What is GRP?

GRP - meaning Glass Reinforced Plastic, is plastic resins (usually polyester or epoxy) strengthened by glass-fibre mat or fabric. The term is often used (rather inaccurately) to include plastics reinforced with materials other than glass; carbon fibres, for example, or aramids such as Kevlar.

Polyester Resin is a thick sticky liquid which, when activated by a suitable catalyst (hardener), sets to a hard, rigid plastic. As a liquid, it naturally adopts the shape of its container - hence it can be poured, painted or injected into a mould, and, once cured, will reproduce the shape of the mould.

The hardened plastic is fairly brittle so glass-fibre materials are added to give considerably increased strength, just as steel rods are used in reinforced concrete.

The combination of polyester resin and glass-fibre produces an incredibly versatile material – strong, durable, weatherproof, waterproof, and non-rusting. Easily moulded to virtually any shape, highly adhesive to a wide range of materials, and capable of making structures of almost any size.

The resin can also be used alone, or with fillers, to make castings which accurately reproduce the finest detail of the mould.

There are numerous instances where GRP can be used as a viable alternative to traditional materials, such as steel or wood, and in many cases, GRP is a more appropriate choice.

Marine GRP products

The range of items which can be made with these materials is almost unlimited... boats, catamarans, canoes, marine grp components, yacht shower trays, tanks, panels.

Items produced include:

Hulls

Decks

Wheelhouses

Fairings

Battery Boxes

Anchor Hatches

Intake Ducts

Water Jets

Bulkheads

Bulwarks

Glass fibre reinforced plastic (GRP) … how to use grp.

Few materials can match the versatility and utility of glass fibre reinforced plastic, commonly called GRP for short or simply fibreglass.

Strong, lightweight and completely waterproof, it can be moulded into free form shapes, such as any of the above, or laminated to make decorative wall panels, sills or roofing.

Formed of woven or felted glass fabric embedded in a plastic resin, the material combines the best attributes of both. The strength of glass fibres in the plastic makes the material rigid and resin gives it a smooth impermeable surface.

The glass fibre fabrics that are used in laminating are mostly made up of rovings, bunched strands of glass filaments.

The rovings are pressed into mat or woven into fabrics of varying densities; density is measured in terms of the weight, in ounces per square foot, or grams per square metre of fabric. The fabric weight as well as the forming method to be used, determines the quantity of resin that will be needed.

Generally, a chopped strand mat takes two and a half times its weight in resin; the ratio of resin to fabric for a woven cloth depending on type can be up to two times.

  Fabric choice depends on the amount of reinforcement needed and on the type of resin being used.

Woven fabrics which are stronger, will not bond as well to each other as they will with mat fabrics, it is best to alternate layers to ensure even strength throughout the laminate, and to produce a smooth surface, a fine glass fibre tissue is often used for the final layer, alternatively coat over the final layer with ‘Flowcoat’ a gelcoat type resin that cures touch-dry.

  Although many resins are suitable for making fibreglass, polyester is both the simplest and most economical; it provides excellent strength and moisture resistance.

When it is laminated, polyester’s normal high shrinkage during curing is reduced to 2 per cent or less, and this slight shrinkage is often an advantage. In moulded laminations it assists easy release from the mould.

With moulds it helps enormously if unobtrusive holes are drilled and taped over prior to applying PVA, into which compressed air can be directed later when curing has sufficiently occurred for the removal of the casting. A bicycle hand pump is often enough.

When you are laminating fibreglass with a mould or form from which it will later be removed, you will need to apply release agents to stop the fibreglass from sticking to the mould, for simple moulds you can coat the surface with PVA (polyvinyl alcohol) release liquids, for larger or complex moulds, a two stage parting compound of non silicone wax and PVA is generally used.

Apply the wax first and let it dry completely, polish up, repeat twice more, then apply the PVA using a sponge or brush to lay on a thin even coat, protect the surface of the mould from dust whilst the parting compound is drying

Polyester resins

Polyester resins are usually available as a two-component system, with resin in one container and the hardener often known as the catalyst in the other.

Be sure to follow the manufacturers mixing instructions exactly, combining the components carefully to avoid mixing air bubbles, which weaken the cured resin.

Usually the resin contains an accelerator, also called a promoter, which speeds the cure time, but sometimes this component needs to be added separately, if you are adding an accelerator, be absolutely sure you mix it into the resin before you add the catalyst.

