Broken blisters in gel coat approx 2mm diameter
I have recently surveyed the hull on a Fisher 25 following the removal of the antifouling by Symblast. The hull was covered in 1-2mm and deep wide broken blisters / bubbles. Some of these were smooth on the inside and some were a bit rougher and these each had a single strand of fibre glass exposed.
My first thoughts are that the aeration bubbles are caused by air in the gel coat during production rising as it sets. It also turns out that they are caused by impurities in the resin mix. I am reproducing here some text from the International paints site.
Understanding and Assessing Osmosis
Introduction
Glassfibre became prevalent in the mid 1960’s. It was the first boat building material that relied on the boat builder chemically creating the material at the construction stage. The convenience and consequent popularity of this method of building is reflected in the speed with which GRP/FRP became the dominant boat building material.
As with many new products, a number of claims were made. Two of these claims – namely that GRP/FRP was both maintenance-free and everlasting – were over-optimistic. The myth about GRP being a no-maintenance material was dispelled within a year or two of its introduction; fouling grew well on GRP and therefore antifouling had to be applied.
It was only in the mid 1970’s, however, that evidence emerged of a much more serious shortcoming of GRP. Simply put, GRP deteriorated underwater, in some cases very early on in the yacht’s life. This deterioration process has become known as ‘osmosis’, after the physical mechanism by which many cases of deterioration are caused, although it is not “osmosis” in the strictest definition of the term.
In the particular context of yachts, the term ‘osmosis’ has come to be applied to a whole spectrum of gelcoat and laminate defects, which usually manifest themselves as blisters in the gelcoat, in most cases after immersion in water.
This text includes:-
An indication of the main gelcoat and laminate defects, and their causes.
Assistance to surveyors, boatyards and boat owners in identifying the type of problem that may exist in a GRP/FRP hull.
Specification of protective and remedial treatment schemes developed by International, based on a detailed scientific understanding of the problem.
Overview of the Osmosis Research Programmes
In the period following the emergence of osmotic blisters as a problem affecting GRP yachts, it was thought that the paint was at fault.
International therefore initiated major research into the subject, with the intention of understanding and eventually solving the problem.
Very early on, however, it became apparent from a thorough examination of many sets of paint flakes that the destructive pressure was building up not in the paint film but behind it, within the GRP, and that it was this internal pressure that was causing the paint to blister. A programme of investigation was launched to look more closely at the laminate and gelcoat used in yacht hulls. The characteristics and differences of osmotic laminates and osmosis free laminates were isolated and compared in order to establish clearly where the problem began and how it was propagated.
Further research identified the major undesirable chemical reactions taking place in the laminate, and has contributed to the development of products and treatment methods that significantly extend the life of yachts.
A series of tests and trials to determine what produced the best barrier properties was undertaken at the same time as numerous practical applications to confirm the characteristics necessary to ensure ease of application in a boatyard environment.
The result has been two key products that are at the heart of the Gelshield system. These were originally Gelshield and Gelshield 200. With the advancement of ongoing research and the availability of new materials technology Gelshield has now been superseded with the introduction of Gelshield Plus.
General Symptoms and Causes of Osmosis
Polyester resin is used in the lamination of most “GRP/FRP” yachts. This resin, which comes in the form of a viscous liquid, is manufactured by the reaction of a polyfunctional acid with a polyhydric alcohol. A linking reaction takes place in which the water generated in the reaction of the basic ingredients has to be removed.
To turn this into a solid, the boat builder adds peroxide, which acts as a catalyst and causes the viscous liquid to react and become converted into a solid.
The process of osmosis is usually attributed to one of three root causes:
Water ingress from outside the yacht.
Water ingress from the inside: bilges, for instance.
Reactive impurities in the resin.
Water ingress has been found to be the cause of the problem in about 85% of all cases analysed. Such cases can now be treated with good effect. In this situation water reacts with the impurities in the laminate or the polyester itself, or the coating agent on the glass reinforcement to form free acidic substances. In this process, known as hydrolysis, the resin becomes broken down into its component parts by the water. However, it is worth noting that all polymeric materials (plastics) are permeable in water and to water vapour to some degree but it is the fact that in the manufacturing process of the resin certain traces of un-reacted constituents leads to breakdown and ultimately the onset of what is referred to as “osmosis”.
