Nivitex Fibreglass and Resins Logo Image

tel: 010 030 0180

Mon to Fri 7:00am - 16:30pm
Sat 9:00am - 12:00am

Request Information

Guide to GRP Fibreglass

GRP stands for Glassfibre Reinforced Plastic – a plastic resin ( polyester resin, epoxy resin, vinyl ester resin, Isophthalic resin and phenolic resin) reinforced with glassfibre mat or woven fabric.

What is GRP?

The term is often used (rather inaccurately) to include plastics reinforced with materials other than fibreglass, carbon fibers or aramids such as Kevlar.

Polyester resin is a treacly 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 practically applied or painted onto a mould. The hardened plastic is fairly brittle so glassfibre material are added to give considerably increased strength (just as steel rods are used in reinforced concrete). The plastic cures at room temperature like polythenes, polypropylenes and other materials which need heat and pressure so it can be easily used in the home environment, school or small workshop.

The combination of polyester resin and glassfibre produces an incredibly versatile material that is strong, durable, weatherproof, waterproof, non-rusting, non-corrosive, 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 like (calcium carbonate, talc powder, cenolite, wolastonite, silika, silika sand and many more), to make castings which accurately reproduce the finest detail of the mould. It can be mixed with metal powders to create realistic imitation of metal castings. The finished item can be sawn, drilled or polished. There are numerous instances where GRP is a more appropriate choice.

The range of items which can be made with these materials are almost unlimited such as costume jewelery, sculptures, garden ponds, shower trays, storage tanks, cladding panels, theatrical props, ballistic vests, racing cars, trucks and many more it is impossible to describe all the techniques involved.

Getting Started

One of the advantages of GRP is that you can start a project with very little in the way of tools, equipment and workshop facilities. Even for quite major projects, the only equipment required is a few brushes, laminating rollers and plastic buckets. This contrasts sharply with the amount (and cost) of equipment needed for carpentry, metal-work and other construction methods.


The following are the materials normally required for most GRP techniques:


Gelcoat is a resin which forms the smooth outer surface of the finished laminate (it is not normally used for castings) and is therefore applied first to the mould.
Approximate quantity per sqm is between 500gsm to 700gsm of gelcoat resin is needed for each square meter of mould surface.

Lay - Up Resin

General purpose Resin is used for hand laminating, casting and for laying-up glassfibre - with a Resin to Fibreglass ratio of 3 to 1 (Example: 1kg of Fibreglass will use 3kg of resin).

1 sqm of Chop Strand Mat 300gsm = 300gsm = 1kg of Resin = 20g of MEKP or Catalyst.
1 sqm of Chop Strand Mat 450gsm = 450gsm = 1,20kg of Resin = 25g of MEKP or Catalyst.
1 sqm of Chop Strand Mat 600gsm = 600gsm = 1,80kg of Resin = 35g of MEKP or Catalyst.


All resins, including Gelcoat, Poolcoat and Flowcoats must be catalyzed with catalyst (MEKP) or hardener before use, un-catalyzed resin simply will not harden. Catalyst is available as a paste or liquid, 20g liquid catalyst is usually needed per kilogram of resin. (Catalyst, especially in liquid form must be handled with special care – see on “Precautions”).

Protective Wear

You are strongly advised to wear plastic gloves, chemical grade respirator and most importantly protective eyewear as well as Barrier cream, when handling resins or glassfibre. This will protect you from any injuries that could occur from working with the chemicals.

Working Area

Whether laminating or casting you should work in an ambient temperature around 20°C as this ensures that the resin will cure correctly. Resins will not cure adequately below 15°C, it is strongly recommended that you do not fibreglass anything as this could cause undercuring of your fibreglass part, and temeratures above 30°C will cure too quickly!

You must not use open flame heaters as resins are highly flammable so maintain the temperature with convector heaters, radiators, closed heaters, air heaters or similar. You will need trestles to support the mould at a comfortable working height – for small jobs, of course, you must use normal workbench or trestle table. Cover it with polythene, Polyester Film or brown wrapping paper to prevent the resin from sticking to the work bench.