The catalyst must be handled with great care at all times; it is a corrosive substance and can ignite spontaneously when in contact with materials like paper and rags.

Gelcoats.

Special resins called gelcoats are always used as the outermost layer in fibreglass lamination.

This un-reinforced resin provides a smooth, glossy, protective layer between the glassfibre and outside moisture.

It is applied as the first layer in the mould, by using a pigmented gelcoat, either pre-mixed or mixed on the job with up to 10 per cent of a suitable polyester pigment paste, you can impart the surface colour of your choice to the fibreglass.

LAYING UP or FIBRE GLASSING using a MOULD (female).

Typically a brushing gelcoat will be either clear or pre-pigmented and have thixotropic (non-drip) properties.

When catalysed it is applied to the prepared mould surface ensuring complete coverage to a minimum thickness of 0.5mm.

The spread rate is around two ounces per square foot.

It now needs some time to cure, a minimum of four hours should be allowed, otherwise the solvent in the laminating resin next applied could attack the gelcoat causing wrinkling.

Cut all the glass fabric to the approximate size before mixing the resin, this can be done with scissors or a trimming knife; glass matting can also be torn.

If the matting edges are to be overlapped, torn edges are most suitable as they will intermingle within the resin and make the joint without a visible seam.

The next step is to paint over the cured gelcoat with lay-up resins, then apply your first layer of matting, to impregnate the glass fibre with resin, use a paint roller or brush, stippling the brush over the fabric so as not to dislocate the glass strands with the sticky, resin coated bristles.

Never apply more than the amount of resin recommended by the manufacturer of the fabric; the heat generated by the resin as it cures can adversely affect the laminate if applied in quantities too great.

Some polyester resins cure completely only in the absence of air. Left exposed, they remain tacky indefinitely.

This can be an advantage in multi layer laminations over a large area, where new coats are usually applied before the previous coat has cured, a forming process called wet on wet.

As the fibreglass layers begin to cure (called the “green” stage) trimming can be done easily (using a sharp blade) but you have to be quick as this stage may only last three to four minutes.

The work can be interrupted for several hours without harming the laminate.

When using such resins, however, the final coat must be sealed with flowcoat or covered with an airtight polyethylene film until the resin cures, after which the film can be peeled off.

The male moulding now needs to cure for several hours but can be shortened somewhat by post curing methods, involving warm air, still or fan applied.

When sufficiently cured, the moulding will be wanting to shrink away from the sides of the mould, this can be helped along by dissolving the blue release agent with water poured along the edges.

Keep tools soaking in solvent so you will be able to clean them when the job is completed, before the resin hardens.

A supply of clean rags is essential for removing spilled resin, and the floor beneath the work should be covered with newspaper to catch the drips, which are difficult to remove once they have hardened.

In addition, the laminating process also requires you to use several specialised tools.

One of these tools is a paddle roller or split washer type for consolidating resin and glass and at the same time pushing away any air bubbles that form between the layers, these rollers come in several forms, with washer or paddle shaped blades, and in sizes ranging from 6 to 300mmm wide.

For use on contoured surfaces there is a flexible roller, with a head resembling a coiled spring.

To trim and finish the completed lamination, if you didn’t do this during the “green stage”, you will need a metal cutting saw, a forming tool such as a file or rasp, and a supply of wet and dry paper from 240 to 600 grit.

Glass fibre work is quite an easy skill to master, providing you have the right tools and materials, and approach it in a methodical manner, taking the required safety precautions.

The basic material components include, release agents, gelcoats, resins, glass mat, glass cloth, glass tissue, and flowcoats.

Chopped strand mat (csm).

The most common fibreglass reinforcement, is made up of randomly arranged glass fibre strands, pressed and held together with a binding agent.

Woven cloth.

Glass fibre filaments, spun into a yarn then woven to make cloth, provides great strength with minimal thickness, is usually used as an alternate layer between CSM, giving added strength without substantially increasing laminate thickness.

Glass fibre tissue.

A very fine mat weighing approx one ounce per square yard, used as the final layer, to conceal the coarser texture of CSM, and give a smoother finish.

HOW DURABLE IS GRP?

The oldest boats made from glassfibre are now over 50 years' old and no deterioration in the glassfibre can be found. Properly constructed glass fibre mouldings will last in excess of 50 years.

