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Guide to Glass Reinforced Plastic (GRP)

GRP (glass reinforced plastic) is a composite of tough resilient, durable plastic resin, and glass fibres of remarkable strength. The resin is a thick, treacly substance which when activated by an appropriate catalyst, sets to a hard but brittle solid. It can be used alone for small castings, or with a variety of fillers, but, when reinforced with glass fibres, becomes a material of exceptional strength and versatility.

Polyester Resins

 The Resins most commonly used in GRP are unsaturated polyesters dissolved in styrene. The polyesters are produced by reacting various organic acids (usually phthalic or maleic anhydrides) with an alcohol such as propylene glycol or ethylene glycol. Depending on the particular alcohol or acid used, various types of resin can be produced-indeed, since a wide variety of both alcohols and acids are available, it is possible to have polyesters tailor-made to specific requirements.

The Polyester/styrene solution sets to a hard, rigid substance, a co-polymer of polyester and styrene, by the cross linking of molecules (i.e. polymerisation). The hardening process is commonly referred to as ‘curing’ Partial cross-linking occurs spontaneously, thereby limiting the storage life of the resins, but, for the process to take place quickly and completely it has to be activated by two additives. One is a catalyst, which triggers the process and the other is an accelerator, which- as the name implies-speeds it up. Typically, the catalyst is an organic peroxide (e.g. methyl ethyl ketone peroxide) and the accelerator  is normally cobalt naphanate. These two substances, if mixed directly together will react violently , even explosively. It is therefore , essential to ad the accelerator to the resin first ( stirring in thoroughly) and then mix the catalyst. In practice, many resins are supplied ‘pre-accelerated’- with the accelerator added by the manufacturer- so that only catalyst is needed to activate the curing process.

Internal heat (’exotherm’)is generated within the resin during curing, and can reach relatively high temperatures about 170oC is typical.

The amount of heat generated normally varies according  the quantity of catalyst used, the volume of resin and other factors such as the presence of fillers and reactivity of the resin.

The curing of polyester resins takes place at room temperature, preferably about 18°C-20°C, and this is one of the great advantages of the material. Most other plastics require heat or pressure (or both) so that their use in any manufacturing process demands expensive and complex machinery. They are therefore only economically viable for producing high volume runs of relatively small articles. As polyester resins can be cast at room temperatures with the simplest equipment , they are practicable for low-volume production, and even for one-off articles with virtually no size limit. Although the cured resin is very hard, it is also quite brittle, which could prevent its use for large articles but this problem is obviated by the use if reinforcements. A large number of materials can be used to reinforce the resin, but in practice, the one offering the best combination of strength, versatility and economy is glassfibre. 

Specification of A typical cured Polyester resin, Without Reinforcement  
Specific Gravity: 1.28
Tensile Strength: 55 MN/M2
Compressive Strength: 140 MN/m2
Youngs Modulus: 3.5 Gn/M2
Elongation at Break: 2%
Specific Heat: 2.3kJ/kg0C
Thermal Conductivity: 0.3 W/moC
Coefficient of linear expansion: 100 x 10 -6/oC
Water absorption: (24 hr at 20oC): 0.15%
Voltage breakdown (0.2mm sample): 22kV/mm


The Glass used commonly for GRP is a calcium-alumina borosilicate with an alkali content of less than one per cent. It is commonly known as ‘E’ type glass, since it was originally developed for use in electrical insulation systems.

Glassfibres are produced by running molten glass from a direct melt furnace into a platinum alloy bushing containing a large number of small holes, from each of which a glass filament is drawn. Filaments for commercial use are normally between  9 and 15 microns in diameter. The filaments are “dressed” with an emulsion before being gathered into fibres. The fibres are remarkably strong-the tensile strength being particularly high. They also exhibit good chemical and moisture resistance, have excellent electrical properties, are not subject to biological attack and are non-combustible with a melting point around 1500oC-all excellent qualities in a plastic reinforcement.


Specfic Gravity

Tensile Strength

Compressive Strength

Thermal Conductivity

Polyester resin (unreinforced)





Chopped Strand Mat Laminate 30% glass





Woven Rovings Laminate 45% glass





Satin Weave Cloth Laminate 55% glass





Continuous Rovings Laminate 70% glass










Mild Steel










High Density Polythene










The fibres can be used in a variety of ways-chopped into short lengths(“chopped strands”); gathered together into loosely bound ropes (“rovings”); woven into a variety of fabrics, produced from yarn made by twisting and doubling continuous strands. In the UK, the most widely used glassfibre material is chopped strand mat, which consists of glass strands chopped together in short lengths (approx. 50mm) and held together in mat form by a polyvinyl acetate or polyester binder. The mat is available in a range of weights, from 225gm2 to 1200gm2, and is a useful general purpose reinforcement.

