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GRP Glossary

As in any other field, the range of different fibreglass words and terms can seem daunting to the composites or fibreglass beginner. We’ve compiled this handy alphabetical list of GRP terms to help you get to grips with all the terminology:

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 reinforcement which can be used in conjunction with glassfibre, and resin.
(or hardener) – This is the chemical added to resin to start the hardening reaction (cure). An organic peroxide, or similar compound which, together with the accelerator, initiates 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 into 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 (CSM)
This is a “cloth” made up of thousands of glass strands bound together by a binder – either PVA emulsion or polyester powder – The binder is dissolved by the resin, leaving just the glass strands embedded in the plastic.
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 most common contact methods are hand lay-up and spray moulding.
Using a metal roller on a glass fibre/resin layer to force out air bubbles, thus consolidating the resin with the reinforcement.
Normal term for the polymerisation process by which polyester resins harden. The process of turning from a liquid resin into a solid plastic.
Curing Agents
Chemicals used to initiate the polymerisation process in resins- e.g. catalyst, accelerator, hardener
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 some time 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.   
(or Topcoat) is a gelcoat with a wax additive, enabling it to cure in contact with air.
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.
Glassfibre Reinforced Plastic, or fibreglass, is what fibreglass is! A plastic resin strengthened with strands of glass. There are various other composite materials made the same way, with differing reinforcements eg. Carbon fibre etc. 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.
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 fibreglass 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 are 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
Polyurethane (PU)
A very versatile material which is used in many different materials such as paints, adhesives, varnishes, resins and foams often used in conjunction with polyester resin based fibreglass. Also used for various casting resins which are stronger and produce less odour than their polyester counterparts.
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, but is increasingly various carbon fibre materials.
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.
Vinylester resins are produced by reacting an epoxy with an unsaturated monocarboxylic acid – they are often thought of as being halfway between a polyester and an epoxy resin – stronger than a polyester, but not as strong as an epoxy etc.
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.