The remaining 15% of cases, where reactive impurities or lamination deficiencies are the cause of the problem, are the most difficult to treat. It is possible that the chemical reaction at the resin-manufacturing stage is incomplete; that is, about 0.1% of the acid or alcohol remains ‘free’ in the resin. In such cases, the resin supplied to and used by the builders is incapable of being completely reacted. Effectively, the ‘free’ molecules of acid or alcohol are available to participate in secondary, undesirable and unintentional chemical reactions within the laminate. Additionally the peroxide catalyst and any other accelerators used do not form part of the solid matrix and similarly exist as impurities in the laminate. Trace constituents are known to react with elements in water generating compounds that create pressure, which is evidenced as blisters in the gelcoat.
The problem often manifests itself in the first two or three years of the hull’s life. Such cases exhibit a great deal of variation, and even when a successful analysis has been made it is not always possible to stop the reaction continuing. Treatment of such cases is, therefore, sometimes impossible and cannot be considered 100% successful although it may well prolong the hull lifespan.
Stages of Osmosis
Stage 1 – Water Penetration
Water passes through the gel coat and Water enters laminate
There are Voids in the Laminate and Gelcoat
Stage 2 – Solutions form in Voids
Initially in the Gelcoat, where the moisture content is higher. And then deeper in the laminate as the condition progresses.
The moisture present starts to break the resin down by hydrolysis.
Stage 3 – Blister Formation
The concentration cells formed draw in more moisture that causes blisters to form and swell.
The increased pressure also tends to accelerate the breakdown of the laminate resin.
Stage 4 – Laminate Failure
Continuing resin breakdown and increasing pressure in blisters leads to some blisters bursting.
Deeper in the laminate, large blisters cause laminate breakdown and eventual failure.
Contributory Factors
There are a number of factors at the building stage that can combine to produce a hull lacking the ability to resist osmosis over an extended period of immersion. These factors include both the standard of raw material used in the construction, the workmanship, or a combination of both.
Raw Material Issues
Emulsion-bound glass; Glass matt requires a coating agent on the fibres to hold the fibres in place prior to use and enable resin to adhere to it. When an emulsion (usually a modified PVC/PVA compound) is used, this is water-sensitive; it reacts with free water in the laminate to produce the characteristic pungent, vinegary fluid (acetic acid) found in blisters. Powder bound matt is preferred for this reason.
Porous gelcoat caused by under-reaction of resin at its manufacturing stage.
Water-sensitive pigments used in the gelcoat. Certain blue and red pigments are known to be hydrophilic.
Water in the resin: During the manufacture of the polyester resin quantities of water are produced. This water should be removed at the manufacturing stage; occasionally, small quantities remain as an impurity.
Glass matt stored in a damp place and then used in a slightly wet condition.
There are a number of cases where the raw materials used in construction are sufficiently far off standard as to have made the entire laminate unstable. Two of the more common examples are:
Acid-rich resin. Occasionally, in the production of the resin, an excess of acid remains after the acid/alcohol reaction stage. This leaves free acid, making the resin prone to blistering. The reverse could happen and there could be alcohol excess.
Peroxide catalyst strength below requirement. The peroxides used as a catalyst in the production of GRP are relatively unstable substances with a limited storage life. Use of old, out-of-date or poorly stored catalysts can result in a seriously under-cured resin.
In these cases, the reaction is irreversible once started, and the hull cannot usually be successfully rectified.
Workmanship Problems
Under-cured, soft gelcoat caused by insufficient catalyst.
Brittle gelcoat caused by excessive amounts of catalyst, often evidenced by ‘star’ crazing.
Aerated or pinhole-blistered gelcoat, reducing its effective thickness.
Gelcoat not bonded to the laminate, due to the gelcoat being allowed to cure for excessively long periods before commencement of lamination.
Matt behind gelcoat not wetted out with resin, allowing moisture to ’wick’ down fibre and into laminate.
Stray matt fibres pushing into gelcoat, reducing its effective thickness.
Poor wetting-out of fibre on the hull interior, allowing water to be drawn in from the bilges.
Resin-to-glass ratio: the manufacturer normally defines the correct resin/glass ratio for a particular laminate. When the resin percentage is allowed to fall significantly, a dry and porous lay-up results.
Poor resin-to-glass adhesion affected by the type of size or coating agent applied to glass fibre during manufacture.