Reinforced Plastic

Provided the glass reinforcement has been thoroughly impregnated with resin, the result, after curing, is a cohesive completely integrated matrix of resin and fibres. The matrix can have a surprising range of properties, depending on the type of glass material and the formulation of the resin. In general, the GRP laminate will display excellent tensile and compressive strength, acceptable thermal conductivity, a low coefficient of linear expansion, reasonable chemical resistance and good dielectric properties. Compared to other materials of equivalent strength, it will be light durable, moisture-resistant, non-rusting and economic.

A Glossary of Materials and Terms


One of the two compounds (the other is catalyst) required to initiate the polymerisation process. See Preaccelerated. Mixed directly with catalyst, the accelerator reacts explosively it is therefore usually added to the resin in manufacture so only catalyst need be added later.


Solvent for cleaning uncured resin from brushes and tools. It is HIGHLY FLAMMABLE. It is a powerful grease solvent and should NOT be used for removing resin from the skin-it destroys the natural oils and can lead to dermatitis.


An extremely lightweight filler powder (it is so light it is likely to become airborne if not dispensed with care). It is used to thicken resins and make them thixotropic.

Air Inhibition

Air inhibits the curing process in some resins, with the result that the exposed resin, with the result that the exposed resin surface tends to remain tacky. This effect is used deliberately in gel coats, but can be a problem with some resins additives which prevent air inhibition discolour the resin and therefore cannot be used for some applications, e.g. clear casting.


In chopped strand mat (the most commonly used glassfibre material), the strands are held together in a random pattern by a binder, either a PVA emulsion or a polyester powder. Powder bound mat gives faster ‘wet-out’ but emulsion-bound gives greater ease of handling.


An unformed, or only partially formed shape, often moulded in a foamed plastic-e.g. a surfboard ‘blank’.

Brush Cleaner

A Solvent for uncured resin, usually acetone. It is highly inflammable. 

Carbon Fibre

An extremely strong, (but expensive) reinforcement which can be used in conjunction with glassfibre, and resin.


Hardener. An organic peroxide, or similar compound which, together with the accelerator, initites the polymerisation process of polyester and other resins. It should NEVER be mixed directly with an accelerator-this can cause an explosion. Catalyst is available as a liquid or paste.

Catalyst is an organic peroxide (a powerful corrosive) and should not come contact with eyes, mouth or skin. Should it do so, wash from the skin immediately under a running tap. If it is splashed in the eyes, flush them with running water for at least fifteen, minutes, and call a doctor.

Catalyst Dispenser

A Purpose-designed instrument for measuring and dispensing liquid catalyst without splashing.

Chopped Strands

Short (6mm or 12mm) lengths of glassfibre. Can be used to make a resin dough, stronger than that made by mixing resin with filler powder.

Chopped Strand Mat

A popular and economical form of glass reinforcement for polyester resins.

Short strands of glass are bonded with a powder or emulsion into a mat available in a variety of thickness.

Clear Casting Resin

A clear resin used for embedding items to make transparent paperweights and other ornaments.

Cobalt Naphthanate

Used in a solution with styrene as an accelerator for polyester resins, it should NEVER be mixed directly with catalyst as the two substances react explosively.

Cold Curing

Able to cure to a hardened state at room temperature, usually when activated by a catalyst.

Compressive Strength

The ability of a material to withstand being crushed. It is found by testing a sample to failure-the load applied, divided by the cross-section of the sample, gives the compressive strength.

Contact Moulding

Any method of moulding glass reinforced plastics without external pressure, as is used for injection moulding. The commonest contact methods are hand lay-up and spray moulding.


Using a metal roller on a glass fibre/resin layer to force out air bubbles.


Normal term for the polymerisation process by which polyester resins harden.

Curing Agents

Chemicals used to initiate the polymerisation process in resins- e.g. catalyst, accelerator.

Curing Time

The period required for a polyester resin to cure fully. In practice, it is taken as the time from the addition of catalyst to the point of full hardening. A resin may actually continue to cure for sometime after it is apparently completely hard.


The compound, which, together with water, results from the chemical reaction between any organic acid and any alcohol. See ‘Polyesters’


Epoxy laminating resin boasts higher adhesive properties and resistance to water, ideal for use in applications such as boat building. Also used extensively in aircraft component manufacture.

Epoxies are widely used as a primary construction material for high-performance boats or as a secondary application to sheath a hull or replace water-degraded polyester resins and gel coats.


