good laminate, all is lost.

With these permeable coatings and resins you may well ask, how can you win? The short answer is, with some difficulty.

There are however enough good craft around with no visible signs of deterioration to prove it is possible to make good GRP craft that do not deteriorate in a short span. Moisture levels can be high in GRP hulls due to the permeability of resins and the water solubility of binders in the glass, but not all hulls will show blistering. The main concern is hulls that blister due to the corrosive, breakdown effect of the resin, and in part, the glass, because these hulls lose significant structural strength.

In the 1970’s most female moulded GRP boats were made with Polyester orthophthalic resins and gelcoats with P-mat chopped strand mat and woven rovings. It is significant that many early boats made with these materials do not, to this day, show any osmosis blistering. They can be high in relative moisture levels, but still not blister. In those days there were few manufacturers of big permanently moored GRP craft.

From polyester to orthophthalic resins, the majority changed to polyester isophthalic, and then in more recent times some manufacturers have used Vinylester resins in parts of the laminate.

Although the resins have changed, supposedly for the better, there is still a lot of blistering these days, and some manufacturers consistently turn them out.

This is far from saying that the resins are solely to blame.

It is significant that most of the early craft used a ‘powder bound’ chopped strand mat (CSM). In the middle of the 70’s, many manufacturers changed to ’emulsion bound’ mat. This binder held the glass together and made the glass far harder, like cardboard, so it was easier to handle. The P-mat we originally used continually fell apart when handled.

I believe osmosis blistering became more prevalent with the introduction of E-mat CSM and the high wet out resins that came out at this time. Many manufacturers have been back using P-mat in recent years and this, with tissue in some cases, has been recognised for several years now as the only mat to put in the initial glass laminate behind the gelcoat. The choice of glass is most important for making quality craft. The standards of various chopped strand mats, and for that matter gunstock, vary significantly and only the best should be used in hulls. I am only referring to CSM glasses, as these (along with tissue from some boat manufacturers) back up the initial laminate before woven roving materials or the improved double bias or triaxial glasses used in more modern craft.

Osmosis problems are invariably located in the initial CSM laminate, although osmosis can get into woven materials if these are located reasonably close to the outside surface and it can, occasionally, get deep into the laminate.

Ratios and Cures

It’s all very well having the materials, but we still have to put them together with good resin to glass ratios and good cures.

The biggest single item missing in GRP craft is resin. Such things as correct temperatures and humidities during construction go towards making good craft, but if the laminate lacks resin, you’re asking for problems. The lack of attention to this detail by some manufacturers amazes me. Lloyds reference figures in their ‘bible’ state ratios of resin to glass, for all the different types of glasses. If these figures were measured and monitored more accurately during production, we would have fewer osmosis problems.

I can grind into craft 15 to 18 years old that might be just developing blisters and invariably, not very deep into the laminate, I find a lovely ‘resin rich’ laminate, bronze in appearance. Craft that develop osmosis blistering in a short time (in one case as short as five months), invariably reveal a milky, fibrous-looking laminate in which I can nearly ‘roll’ the fibres with my fingers – decidedly lacking in resin. Craft that blister early in their lives are prone to be bad for the rest of their days.

The other disappointing aspect in many laminates I grind into is the amount of air in the laminate. Many boats have the appearance of not being metal rolled to pack the glass down and eliminate the air, or they have been metal rolled by running the roller round in circles and breaking the air up. This leaves the air in the laminate and doesn’t push it to the sides and out of the laminate.

Many hulls have the appearance of being ‘bulk’ laid up close to the gelcoat, i.e. five or six ounces of glass have been laid at once. Resins are made that allow for this without too much heat build up, but it is far easier to get air out of thin laminates than thick laminates.

Laminates next to the gelcoat must be in thin layers and allowed to gel before proceeding.

Five Blister Types

I divide osmosis and related blister problems into five key areas, which I will discuss in order from surface coating blistering, through to deep osmosis blistering. I have developed successful methods of remedial repair for all of these blister conditions. I am a firm believer, with this type of work, in repairing what is necessary and leaving areas that are not showing any signs of deterioration. If partial work is done, annual inspections are recommended to help keep the underwater areas in as sound a condition as possible.

First, it must be understood that all materials are permeable in GRP boats. Some blistering is in the cosmetic coatings only and although not a structural problem, is eliminated because it is highly recommended that moisture be kept as far from the laminate as possible and practicable. When blistering develops to what I call osmosis type in the laminate, there is a breakdown of the resin in the blisters – and partly in the glass – and significant amounts of strength are lost from the structure. The osmotic effect creates pressure within the void and the ‘breaking down’ and corroding effect of the resin and glass cannot be treated lightly.

To do all the following repair procedures, you have to find sound, solid, dry laminates and surfaces to rebuild on and it takes experience to determine the required surface. Not all repair work is done with the same type of resin. It is necessary to use resins with different characteristics for different situations.

Hull Analysis

I analyse hulls by grinding into the surface coatings, then down to the gelcoat, then into the laminate and progressively deeper to determine the layers that are blistering, and exactly where the problem is. I decide on the appropriate course of action to take from there. This takes experience and is not to be analysed lightly, as you have to be able to recognise the different blisters and the different coatings and replace for the appropriate situation. The correct analysis is imperative for all interested parties as all repairs cost money and it must be spent wisely.

Surface Blistering

The first type of blistering is in surface coatings on the gelcoat. Over the years, GRP hulls have been coated in a variety of paint systems to protect hulls. They range from Reaction Lacquer in the 70’s, followed by a progression to PGH subcoat (a rubberised coating), to Pita coat (an epoxy coating used on swimming pools), to straight epoxy resins – some with solvents, others without. Some manufacturers have progressed through many coatings over the years. Some have been successful at times, but on other boats, they have had no noticeable effect. The underlying laminate is usually the reason for the success or otherwise, of different types of coatings on particular craft. If boats are badly laid up, lacking in resin and riddled with air voids, moisture will get to the bad laminate despite the coatings.