Water Temperature and Salinity
For all types of osmosis, the laminate temperature, defined by the temperature of the water in which the hull is immersed is a key factor. The process of osmosis is based on a series of chemical reactions and as such a warmer laminate will degrade faster than one in colder conditions.
The salinity of the water is also important. Water not only seeks to fall to a level but also will seek to dilute any concentrated solutions. Thus, the moisture attracting effect of the highly concentrated solutions in the voids and blisters is most severe in fresh water where the concentration difference is greatest.
For these reasons, two identical boats will display apparently different resistance to osmosis if one is kept in a fresh water lake where it is warm enough to stay afloat all year around and the other is in a salt water environment where it freezes each year and the boat spends several months out of the water.
Identifying Potential Problems at an Early Stage
Visible Evidence
By inspecting a hull with the aid of a strong magnifying glass (10x or greater), it is possible to see on the hull surface indications as to whether the laminate is prone to water absorption. The following defects, if seen above the waterline, are likely to be repeated below the waterline, underneath the antifouling where they can often escape notice.
Star crazing. Evidence of star crazes indicates that the gelcoat is brittle and may have been rapidly reacted. Water will seep in through the cracks.
Micro-cracks. Any micro-cracks in the gelcoat will exhibit the same tendency.
Pinholes. Small pinhead-sized bubbles in the gelcoat that will either have burst or shown up as little voids. This indicates that the effective gelcoat thickness is much less than it should be below the waterline. In turn, this will allow water to penetrate the hull more easily; the thinner the gelcoat the quicker water will be absorbed rominent fibres. These can sometimes be seen protruding either beneath or Surface of GRP laminate magnified 32x showing pinholes in the pigmented surface through the gelcoat. This allows ‘wicking’ to occur, this being defined as the process by which water is drawn into the hull by capillary action.
Surface of a GRP laminate showing prominent fibre magnified 32x examination showing prominent fibre in pigmented gelcoat.
Blisters. These normally occur only below the waterline and are usually evidenced as bubbling of the antifouling. In this eventuality the antifouling will need to be removed to enable the underlying cause of the problem to be ascertained.
Invisible Evidence
A number of laminate defect symptoms are not visible to the naked eye. Such problems as under bound glass and weak structure can probably be found only by more thorough analysis. The removal of a glassfibre core – such as the ‘plug’ that is removed when a skin fitting is installed – can give a great deal of information.
For example:
There may be evidence of de-lamination in the structure.
Further bubbles, previously hidden by the pigment, may now be visible within the thickness of the gelcoat.
The resin/glass ratio may be incorrect. It is possible to determine the resin/glass ratio by performing a relatively simple test.
Evidence of Water Absorption
It is not easy for the owner to know whether water absorption has taken place unless this has occurred to such a degree that the vessel floats visibly low. On larger vessels this will be less noticeable.
Meters used to detect moisture in Yacht hulls typically work on the principle of electromagnetic conductivity. Often it has been found that a high reading has not been due to moisture content but some other factor.
Examination of Blister Fluids
The appearance of blisters below the hull waterline is the key indication that a laminate problem exists. The blisters take the form of a dome or, if pressure has caused them to break, possibly a crater.
Analysis of the blister fluid is the usual method of determining the type of problem within the hull. There are three characteristics that can be quite easily tested on-site:
Its acidity or alkalinity is determined by the use of pH paper. Testing the blister fluid best takes place at the moment the blister is broken and the fluid released. The neutral reading – as in the case of distilled water – is pH 7. Readings usually found for GRP blisters are:
pH 5-pH 6: Acid reading. This is by far the most common result, and indicates that free acid has hydrolysed off the emulsion coating forming acetic acid and other acetic compounds. Occasionally readings as low as pH 4 have been found.
pH 7 is neutral and is sometimes found in vessels located in brackish or freshwater moorings.
pH 8-pH 8.3: Seawater-filled blisters. Water has penetrated the gelcoat.
pH 9: Alkaline reading. This is very rare and indicates that amine accelerators have been used either at the laminate stage or, just possibly, at the resin manufacturing stage. This is very unusual in more modern hulls, so the implications are not discussed here
Assessing the Extent of the Problem
The recommendation from International to owners of a yacht affected by blisters is to request an inspection by a surveyor. The aspects that will typically be considered by the surveyor in the course of his inspection will include the age of the vessel, the nature and extent of the blistering, and an assessment of how much of the gelcoat will need to be removed.
