Structural Integrity and safety advice for boat or yachts suffering high moisture caused by Osmosis

Osmosis, the pox or what ever name one wishes to give it, instills fear and trepidation in most boat owners of GRP vessels and is one of the most talked about subject when buying a used boat. Below is an article written by Nigel Clegg an expert on the subject whose comments and conclusions I concur with.

A Short Guide to Osmosis and Its Treatment
Written by Nigel Clegg

Mankind has been building boats for thousands of years, but despite the vast experience gained during this time, the ‘perfect’ boat-building material is still as elusive as ever. Indeed, just about every boat-building material ever used suffers at least one major shortcoming; whether it be rot and nail-sickness in wood, corrosion and fatigue in metals, efflorescence in ferro-cement, or our subject here, ‘Osmosis’ in glass reinforced plastic (or GRP) composites.
‘Osmosis’ first caused panic amongst owners of GRP composite boats during the late 1970’s, and has been widely exploited by the marine trade, and the yachting press ever since; but despite this anathema, GRP (or glass-fibre as it is incorrectly known) must be the nearest thing to the perfect boat-building material yet. After all, GRP is comparatively cheap to fabricate; light in weight yet remarkably strong; and can easily be moulded into complex shapes. And despite what you may be thinking as you read this paper, it is almost maintenance free!
Nevertheless, osmosis can be a very real problem, and it only takes a handful of high moisture readings at survey time to render an otherwise sound boat un-saleable, and possibly uninsurable. And herein lies the greatest challenge of all. After more than thirty years of experience, osmosis is a subject that is still surrounded by misconceptions and old wives tales, even amongst those who should know better!

The purpose of this guide is to look at the causes of osmosis, and at ways of maximising the success of remedial treatment schemes, while also considering some of the pitfalls that can be encountered when carrying out repairs. We shall also consider recent advances in treatment techniques such as the HotVac Hull Cure® system.
Those wishing to ask more searching questions on the subject are advised to obtain a copy of “The Osmosis Manual” also written by the Nigel Clegg, which covers this fascinating subject in much greater detail. This document will also be updated from time to time as new information becomes available and opinions change.

So what is Osmosis?
According to my old school books, osmosis is defined as “The equalisation of solution strengths by the passage of a solvent (usually water) through a semi permeable membrane”.

In the natural world, osmosis is used by plants and trees to draw moisture and nutrients from the soil, and plays an essential role in the function of cells in body tissues. The basic principle is shown in Fig 1 below, where a hypothetical container is divided into two separate chambers by a ‘semi permeable’ membrane to form a simple osmotic cell. For our purposes, the membrane could be a polyester gelcoat or an epoxy paint scheme, although many other natural or synthetic materials would work just as well.

If both chambers were filled with an identical fluid, our cell could be said to be ‘in equilibrium’, and there would be no flow of liquid in either direction: but if we increase the density of the fluid in just one of the chambers by adding a solute such as sugar or common salt, the ‘solvent’ will be drawn through the membrane towards the chamber having the greatest density, in an attempt to restore equilibrium.

The fundamental principle here is that ‘stronger’ solutions will always try to draw solvent from their weaker neighbours – but as the more concentrated solution becomes diluted, it must also increase in both volume and pressure – which in the case of GRP boats leads to the all too familiar gelcoat blistering!

This osmotic process (but not osmotic breakdown) can be reversed, either by applying greater pressure than the ‘osmotic pressure’ (as in reverse osmosis water treatment systems), or by simply swapping the two solutions around. But while this phenomenon provides us with a convenient, if rather simplistic explanation for the maladies suffered by GRP hulls, it also leaves some important questions unanswered. The first, and most obvious question must be where do the solutions found in osmotic hulls come from, and how are they formed? After all, a fully cured GRP laminate should be chemically inert (or passive) when manufactured, and so in theory at least, should be incapable of creating an ‘osmotic’ cell. The second, and rather more fundamental question must be how can water pass through a polyester gelcoat or epoxy coating anyway, especially when these materials are widely marketed as being totally impermeable to moisture? To answer the second question first, all organic paint coatings are capable of transmitting small quantities of moisture at molecular level owing to the tiny gaps or ‘holes’ in their molecular framework. Densely cross linked coatings like epoxies and two pack polyurethanes exhibit the lowest moisture permeability, while ‘loosely cross linked’ polymers such as those used in conventional alkyd paints are very much more permeable, and provide only minimal protection for GRP hulls. Similarly, the biocide release mechanisms in most modern antifoulings depend on the free movement of seawater within the antifouling system, and therefore provide virtually no moisture barrier or anti corrosive properties to speak of.
In practice, the size of the ‘holes’ in organic polymers means that gelcoats and paint coatings could be described as ‘selectively’ permeable rather than semi permeable: or in other words, rather like a very fine ‘filter’.
Consequently, while gelcoats and paint films can transmit small amounts of simple compounds like water (H2O) with comparative ease, their permeability to complex compounds like high molecular weight alcohols is significantly lower. This point is especially significant in the formation of blisters in boats hulls.
Some readers may regard these points as rather academic, but they do help to prepare the ground when explaining the causes of osmosis in GRP, and more importantly, when prescribing remedial treatment.

