ABL (STEVENS) Resin & Glass

General info & How-To guide on GLASS FIBRE REINFORCED PLASTIC (GRP)


Few building 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 boats, car wings, ponds, basins or furniture, 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 a release agent to stop the fibreglass from sticking to the mould, for simple moulds you can coat the surface with PVA (polyvinyl alcohol) release liquid, 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 resin is 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 some times 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.


A special resin called gelcoat is 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 gel coat, 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.


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 resin, 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.


Because working with resin is messy, use disposable utensils for measuring and mixing wherever possible.

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.


Most of the components used in laminating fibreglass are irritants.

Always wear a respirator and goggles when you are cutting or sanding fibreglass to protect yourself from the fine glass fibres released into the air.

Protect your skin with gloves when you are handling the material.

Both the resin and hardener are caustic, inflammable and toxic.

Work with them in a well-ventilated place, away from flame, and keep them away from skin and eyes.


Because of the large quantities of resin often involved, laminating is best done outdoors, where the fumes are less dangerous. But do not work outdoors when temperatures are below 12° C, or the cure may remain incomplete.

Also, avoid direct sunlight and hot or cold draughts, as these can adversely affect the curing process, the risk being rapid gelling or retarded cure.


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.

Some of the basic hardware you need to work efficiently are, plastic buckets, brushes, rollers, cleaning solvent (acetone), cleaning rags (stockinette), wet & dry abrasive paper, Stanley knife, coarse file or small grinder, latex gloves, face mask, and also a little knowledge gained from reading STEVENS hints & tips and guide sheets, on fibreglassing.

The basic material components include, release agent, gelcoat, resin, glassmat, glass cloth, glass tissue, and flowcoat, with the above you can produce many different jobs or constructions, and with a little practice achieve highly professional results.



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

Available mainly, in three different weights, one, one and half, and two ounces per square foot. Metric equivalents are 300g, 450g, and 600g per square metre, or GSM.

CSM lends itself ideally to most laminating jobs including, boatbuilding, car body, roofing, ponds, etc.



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.



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.












These notes are given in good faith for general guidance purposes only and since operating conditions, methods and application techniques are beyond our control, we cannot accept liability for any losses however they may occur.

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