The study of the osmosis problem, by International, leads to the following observations: –
Age of the vessel
A blistered yacht, which is less than 3 years old, should be referred to the builder, as this is unusual and therefore it is likely that the cause of the problem will be of particular interest to them.
Blistered gelcoats in yachts that are more than 5 years old may indicate that the laminate quality is compromised in some way, but the problem can probably be treated effectively.
Hulls which have turned in a satisfactory performance for over 10 years are probably well built but suffering from gradual breakdown of the gelcoat and consequent water penetration. There is a high probability that treatment will extend this trouble free performance.
Nature and Extent of the Blisters
The surveyor should define whether the blistering is localised or general; there is no point in removing the complete gelcoat if only a small area is affected. In this case it is important to identify the type and probable cause of the blistering to understand if there are implications for the remainder of the hull:-
Pinhead blistering emanating from pinholes can indicate a badly mixed gelcoat; other signs will tell whether a chemical reaction is the cause.
Blisters should be broken to discover whether they are fluid-filled or dry. If the former, the fluid should be tested with pH paper.
The blister crater should be examined for evidence of stray fibres and dry matt laminate. If considered necessary, a small part of the gelcoat should be removed to find out how well it is adhering to the matt in the laminate below.
If the laminate looks very dry, a resin-glass ratio test should be undertaken.
Area of Hull to be treated.
In a severe case affecting a large area, most or all of the gelcoat will have to be removed if the hull is badly affected and there are possibly some underlying laminate causes. In the case of a few random blisters, treatment may be a relatively simple matter of cutting them out and treating individually.
Treatment and Protection Options
Depending on the results of the survey three basic options can be considered:
Option 1.
Survey result: Gelcoat in sound condition, no evidence of osmosis.
Recommendation – Protection system.
Even though no osmosis appears to be present there is a continuing risk for all GRP/FRP boats. To minimise this, apply a thick epoxy coating (Gelshield 200) to the hull to create a separation layer between gelcoat and water. This will delay the onset of any osmosis that is likely to occur.
Option 2.
Survey result: Evidence of osmosis including blistering in the gelcoat.
Recommendation – Treatment system
Full removal of the gelcoat is likely to be required and replacing the gelcoat with the recommended film thickness of Gelshield Plus epoxy barrier coating after thorough cleaning and drying.
Option 3.
Survey result – extensive blistering and craters in gelcoat and underlying laminate.
Recommendation – Removal of gelcoat and affected areas of laminate.
Thorough cleaning and drying prior to re-lamination with Epiglass epoxy resin system and application of Gelshield Plus epoxy barrier system
Protecting Hulls and Rectification of Osmosis
Option 1 – Protection System
Description of System
A vessel with no signs of osmosis will benefit from protection against osmosis. The earlier this is done, the greater the benefits, and some manufacturers are applying Gelshield 200 to their hulls either as an option or as standard equipment.
Gelshield 200 is an epoxy barrier system that serves the dual function of the barrier system and antifouling tie coat. Once the recommended thickness has been applied it can be over-coated by any of the antifoulings in the International Range.
Gelshield 200 is an epoxy primer with specific formulation characteristics developed to offer easy application by a variety of methods whilst consistently delivering a high level of protection against osmosis. The solvent based epoxy technology delivers extremely good adhesion to the surface and very good tolerance to a wide range of application conditions and temperatures. To augment the barrier properties of the epoxy, layered mica has been incorporated within the formula that creates an interlocking Microplate® effect within each coat, which is highly effective in combating water penetration.
It will also protect metal fittings from corrosion, and so a single product can be applied to hull, sterngear and skin fittings to provide maximum protection to all underwater parts of the vessel.
Gelshield 200 can be applied directly to the gelcoat after the appropriate preparation. Roller, brush or conventional sprays are appropriate application methods, whilst airless spray has the advantage of being able to deliver the greatest coating thickness in each application, reducing the time to complete the project.
Once cured, the full range of antifoulings from International can be applied direct to Gelshield 200 as it functions as an antifouling tie coat as well as a primer. If the application is completed within the overcoating time specified, no sanding is required between coats of Gelshield 200 or subsequent antifouling layers.
Surface Preparation – GRP/FRP and Composites
Introduction
As with any painting project, correct preparation is essential if early failure is to be avoided. Depending on the construction method employed, the surface to be coated will have characteristics which will require specific steps in preparation before the project can proceed.