Why do GRP hulls blister?
So if GRP boat hulls do not behave like trees or plants, how do they get ‘Osmosis’, and more importantly, why do they blister?
To answer these questions, it may be helpful to explain that there are three quite separate stages in osmotic process, starting with a brand new GRP hull, (which should, in theory, be chemically inert), and ending with the all too familiar gelcoat blistering.
A brand new GRP yacht will start to absorb moisture through her gelcoat almost as soon as she is launched, and will suffer a gradual increase in laminate moisture content all the time that she is afloat. Initially, this moisture will cause very little damage, and for the first two or three seasons at least, will pass slowly through the laminate and into the bilges, where it will disperse harmlessly and invisibly as water vapour.
When lifted out, any moisture absorbed by the hull should evaporate fairly quickly, as there will be nothing to hinder or retain it. Moisture meter readings can therefore be expected to fall rapidly after lifting.
Yachts built with modern Isophthalic and Vinyl Ester gelcoat resins, which have been used widely since the mid 1990’s, will often show satisfactory moisture meter readings within an hour or so of lifting out. However, the older Orthophthalic resins tend to absorb and retain moisture; so yachts built with these materials can be expected to show ‘high’ readings for at least a week or two after lifting out, even where the laminate is perfectly sound.
This is ‘Stage One’, where the laminate could still be regarded as chemically inert or ‘passive’. Assuming that moisture meter readings fall quickly as described, this would be an excellent opportunity to protect the hull with an epoxy coating scheme, such as Blakes Gelprotect SFE200 or International Gelshield schemes whilst the hull is still in good condition: – and before its too late! However, don’t forget that boats laid up with older Orthophthalic resins can take weeks rather than days to dry, so avoid jumping to conclusions too soon! In this respect, boat owners are often in a better position to use moisture meters and interpret their readings than surveyors.
A correctly applied epoxy coating scheme will provide a better moisture barrier than virtually any gelcoat, and will reduce moisture absorption to levels which are almost insignificant. In most cases this tiny quantity of moisture should pass safely through the hull and into the bilges without causing damage, and on a new boat should prevent osmosis from ever occurring.
A correctly applied and cured epoxy scheme should have a lifespan of at least ten, and possibly up to twenty years, and so will not need to be replaced very often unless it is damaged. But without this protection, the effects of continual soaking in water will eventually take their toll.
The most common problem is that tiny quantities of hygroscopic (i.e. moisture absorbing) solutes within the resin are drawn together under the influence of incoming moisture to form what are called ‘foci’. This usually occurs within poorly consolidated reinforcement or ‘air inclusions’ immediately behind the gelcoat.
In more severe cases, elements of the gelcoat and laminating resins themselves can be ‘hydrolysed’ (or broken down into more basic constituents), to liberate a series of chemically active breakdown products. However, this is unlikely unless the resin is very badly under-cured or formulated.
The onset of this breakdown could well take six to seven years in a hull built with Orthophthalic resins, or perhaps twelve to fifteen years where the newer Isophthalic resins have been used; – but it’s at this point that osmosis starts!
We are now at ‘Stage Two’ in the osmotic process. Outwardly, the hull may still appear to be in perfect condition, but small amounts of moisture will be working overtime beneath the gelcoat, busily trying to destroy the laminate by breaking it down into its original constituents. Laboratory analysis would reveal a plethora of breakdown products in a laminate in this condition, including a variety of acids, alcohols and metallic compounds.
In older boats, the most obvious breakdown products will be acetic and hydrochloric acids, which are liberated by the emulsion binder used in the manufacture of glass reinforcing cloth. These acids give osmotic blister fluids their characteristic ‘vinegary’ odour, and can be readily detected with litmus papers. These acids can also contribute significantly to ‘osmotic pressure’; so most boat builders now avoid using emulsion bound glass behind the gelcoat in an effort to reduce the risk of blistering.
However, some of the most harmful substances are liberated by the laminating resins themselves, and being hygroscopic, (i.e. water absorbing), they actively help to accelerate the rate of moisture absorption, and ultimately, the rate at which blistering develops.