The internal heat generated within a resin by the polymerisation process. As the resin cures it becomes noticeably hotter. This can create problems in resin casting, since the temperature can be high enough to crack the casting.

Female Mould

A mould in which the internal surface decides the form of the casting or laminate takes from it. A child making sandcastles with a bucket is using a female mould! See ‘Moulds’


Any Substance added to a resin to extend it. A typical filler is an inert calcite (talc) which increases the bulk of the resin without affecting its chemical properties. Fillers can also be used to alter the texture of a cast piece by creating realistic metallic or stone effects. Almost any dry substances can be used as a filler- stone or slate powder, metal powder, sawdust, sand, gravel etc. Most fillers have the advantage of reducing exotherm.


Glassfibre materials once hardened can be polished, sanded, drilled, sawn or filed. Diamond carborundum or metal finishing tools generally are required. Since the dust produced can be extremely hazardous to eyes and lungs protective goggles and breathing masks should be worn at all times when machining hardened resins, with or without glassfibre reinforcement.   


Anything round which a GRP lamination can be laid –e.g. a cardboard tube can be used as a former for a laminated stiffening rib. The term could be applied to such items as a surfboard blanks, and is also used for the ‘pattern’ or ‘plug’ from which a mould is taken.


Resin often used on a plug (especially a wooden plug) to give a highly glazed surface.


Before hardening completely, a catalysed resin first reaches a thick jelly like consistency known as the ‘gel’ state. Once it reaches this stage, the resin is impossible to spray, paint or pour. Stored resin which has passed its shelf life may gel without being catalysed.

Gel Coat

A thixotropic resin invariably used as the first coat (applied without glass reinforcement) on the mould surface. It forms the hard, smooth shiny surface of the finished article and is usually pigmented. It paints on easily but does not drain from vertical surfaces. When 2% wax solution is added it becomes a Flowcoat.

Gel Time

The Period between a resin catalysing and reaching gel state- in effect, the time in which it is still workable. Gel time varies from one type of resin to another. It is also known as ‘setting time’.

Glass Finish

Treatment of glassfibres, during manufacture, to improve adhesion to plastic resins.


Glass Filaments drawn together into fibres and used to reinforce polyester resins, to produce a strong, lightweight, versatile material. The fibres can be woven into a variety of fabric types or used as a random matrix of short (‘chopped’) strands held together by a powder or a emulsion binder.


Abbreviation for Glassfibre Reinforced Plastic. The plastic is a resin sometimes epoxy but usually polyester.

‘Green’ Stage

A point reached by a GRP laminate after the gel-time but before it is fully hardened. When ‘green’, the laminate is fairly firm but can be cut with a knife- it is therefore easy to trim at this stage.

Hand Lay-up

The process of applying the resin/glass laminates to the mould manually with brushes and rollers- an economical but effective method, requiring no specialised equipment, and therefore the most popular DIY method.


See Catalyst.


Any substances which slows, or stops the curing process. Air is an inhibitor to the surface of some resins, oil or water will inhibit most.


A proprietary cleansing cream which removes resin, etc., from the skin- it is rubbed in prior to washing in soap and water.


Any material in which separate layers of material are bonded together. In GRP work, the layers are resin and glassfibre.

Latex Rubber

Liquid compound which air dries to a flexible rubber-popularly used for making small moulds for craftwork.


The process of applying the resin/glass laminates in the mould. See ‘Hand Lay-up’ and ‘Spray Lay-up’.

Male Mould

A mould having external working surface on which the laminate is laid-up.

Maturing Time

The time taken for an apparently hardened resin to become completely cured and stable. This is only important in mould-making and in certain specialised applications-e.g. the construction of fish ponds or of food and chemical containers, where a not fully cured resin may release traces of chemicals, such as styrene.


Methyl Ethyl Ketone Peroxide, an organic peroxide-the main constituent of a widely-used catalyst for polyester resin.


A plastic polyester film which does not adhere to resin, and therefore, can be used for self-releasing formers, etc.

Metal Powers

Powdered metals used as a filler, giving a realistic metallic finish and texture to resin castings.

Metallic Jewels

Tiny flakes of epoxy-coated metal foil, available in a wide range of colours. Can be mixed with resin to produce an attractive sparkling effect.


A substance which is capable of being polymerised.


Many glassfibres projects require a mould in which to cast the resin, or lay-up the laminations. A mould can be made in almost any material as long as it is sufficiently rigid, has a smooth surface, and will not adhere to the resin (or can be treated so that it will not do so). The most usual mould materials for laminating wood or GRP. Various flexible rubber compounds are popularly used for small castings. Moulds can be male or female (the lamination is laid up on the outside of a male mould and the inside of a female mould).