Where the glassfibre has been kept immersed in water, as in boat hulls, or exposed continuously to the elements, as in boat decks, there is no evidence of deterioration of the laminate in either application.

The surface finish and colour of a "GRP" laminate can be modified to simulate almost any appearance: shiny or matt, textured or smooth, any colour - including translucent finishes or metallic. Resins are already used commercially to manufacture synthetic marble, onyx, granite and any stone imaginable. "GRP" is also used for translucent roof lights and can be tinted to give the appearance of stained glass.

A water tank or boat hole may be repaired by forming a glassfibre patch over the damaged area.

From levelling up the hull, selecting the tools and timbers, to fitting grp bulkheads, grp bulwarks, tanks, engine compartments, steering, deck fittings, bunks, galley and much else.... all these are the next stage of boat construction.

Osmosis treatment.

Despite most people’s assumptions to the contrary, fibreglass mouldings (yacht hulls for example), no matter what resins are used, are not actually totally waterproof. Individual water molecules are so small that they can actually find their way into and ultimately right through the layers of glass and resin forming a GRP boat hull.

Problems start to occur when the water molecules migrating into the GRP encounter other chemicals inside the laminate, primarily water-soluble materials (WSMs) such as the the emulsion binders used to hold the glass mat together before it is moulded, or pockets of uncured or only partly cured resins in the moulding.

The water molecules can then have a chemical reaction with these substances, forming larger molecules of a new chemical, often acidic - which unlike the original small water molecules, cannot carry on passing through the GRP. These larger molecules are then trapped. This is the point at which osmosis actually starts.

Pressure builds up inside the laminate. If this process takes place in a solid part of the laminate, there is usually no problem as the structure is strong enough to contain the pressure. If however it takes place on the boundary of a small air-bubble in the moulding, or at a point where layers of GRP are poorly bonded, the resultant new chemical compound or compounds slowly fill up the bubbles or the minute gaps between layers with liquid.

Almost all mouldings have these air bubbles and small areas of poor bonding, although they should not. Ideally the resin should totally fill the gaps between the glass strands, and every layer should perfectly bond to the next. In practice, however, this is extremely difficult to achieve with conventional moulding techniques. 

The process of osmosis in GRP is however very slow, unless the moulding is appallingly badly made, and no matter how long it remains in water a typical GRP laminate cannot absorb more than about 2-3% of it’s own weight of water.

Surveyors and boatyards (and some brokers) put moisture meters on yachts hulls to check the moisture content, on the basis, often but not always correct, that high moisture levels in the GRP are a precursor to the development of blisters.

If this osmosis (using the term in it’s correct manner for once) was all that happened, it would be a very minor problem. Even completely saturated with water molecules, a GRP laminate still retains most of it’s strength, although it does become slightly more flexible.

Racers who want stiff hulls with the absolute minimum weight already mostly keep their boats ashore when not sailing, and for any properly built cruising boat small extra weight and a trace more flexibility in the structure should not be problems. 

If the air bubble simply filled with this acidic compound, the problem would still be relatively minor. However the nature of the osmosis process is that water molecules keep osmosing through the laminate, and join the chemicals in the bubble, steadily building up pressure. Eventually this causes the surface of the moulding to blister. 

These blisters are the typical sign of what boat-owners usually refer to as ‘osmosis’. When pierced these blisters will give off a small amount of chemical-smelling (usually vinegary) liquid - which is the juice built up inside the pressure-raised blisters.

The term ‘blister juice’ is often used. This ‘blister juice’, which is usually acid, can break down the polyester. This breakdown process is known as hydrolysis, and causes a reduction in strength of the laminate.

Once blisters in the gelcoat have appeared, a period of storage ashore, particularly in warm dry weather, may cause them to apparently disappear, as most of the water in the blisters dries out. What is left behind, though, is a highly concentrated solution of the 'blister juice', which will usually rapidly re-absorb water once the hull is put back into the water.

The process of osmosis is advanced by the time visible blisters start to appear on the bottom of a hull. It is the case, however, that the process starts the minute a new yacht is craned into the water, or even when it’s hull and deck first gets rained on as it is wheeled out of the factory. 

Another facet of water absorption into a hull is known as ‘wicking’. This refers to the ability of water molecules to creep along the boundary of the individual strands of glass within the moulding.