One of the chief culprits here is Propylene Glycol (Propane 1,2 diol), a high molecular weight alcohol used as a ‘water scavenger’ to remove unwanted water from batches of polyester resin after esterification and ‘cooking’. Glycols may also be used as an inert base (or vehicle) for the colouring pigments in tinting pastes; although these are generally avoided in marine resins.
Significantly, propylene glycol was once used by a major French yacht manufacturer as an ‘extender’ for organic peroxide catalyst – in an attempt to improve laminate quality and consistency by simplifying the resin to catalyst mixing ratio. Sadly, the results of this practice were quite disastrous, but they probably did more to confirm the connection between glycols and osmosis than any research project could ever have done.

Three of the more interesting properties of propylene glycol are that it is particularly hygroscopic,1 and has a boiling point of around 188 °C – or nearly twice that of water. And like all alcohols, propylene glycol is ‘polar’; which means that it is readily soluble in water; and once in solution will conduct electricity; so we should be able to find it with a good moisture meter.
Once liberated, these hygroscopic solutes tend to promote a steady increase in hull moisture content, which will fall only very reluctantly after lifting out. Long periods ashore or a spell of warm, dry weather may well show some temporary reductions in moisture content, but re-launching or a few days of heavy rain will soon send readings upwards again. Indeed, having reached this stage, there is very little that can be done to prevent further breakdown.
Sadly, the most widely accepted answer to this problem is to ‘dry the hull out’ with infra red heaters and a dehumidifier, after which the hull is quickly painted with an epoxy coating scheme “before the water can get back in again”!
Unfortunately this rather simplistic approach to high moisture meter readings usually causes more problems than to solves; for whilst a correctly applied epoxy may slow down the rate of moisture ingress, its densely cross-linked polymers will also prevent the escape of hygroscopic solutes – which means that blistering is even more likely to occur than if the gelcoat was left unprotected!
Furthermore, ‘drying’ may provide temporary reductions in moisture content, but it does nothing to remove the solutes which are the real cause of our problems. Osmotic breakdown is not a reversible process, and simply removing moisture will never cure it!
I am not suggesting that abnormally high moisture readings should be ignored; but there is usually no need for panic. In practice, many yachts are sailed for with ‘high’ moisture readings for perhaps ten or twenty years without their owners even being aware that anything is wrong; so probably the best advice at this stage would be to leave well alone, whilst keeping an eye on the problem, and delaying further damage by wintering ashore if at all possible.
In typical UK boatyard conditions, Propylene Glycol will absorb more than 65% of its own weight in atmospheric moisture if allowed to stand in an open glass beaker for a prolonged period.
It is also important to note that, at this stage, the osmotic process is almost entirely ‘chemical’ in nature; therefore mechanical strength – which is what we should be most concerned about – is little affected.
Eventually though, this cycle of moisture absorption and laminate breakdown will accelerate to the point where moisture is absorbed more quickly than it can escape, with the result that hydraulic pressure develops within the laminate, and blisters appear in the gelcoat. This usually happens quite slowly, perhaps starting in a small area of the yachts bottom, but it will gradually become more widespread as the years go by. This is the third stage in the Osmotic process.
Localised treatment can sometimes be carried out, but this must only be regarded as a temporary measure. After all, if it has taken fifteen or twenty years for the port bow (for example) to start blistering, it is likely that the remainder of the yachts bottom will be in a similar condition.
So to sum up, in practical terms, osmosis is probably best defined as “migration of hygroscopic solutes within a laminate owing to moisture ingress, which ultimately results in blistering of the gelcoat”.
However, it is important to stress that the osmosis process progresses very slowly in most boats, and the timescale between stage two, (when high moisture readings are first noted), and stage three, (when the first blisters occur), may be as long as thirty years.
Nevertheless, if we are to treat this osmotic condition successfully, we must remove all hygroscopic solutes from the laminate to restore it to its original neutral (or “passive”) state before a new coating scheme can be applied.
Inspection and Diagnosis: —- Polyester Gelcoat 3) Once an ‘Osmotic’ cell has been created, moisture will pass through the gelcoat in an attempt to equalize solution strengths. The ‘osmotic pressure’ created by this process can exceed 60 psi, and may eventually cause gelcoat blistering. With time, larger blisters and ‘swellings’ may also occur owing to de-lamination and swelling within the laminate mass. Nevertheless, the osmotic process usually progresses comparatively slowly, and it may take anything up to thirty years before the first blisters become visible.
Any type of blistering in GRP hulls tends to be labelled as osmosis, although this is not always an accurate description of the problem. We therefore need to look for symptoms that will give us a clear indication of laminate condition, and any remedial work required:-
Osmotic blisters can vary in size from pinhead to 8 ~ 10 CM in diameter, and will nearly always be fluid filled. This fluid should be checked for acidity by using pH papers; an acidic reading anywhere between pH 0 to pH 6.5 would suggest an osmotic condition, although figures of pH 3 ~ 5 would be more usual. Alkaline readings may be encountered if amine accelerators were used in the lay-up and/or gelcoat resins. Alkaline readings may also indicate failure of any epoxy coating scheme.