Moulded GRP Surfaces
Fibreglass hulls and components are often made in a mould and therefore it follows that to release them from the female mould, a release agent of various types will have been used. These can vary between silicone-modified waxes, hard pure waxes and water miscible polyvinyl alcohol release agents. In the case of some GRP/FRP composites the mould is a male mould and the release agent is therefore on the inside of the structure rather than the outside.
In either case, the release agent must be removed before painting can commence by emulsifying with detergent before thoroughly washing. A key indicator to thorough removal of release agent is that the surface will become fully wetted with water if all mould release agent has been removed. If release agents remain, water will remain in droplets on the surface. In this instance the process should be repeated.
GRP Laminate Surfaces
Occasionally, an entire hull may be made on a male mould using polyester resin. In this instance, the outside of the hull will not be a smooth gelcoat finish but rather a rough laminate finish. Typically this type of surface will require abrading to remove the outer layer of resin. This will often be slightly tacky as a result of the ‘air inhibition’ of cure at the surface of polyester
laminates and this can be a guide to determine if the outer layer has been removed correctly. The water-wetting test is also effective in this situation.
If a peel-ply has been used, abrading will not be required, as the outer layer will be removed with the peel-ply.
Preparation Summary
For the moulded face of the substrate, any mould release agent must be thoroughly removed by detergent cleaners or special solvents, and possibly sanding.
Gelcoat surfaces should then be checked for:
Pinholes If present these should be filled prior to painting.
Star Crazes Very difficult to detect and sometimes only show up after the first coat of paint has been applied. They should be ground out and filled with Interfill 830
Blisters
This may indicate moisture is present, so the hull should be checked for osmotic attack using a moisture meter. If osmosis is present the gelcoat will need to be removed and an osmosis treatment scheme applied.
All surfaces should be abraded with 180 – 220 grade papers to ensure a good mechanical key is present.
Application
Brush/Roller are widely used methods of application. It should be noted that the film thickness of Gelshield 200 when applied by this method would generate dry film thickness in the order or 50 microns. For suitable degrees of protection, ensure the overall dry film thickness recommended is achieved.
Conventional Spray application demands that the material is thinned to allow for passage through a conventional spray gun. It is advised that wet and dry film thickness are regularly checked to ensure adequate amounts of material are deposited on the surface Airless Spray allows for the material to be applied un-thinned with the result that much higher dry film thickness can be achieved per coat. Given the higher build it is advised that you check the product data sheet is carefully checked to ensure overcoating times are met. They differ from those resulting from the methods of application mentioned previously that deposit a lower thickness of material.
Removing the Gelcoat
There are five general methods of removing the gelcoat:
They operate on the principle of controlled heat combined with high vacuum conditions that speedily vaporise many impurities. The heating blankets conform tightly to the hull surface ensuring an evenness of substance removal.
Drying Out
After removing the gelcoat, the hull should be thoroughly washed off with fresh water to remove salt, dirt and any soluble residues. If this can be done with hot water or steam cleaning a better result will be achieved. The hull should then be left to dry out. The purpose of the drying stage is to allow all water in the hull and any remaining surface chemicals to evaporate into the atmosphere. If drying is to take place in the open air, the hull should be fresh water pressure washed or steam cleaned on a regular basis. Use of a dehumidifier chamber, accelerated drying (dehumidifier, infra red, vacuum or Hot Vac treatment) will considerably shorten this period.
After the drying period has elapsed, the hull should be tested for dryness. This can be done by using a suitable moisture meter but in all cases it is recommended that the following physical check is also carried out: –
Fix a 1-foot square (30 centimetres square) piece of transparent plastic sheeting to the hull with adhesive tape. Leave for 1 hour. If after this period has elapsed no condensation appears on the plastic sheet, it can be assumed that this area is free from moisture. This is not so effective at elevated temperatures and can only be used as a general guide.
The purpose of this test is to see if there is moisture deeper in the laminate that may not revealed by the meter test. As shown in the diagram, moisture quickly evaporates from the surface whilst remaining deeper in the laminate. This situation can also develop if the solutes and impurities are only washed out of the outer layers of laminate. In this instance it may be necessary to remove more layers of laminate.
see the original article here on the International paints website http://www.boatpaint.co.uk/datasheets/International/Extras/International_Manuals/Gelshield%20Plus%20Application%20Manual%202004.pdf