A totally dry moulding, if moulded with clear resin, will be virtually transparent. If you can see individual strands of glass as whitish threads, what you are seeing is not the glass strands themselves, but water around the strands.

This ‘wicking’ is an indication that there is a significant amount of moisture in the resin, and is often a precursor to or accompanies blistering. 

All GRP yachts, from the day they are built, suffer from osmosis. Manufacturers now typically offer five year hull warranties, and it has been said - cynically but probably accurately - that their main concern is to do just enough that they don’t get visible blisters within the warranty period, which for most yachts is 5 years.

Several US manufacturers of small power boats, however, now state in their warranty conditions that the boats are not to be left afloat for more than 2 weeks, or the warranty against blisters is invalid! At least one US builder will not offer any warranty at all against blisters.

The fact remains that in practice some yachts ‘get osmosis’ - ie blisters, and some don’t. It is known that several factors increase the likelihood of blistering. These are  Long periods afloat without layups   Warm tropical waters. Fresh water is worse than salt water  

Coloured resins (including white - the most common) are worse than clear resins. 

Some builders, including those who produce some very expensive boats, have had runs of boats prone to blistering, they have also turned out apparently identical boats that have not blistered. Current thinking is that cleanliness, temperature and humidity control in the moulding shop, and precision of the mix of resins, are the key to building boats that will not blister. No-one really knows.

While all GRP boats slowly absorb some water, it should not be fast. Visible blisters or wicking are an indication of a well developed absorption of water, and if they occur in the first few years of a boat's life are an indication of a moulding problem of some sort, whether it be poor materials, poor workmanship by the laminators, or any other quality control problem ranging from sawdust getting into the moulding to a prolonged delay between laminating up the various layers that form the hull.

Osmosis treatment involves stripping off the external gelcoat, drying out, and recoating with epoxy fillers.

As this new external coating is essentially ‘glued on’, and not chemically part of the original moulding like the original gelcoat, you could argue that the boat is substantially devalued by this repair. 

After ten to fifteen years,it is common to find that yacht hulls have a moderate to high moisture content.

Some may also have developed a few blisters. This is normal, and not necessarily a sign that there is anything terribly wrong with the boat.

If a yacht reaches twenty years of age without high moisture content or visible blisters it is actually a surprise. 

These timescales assume a standard mass production yacht, given average use of perhaps seven or eight months afloat a year, with just antifouling paint on the bottom. Painting with polyurethanes or epoxy coating the bottom can considerably, but not totally, slow down the rate of water absorption, and some builders do this from new. Opinions vary as to the effectiveness of epoxying or hull painting once there is already some moisture in the moulding. 

On boats which have been painted or epoxied, it is not uncommon to find, after a few years, blisters in the interface between the epoxy/paint coating and the gelcoat. This is obviously less of a problem than blisters under the gelcoat itself. Some experts believe that paint or epoxy coatings should be renewed regularly to maintain effectiveness.

Osmosis treatment can be local treatment, cutting or grinding open individual blisters, repeatedly washing out with hot water or steam, to remove the ‘blister juice’ from any blisters, drying thoroughly and filling with epoxy paste.

Osmosis Treatment Centres doing a whole hull remove all the gelcoat, wash and dry out, and recoat the hull with epoxy. The smaller and older the boat the less cost-effective this is.

If you are trying to sell the boat, buyers almost always prefer boats without blisters.

Boatyards like doing the work... it is profitable, and can be scheduled in to when staff have free time. Some surveyors like to recommend it, as it means they’ve ‘covered their backs’ against a later claim that they didn’t pick up a defect. 

It is certainly easier to sell a boat with no blisters and a low moisture content.

If you are buying a boat, it is perhaps preferable to have one with no blisters and a dry hull (low moisture levels on the magic meter).

If this condition is achieved by the original hull surface, with no repairs, it is better than a similar boat that is also dry and with no blisters, but achieved by having recently had an ‘osmosis treatment’.

The treatments are not cures - they simply 'restart the clock' on a progressive absorption of water again, as even epoxy coatings are not totally waterproof.

Occasionally buyers will happily accept a boat with high moisture content or blisters - on the grounds that they can haggle down the price as a result.

They then may or may not get some form of treatment done - perhaps just before they sell it on a few years later.  The “game” is perpetual and ongoing.