Any fluid should be rubbed between thumb and forefinger to check whether it has a sticky or greasy feel, indicating the presence of glycol. If glycol is present, (which is nearly always the case), it must be completely removed if the laminate is to dry satisfactorily, and be successfully treated.
If possible, (and with the owners permission), one or more sections of gelcoat should be removed with a sharp wood chisel to allow examination of the laminate itself. Any laminate that has not been properly invested with resin must be removed before re coating, as it may be physically weakened, and is also likely to harbour hygroscopic solutes.
When examining the laminate, it is also useful to cut deeper into the hull to ascertain just how deeply seated the problem is. In many cases, osmotic activity will have been confined to the layer(s) immediately behind the gelcoat, and removal of these layers in isolation will prove more than sufficient to allow successful treatment.

However, I am now seeing an increasing number of older, heavily laid up yachts where osmotic activity has affected multiple layers of the hull, sometimes extending to a depth of 5 mm or more. Closer examination will usually show that several layers in the laminate are starved of resin, and are therefore comparatively permeable to moisture and any solutes. The shortage of lay-up resin means that any osmotic pressure generated within the laminate is quickly lost, (rather like a punctured tyre), so the osmotic condition must be well developed, and quite vigorous before sufficient osmotic pressure is developed to form visible blisters.
Hulls in which the outer layers are laid up with pigmented resins are especially prone to problems, as any pigment or extender tends to inhibit the ‘wetting’ qualities of the resin, preventing thorough consolidation or ‘investment’. Moreover, some colouring pigments (most notably phthalocyanine blues and greens) are themselves soluble in water, and are known to encourage blistering.

Where these problems are identified, it will often prove necessary to remove all of the affected layers before the laminate will ‘dry’ satisfactorily. Also remember that well consolidated resin on the surface of a laminate may be ‘hiding’ and ‘protecting’ poorly invested material beneath, effectively preventing the removal of solutes.
Removing significant thicknesses of laminate usually means that expensive re-lamination is needed to restore mechanical strength; nevertheless, it is usually much more cost (and time) effective to prepare the hull adequately to begin with, rather than paying for a second peeling operation when the first has been found inadequate.
I would also add that, in my experience, yachts which have been re-laminated with a sheath of epoxy glass fibre after heavy peeling have proved very reliable in service, even where satisfactory moisture readings could not be achieved.