Sometimes the problem is caused by lack of sanding of the gelcoat or not sanding with the right grade of paper. Other times it is under cured coatings. Experience is necessary in what the different types of coatings look like, to evaluate problems.

If blistering is in the coatings, all blistered layers are scraped or sanded off, often including the layer below, and are replaced with a suitable paint system.

Epoxy is not recommended over gelcoat on second-hand boats as boats that have been in the water six months or more will have absorbed too much moisture into the gelcoat and 18 months or 2 years later, off it all comes. Epoxy is too waterproof a coating to put on moisture-contaminated, aerated coatings like gelcoat.

We have had success with epoxy over gelcoat on ‘new’ hulls, as long as they are sanded with the right grade of paper. If using epoxy it is also imperative that all the coats are applied wet on wet to eliminate chances of the epoxy delaminating between coatings. Many of us have thought that epoxy would solve all permeability and blister problems, but it’s not quite so simple. There are often problems thrown up that even the chemical experts cannot explain.

Gelcoat Blisters 

Next, moving progressively towards the laminate, are gelcoat blisters. These are blisters within the gelcoat and there is usually moisture in them. They are usually found in clusters, regular in size, about 1/8″. Invariably they are in areas where the gelcoat is very thick (maybe 40 or 50 thousandths of an inch or more). Lloyds recommend 20 to 25 thousandths. The blisters are found more often in sprayed gelcoats than brushed or rolled ones. The gelcoat has probably been put on with too heavy a pass and trapped air has not escaped before the gelcoat has gelled.

To eliminate these, I recommend sanding back through the gelcoat to the bottom of the blisters, i.e. eliminate, maybe, half of the gelcoat, before washing, drying, priming, fairing and re-antifouling. It is desirable to eliminate these blisters, (although not structural) as there is usually moisture in them which is getting closer to the laminate. Again epoxy resin is not recommended.

Small Sparsely Placed Blisters 

The third type of blisters are small, sparsely spaced osmosis type blisters in the initial layers of glass.

I always evaluate the extent of the blisters first. If blisters can be counted either in dozen lots or even hundreds (depending on the size of the hull) and they are individual enough, they can be treated on an individual basis. I believe in doing the least work required and then having annual inspections to keep problems to a minimum. I developed this repair method many years ago and although the types of fillers used have been changed, the method has not. While my method may look daunting it is effective and works. With older hulls especially there is no real problem in scalloping out small blister areas on an individual basis, as invariably there is plenty of glass in their hulls. Craft of the 60’s and 70’s and even those of the early 80’s fall into this category with many overbuilt. Unfortunately many hulls of the mid to later 80’s are much lighter in construction, for obvious performance and economic reasons, and this means this method of repair cannot be contemplated. Repairs of these lighter craft can be much more expensive.

We have all seen owners under their boats drilling out blisters. While this has worked in some cases, it is a bad practice and one prone to failure. In all osmosis blisters there is moisture that has wicked along the glass strands and moved out into the laminate from the blister centre. The resin is usually badly ‘corroded’ away in these blister areas. I have seen these little drill plugs literally fall out and I’ve seen people literally drill right through their hulls.

Deep Osmosis Blisters

The fourth type of blister, less common than the other types, are larger, deep in the laminate type but not in great numbers, and they can be treated individually. When blisters show large, on the surface, they are always deep in the laminate. These blisters are often half to three-quarters or so through the laminate and are very destructive. Intense pressure develops inside them and they literally ‘blow’ the laminate apart. When the moisture level is reached during grinding, the release is like from a water pistol. You can readily see where the laminate has been ‘blown apart’. We grind these out by going down into the laminate, past the corroded resin and wet, soggy glass, and then ground outwards until we find a sound perimeter.

Again, you have to be experienced to know what the laminate should look like and preferably know the relative moisture levels before rebuilding can take place. Rebuilding areas deep in the laminate is done with vinylester resin and P-mat CSM. If the blistering is so deep we have gone through woven layers, these must be replaced also. Large ‘feathered’ edges must be presented, so that we get bonds over a good distance and not short steep edges which can cause strength problems in the specific areas. The glassing is brought up almost flush with the hull before fairing with a thin filler mixture, then priming before antifouling.

Vinylester resin is used because of its great ‘stickability’ – one of its best features – and its great resistance to permeability and corrosion.

Extensive Osmosis Blistering 

The final stage is extensive osmosis blistering. These can be reasonably deep in the laminate and cover all, or virtually all the underwater areas. They often travel up into the boot top area.

These are the expensive ones as total re-glassing is necessary. The outer laminate is badly corroded and wet. There is no way to differentiate between where the blisters start and where they finish and there is no alternative but to eliminate all coatings and contaminated glass. Often boats that have had full osmosis jobs done previously, and are blistering again, come into this category. All coatings and contaminated glass are removed to find a sound, solid and dry base. Remedial rebuilding is done with vinylester resin and P-mat CSM, before fill fairing, priming and reantifouling. Again if woven type materials are eliminated, they must be replaced.


Boats are expensive items and becoming more so, and I am a firm believer in regular inspections of the bottom of GRP craft. Problems can be detected and if blisters are developing, removal with proven procedures and materials means the hull should not blister in the repaired area again. GRP is an excellent medium to construct in, but it has its side effects and for owners to protect their investment certain maintenance must be addressed.

It is a continually changing field and one day, no doubt, there will be “perfect” resins to mix with “perfect” glasses and “perfect” people, perhaps robots, to put it all together and produce perfect hulls.