Paddle Roller

Type of metal roller used in laminating.

Parting Agent

See ‘Release Agent’.

Paper Rope

Paper wound on a wire core. Widely used as a former over which to laminate stiffening ribs, etc.


See ‘Plug’.


Conventional polyester resins are translucent cloudy. They can be coloured with the addition of a wide variety pigments,-opaque, translucent, metallic.


An additive which increases the flexibility of resins.


Also known as a pattern or former-a full size model or mock up from which a mould is taken. The mould is then used to produce the finished glassfibre article.


Substances produced by reacting certain glycols (alcohols) with anhydrides (organic acids). Conventional GRP resins are polyesters dissolved in styrene.


Commonly called polythene. As it is completely resistant to polyester resins it can be used for buckets, mixing containers, and small casting moulds. Polythene sheet is a useful release agent in some circumstances.


In general, the remarks regarding polyethylene also apply to polypropylene, which is widely used for expensive heavy duty kitchenware etc.


Polyester resins attack and dissolve polystyrene, which cannot therefore be used for storage containers, moulds etc.

Post cure

The application of heat to accelerate the complete curing process and shorten maturing time.

Pot life

The working time of a resin – the period between the resin being catalysed and becoming gelled.


(Polyvinyl Alcohol): Used as the basis of some release agents.

Reinforced Plastic

Any plastic reinforced with additional material. The term is now virtually synonymous with ‘glass-reinforced plastic’ due to the universal popularity of glass as the reinforcement.

Release Agent

Since polyester resins adhere to most substances to some extent (with the exception of a few plastics), moulds invariably have to be treated to enable the laminate to be released easily. The treatment usually takes the form of polishing with purpose designed waxes called release agents. Moulds made of polypropylene, polythene or silicone rubber will sometimes be self-releasing, as will those protected by a sheet if polythene or Melinex film. 


Occur in nature as organic compounds, soluble in organic solvents but not in water – e.g. amber, shellac. Synthetic resins have similar properties and are normally produced by polymerisation. The resins most used in GRP are polyesters


Breathing Mask incorporating filter designed to protect the lungs from minute dust particles and/or harmful fumes. Essential when machining glass fibre laminates. Make sure you are using the correct OEL ( occupational exposure limit).


Either Aluminium paddle, PTFE, nylon ect,  are used for rolling resin onto working area or consolidating laminate.


Long fibres of glass, held together by dressing, supplied in a ‘cheese’.

Saturated Compound

A chemical compound incapable of polymerisation.

Setting Time

See ‘Gel Time’.


The Process by which an item is given a protective skin of resin impregnated glassfibre. Often applied to boat hull and roofs.

Shelf Life

Catalysts and accelerators speed up the polymerisation process-even without these additives, resins will tend to polymerise slowly by themselves. Hence, all resins have a limited shelf life, from three months to a year or so depending on the type of resin and storage conditions.

Silicone Rubber

A Synthetic rubber used for flexible moulding compounds, sealants etc. It is usually cold-curing. Por-a-mold is a typical example.

Spray Lay-Up

Mainly used in industrial workshops, spray lay up involves spraying the resin, catalyst and finely chopped glass fibres into the mould form a special spray gun.


Benzene feed stock product (phylethylene) used as a solvent and thinner for polyester resins. It is highly inflammable and produces fumes which can be dangerous in excess. During curing, the styrene molecules cross link with polyester to produce a co-polymer.

Tensile Strength

Ultimate strength of a material measured under tension, normally expressed in MN/m2.


Plastic which can be softened (by heating) and hardened (by cooling) and still retain its properties – e.g. polythene, PVC, celluloid.


Plastic, which hardens by a non-reversible chemical reaction  initiated by heat and / or curing agents. Once hardened, it cannot be melted without being altered or destroyed – e.g. polyester, epoxies.


The condition of a resin which has failed to harden completely  due to very low temperature, insufficient catalyst, or the presence of inhibitors. If any of these factors are excessive, the resin can be permanently under-cured and never harden.


The ability of a liquid to resist flow – a thick, treacly liquid has high viscosity, a thin, runny liquid has low viscosity.


A proprietary, hot-melt, vinyl compound used for flexible ‘rubber’ moulds. It can be re-melted and used several times.

Wetting Out

To achieve full strength, the glassfibre materials in a laminate should be thoroughly impregnated with resin – the glass is said to be wetted out when it is so impregnated.


Petroleum based compound from which are derived the organic acids used in polyester resin manufacture.

This information is, to the best of our knowledge, true and accurate. Recommendations are made without warranty or guarantee. Users are advised to make their own tests to determine the suitability of specific materials or methods.