Will it Sink?
We know that GRP boats with osmosis can be difficult to sell, and may have their value significantly reduced. However, one point that we have not yet considered is whether osmosis causes any significant loss of strength or buoyancy in GRP boats.
Many people will have heard horror stories about boats that have sunk at their moorings because of osmosis, and how others have absorbed so much water that they can hardly float! Whilst I cannot say for sure that these rumours are completely untrue, many years of practical experience in the field would suggest that they are extremely unlikely! Nevertheless, safety must always be uppermost in our minds; so these questions deserve serious consideration.
Blistering: Osmotic blistering is usually a comparatively superficial problem, which only affects the protective gelcoat layer on the outside of the hull. The gelcoat itself is rather like a thick coat of paint, typically about 500 μm (½ mm) or 20 thou thick; although there can be significant variations in thickness from one part of a hull to another, and between different hulls.
Crucially though, the gelcoat layer itself has very little mechanical strength, and is used only to provide a glossy, hardwearing exterior finish and to help protect the structural laminate beneath it from the effects of water ingress and ultraviolet degradation. Gelcoats are also notoriously brittle, and will readily crack or shatter if stressed, resulting in the characteristic ‘spiders web’ effect.
Most yachts could be sailed perfectly safely with their protective gelcoat layer completely removed; although they would attract marine fouling very rapidly, and would look unattractive. (Indeed, many Naval vessels such as Minesweepers are designed and built without gelcoats, but are protected with epoxy and polyurethane paint coatings from nebuild).
In most instances of osmosis, where only moderate blistering is visible, the structural laminate will be barely moist, and any reduction in mechanical strength will be negligible. Furthermore, the very nature of osmosis in GRP usually means that moisture and any solutes are concentrated in the layer(s) of structural laminate directly behind the gelcoat, (sometimes known as the ‘skin coat’), so any negative effects are confined to this region.
However, in some ‘very bad’ or ‘advanced’ cases, several layers (or plies) of laminate can become quite ‘wet’, with the result that bundles of glass reinforcement become swollen, and adhesion between the glass filaments and lay-up resin is reduced, resulting in some loss of mechanical strength.
De-Lamination: Osmosis does not cause De-lamination, but if the laminate is poorly consolidated, the internal hydraulic pressure generated by the osmotic process may separate (or de-laminate) poorly adherent layers (or plies) from one another, severely weakening the hull. This effect will usually be identified by visible undulation or large ‘swellings’ in the hull surface, although classic ‘Osmotic’ blisters need not be present. The hull may also appear slightly ‘soft’ if pressed firmly with a thumb nail or a tool, and will sound ‘dead’ or ‘dull’ if tapped gently with a plastic faced hammer.
In this context, the shape and size of any blister formations can give a very good indication of the laminate condition beneath:
Small or well formed blisters usually indicate that the gelcoat is adhering well to the laminate, and that the laminate itself has good inter laminar adhesion.
By contrast, large, shallow and irregularly shaped blisters are usually formed where adhesion between the gelcoat and structural laminate is poor. In some instances, two or more blisters will merge together to form larger blisters, again indicating poor adhesion between the gelcoat and laminate.
Very large or shallow blisters, and unevenness in the hull surface generally indicate de-lamination.
Whilst most yachts can be sailed perfectly safely with blistered gelcoats, symptoms of de-lamination must be investigated by a Surveyor as a priority.
Buoyancy: Contrary to popular belief, the quantities of moisture involved in these processes are comparatively small, and are most unlikely to have any adverse effect on buoyancy.
To put this into perspective, a typical ten metre (33 foot) yacht, will have an underwater area of approximately twenty square metres. If we assume a (generous) average laminate thickness of 10 mm, and an average hull moisture content of 20% by volume (which would be exceptionally high), that gives us a total of forty litres of water; which is significant, but certainly not enough to cause sinking!
In practice, the true moisture content of even the worst laminate is most unlikely to reach 20%. Unfortunately, electronic moisture meters are somewhat misleading in this regard, as they are calibrated for softwood rather than GRP, and their high sensitivity means that just 2.5% of moisture in GRP would give readings of 20% H2O or more on either the Tramex or Sovereign instruments.
Moreover, moisture meters tend to indicate the moisture content of the ‘wettest’ layer(s) in the laminate; and as we have already discussed, moisture in osmotic hulls is usually confined to just one or two plies of laminate directly behind the gelcoat, and so represents a small proportion of the overall hull weight.
De-Humidifiers: Leaving aside moisture meters for a moment, one often hears claims of huge quantities of water being extracted from boat hulls by de-humidifiers. Apart from the dubious benefit of using these machines to help ‘dry’ osmotic hulls, much of the water allegedly collected tends to come from the workshop floor, and the workshop atmosphere generally; rather than from the hull itself.
In practice, the total quantity of moisture in any GRP hull is unlikely to amount to more than a litre or so; equivalent to less than two bottles of wine!
Nevertheless, even small quantities of moisture will have a ‘plasticizing’ effect on polyester resins, reducing their hardness and Tg (Glass Transition Temperature). Apart from increasing the mobility of solutes within the hull, (thereby accelerating the formation of blisters), excessive moisture content may reduce the responsiveness of some racing craft, perhaps making them feel heavier than they really are.

Will Osmosis Do Any Other Damage?
As we have seen, the effect of Osmosis in GRP boat hulls is usually quite superficial. However, I have often heard it said that Osmotic acids will destroy the lay-up resin, making holes in the laminate, and severely weakening the hull.
While it is true that many Osmotic hulls are found to have hundreds of ‘voids’ when their gelcoats are peeled off, these voids will almost certainly have originated as simple ‘air inclusions’ at new-build, which remain hidden within the laminate until they are exposed during osmosis treatment. Similarly, laminates that are found to be severely lacking in resin, and are de-laminating were almost certainly laid up this way.
In this context, polyester resins actually have very good resistance to strong acids, (they are less resistant to alkali), and are therefore most unlikely to be dissolved by the [comparatively weak] acids found in osmotic boat hulls. Furthermore, even if this were possible, there is no means by which the dissolved resin could be diffused through a thick gelcoat, and several layers of antifouling!
Nevertheless, laminates which are poorly laid up, and incorporate one or more layers of poorly consolidated reinforcement are prone to suffer de-lamination as a result of osmotic pressure within the laminate, which will tend to separate the poorly adherent layers causing large swellings. However, we need to be clear that any such damage is not caused by osmosis per se, but by the internal hydraulic pressure generated by the osmotic process.

To Treat or Not To Treat?
Having carried out the basic tests outlined above and assessed the laminate condition, a decision will need to be taken whether to treat the boat or not.
My own experience is that early treatment of osmotic boats tends to be less successful than treatment of vessels with advanced blistering. While this statement may seem to contradict normal precautionary practice, experience has shown that the breakdown process in GRP laminates take some time to reach its conclusion; therefore if treatment is carried out prematurely, it is much more difficult to remove solutes from the laminate, and a reoccurrence of osmosis is much more likely to occur.
Another useful point to remember is that laminates are much easier to treat shortly after a season afloat than after a long period on hard-standing, simply because any solutes in the laminate will be more dilute, and hence easier to remove.
Remedial treatment is sometimes recommended on the basis of high moisture meter readings alone; although in my view this is unwise, as moisture content has no direct correlation to laminate condition; and in any case, moisture meters do not give a sufficiently accurate indication of moisture content to allow a judgement of this importance.
However, the one overriding factor must always be the integrity (and safety) of the hull. Osmosis in its early stages is very much a chemical condition, which has very little effect on hull strength, but if allowed to deteriorate too far, the laminate will eventually start to de-laminate (i.e. separate into individual layers), with a corresponding loss of hull strength.
Fortunately, de-lamination is quite easy to spot owing to the large “swellings” that invariably appear in the gelcoat, which are quite distinct from the smaller, and well defined blisters more usually associated with osmosis.
Clearly then, it is important that diagnosis is only made after careful evaluation of all symptoms, and to be certain that the symptoms really do warrant remedial treatment. The flow chart (Fig 10) overleaf may be found helpful when taking this decision.

Correct and Effective Preparation:
In my experience, abrasive grit blasting and slurry blasting are the two most effective methods of laminate preparation.
The advantage of grit or slurry blasting over other methods is that the blasting process selectively removes soft, damaged or weak areas of laminate, while having little effect on sound areas nearby. Blasting also produces an excellent surface profile, which helps to promote good adhesion of paint coatings, while the enlarged surface area also encourages drying and removal of solutes.
Unfortunately, blasting is a slow, noisy, messy job, consuming large quantities of abrasive grit that must be disposed of safely after use. As a result, grit blasting and slurry blasting operations are now severely restricted in many modern marina complexes, and have been effectively banned altogether in some European countries such as the Netherlands.

A further practical disadvantage is that grit and slurry-blasting methods can produce a very uneven hull profile, which may require extensive filling and fairing if a satisfactory hull profile is to be restored.
These drawbacks have led to the increased popularity of gelcoat peelers in recent years, which are comparatively fast and clean, and produce a smooth hull profile requiring only minimal filling.
Unfortunately, gelcoat peeling has the major disadvantage that it only removes a pre-set thickness of material, and so does not remove or identify those areas where the laminate is soft, weak or under-bound. This is akin to peeling an apple with a knife, and not removing rotten areas; although unlike an apple, weak areas of GRP laminate are not always readily visible!
A further drawback of gelcoat peeling process is the comparatively smooth surface produced, which makes drying and removal of solutes an even more difficult task.
Where possible, the best compromise is to use a gelcoat peeler to remove the bulk of unwanted gelcoat and laminate, followed by moderately aggressive grit blasting to selectively prepare the laminate surface. A pressure washer fitted with a grit blasting attachment is often ideal for this purpose.

Alternative Methods:
Alternative methods of preparation include grinding and the use of heat guns.
Grinding is generally unsatisfactory for preparing large areas as it disperses significant quantities of dangerous dust into the atmosphere, and also produces a very smooth surface. Nevertheless, grinding can be useful for preparing limited areas of a yachts hull, and especially around the keel up-stand and fittings where a gelcoat peeler cannot be used.
Where grinding methods are used, all personnel in the vicinity must wear suitable respiratory protection to guard against the inhalation of irritant glass particles.
Heat guns should not be used on GRP as they can easily heat the laminate beyond the Glass Transition Temperature (or Tg) of the laminating resin, resulting in distortion and de-lamination of the lay-up; especially if it is very wet. Heating a GRP laminate strongly is also likely to generate toxic fumes, especially where old antifoulings are present.

Blisters Types explained
There are basically four types of osmotic blisters one of which has three sub-types.

Type A
Blisters range in appearance from minute protuberances which appears as a general surface roughening of the gel coat in small blisters of about 1 mm diameter. The observed state could represent the early stages of other types considered later or the condition could remain static for a considerable period without any further development in either size or extent of the blistering. This condition has no significant effect on the strength of the hull laminate, nor, unless widely spread, on the protection afforded by the gel coat and needs no remedial action.

Type B
These characteristically follow the line of the glass fibre strands and may either be a train of small pinhead blisters or, alternatively, may form an elongated ridge. This defect can be anticipated to develop by a coalescing of the blisters into a ridge like form cracked at the apex. Like Type 1 this condition has no significant effect on the strength of the hull laminate, nor, unless widely spread, on the protection afforded by the gel coat and, again, needs no remedial action.

Type C
Blisters can only occur on vessels built with a double gel coat. They are generally 5 to 15 mm in diameter and on a boat that has recently slipped are typically dome shaped. With time out of the water the blisters tend to flatten. When not already broken, the blister may be ‘popped’ by applying pressure with a sharp pointed tool whereon a characteristically vinegar smelling clear fluid emerges. Underneath the bottom of the pit will have a smooth, often glossy, appearance with no evidence of a glass fire or fibre pattern showing. Blisters of this type if rectified may not recur or, again, may break out on a different part of the hull later. There is some merit, therefore, if the defect is not widely spread in letting the defect develop to its full extent and undertaking a single repair at a later stage. The defect, though unsightly, is not structurally significant and repairs may be safely deferred until a suitable time.

Type C2
Are similar in appearance, when the blisters are unbroken, to Type C1. The difference appears when the bottom of the pit is examined in that the glass fibres are exposed though they may well be covered with resin. No dry glass will be visible. In general the pit will also be deeper than that for Type C1. This type of blister leaves the laminate susceptible to moisture ingress and wicking and is structurally significant and does need remedial action.

Type C3
When unbroken, the Type C3 blister presents the same outward appearance as the previous Types C1 and C2. On breaking open, however, this type is characterised by the presence of resin free, often fluffy, glass fibre strands. The cavity beneath is usually deeper than that found beneath the blisters of Types C1 and C2 and may contain foul liquids even for a considerable amount of time after the vessel has been slipped. This type of blister requires early rectification and remedial treatment should not be delayed for any significant time. With this type of blister progressive increase in the defect size may be anticipated and the possibility of an area of delamination developing cannot be discounted if the repair is unduly delayed. This type is also frequently associated with wicking. Whilst not of immediate significance to the hull’s integrity when first detected, leaving a blister of this type to develop over a number of months may lead to a localised weak area in the hull.

Type D
As opposed to the sharply raised dome shaped blisters considered earlier, the Type D blister is characteristically broad and flat, often most readily found when viewing along the surface of the hull laminate or by touch when running the finger tips over an apparently smooth surface. Blister size is typically 10 to 50 mm in diameter and raised some 1 to 3 mm above the surface of the surrounding laminate. This broad flat blister is typical of defect which lies beneath the gel coat and the first reinforcement layer or even deeper still. This defect does not mean that the hull is in imminent danger of structural failure, although it is indicative of a void within the laminate where water may be accumulating and, by osmotic pressure, tending to extend to the affected area. In time, laminate integrity will be affected and early remedial action must be taken.