Estimating Quantities

To estimate the quantity of materials needed for a specific laminating project, you will need to know the surface area of the mould you will be using, and the thickness of the finished laminate (ie. The number of layers of glassfibre). From this, the required materials can be calculated.

The surface area (in sq meters), multiplied by the number of layers, gives the total amount of glassfibre material.

Area in sqm x fibreglass material gsm x number of layers = fibreglass needed in kilograms.
10 sqm x 0.450 gsm x 5 layers = 22.5kg of fibreglass.

Using the table below, you can estimate the amount of lay-up resin needed.






Chop Strand Mat

















Divide the surface area by ten. Then multiply by six, to obtain the maximum amount of Gelcoat in kilos. Add the gelcoat and lay-up amounts together to find the total resin quantity. You will need 20ml of catalyst for every kilo of resin , gelcoat , poolcoat and flowcoat. You will need a maximum of 100g – 150g pigment paste per kilo resin, unless you are using black pigment, then the maximum is 50g per kilo of resin.


These materials are safe, provided they are used with proper care and adequate precautions are taken:

  • GRP materials produce vapors which are flammable and should not be inhaled.

  • ALWAYS work in a well-ventilated area, and NEVER smoke or use naked lights or fires in the work area.

  • When you have finished working, make sure materials like catalyst and brush cleaner (acetone) are securely stored away. Never store acetone and catalyst MEKP together.

  • Wear gloves and goggles – do not let any materials contact the skin , and especially the eyes or mouth.

  • Do not swallow any of the materials.

  • Catalyst should be treated with special care: it is flammable and corrosive – do not let it contact the skin, mouth or eyes. Should it do so, wash from the skin immediately under running water. If it contacts the eyes, flush them under running water for at least 15 minutes and obtain medical attention at once.

  • Glassfibre, Resins and ancillary materials should not be used by children. Please keep in a safe place.

  • Treat liquid catalyst (hardener) with special care. Measure out small quantities with a syringe. For larger quantities, use a purpose-designated safety dispenser bottle.


Although laminates and castings can be painted, it is simpler to make them self-coloured, by adding pigment paste to the resin and/or gelcoat and poolcoats.

Glass Fibre

A variety of glassfibre materials ( mats, rovings, woven fabrics, gun rovings etc.) are available to give a choice of strength-to-weight ratios. The most widely used is probably Chopped Strand Mat.


Add filler powders to a lay-up resin to produce a general purpose body filler / stopper paste or to make a “slurry” for resin casting. Fillers are not needed for many laminating projects or applications.

Release Agents

Release agents are applied to the mould prior to laminating, to prevent the laminate adhering to the mould surface. They are not usually required for silicone or latex moulds where self releasing flexible moulds are used.

Cleaning Solvents – Acetone

Acetone is used for removing uncured resin from brushes and tools – it can be harmful to the skin and should NOT be used as a hand cleaner. It evaporates quickly and should not be kept in warm or hot areas. Acetone is only used for cleaning and not for lowering the viscosity of the resin, gelcoat, poolcoat and flowcoat. Acetone will dilute the molecular structure and weaken the finished product.

Barrier % Cleansing Creams

To protect or clean the skin – Barrier cream should be worn in conjunction with gloves when handling resins or glassfibre, and cleansing cream (NOT - Acetone) should be used to remove resins, etc, from the skin.

Some GRP projects may also require one or another of the following:

Specialist Resins

Apart from the standard gelcoat and lay-up resins, a wide variety of other resins are available for specialist users such as making chemical-resistant or fire-retardant laminates, clear castings and castings better known as polymer concrete, etc.

Specialist Reinforcement

Reinforcements other than glassfibre are sometimes used, especially for high-performance laminates. Typical examples are Kevlar, a synthetic aramid material which is even stronger than glassfibre, and has excellent impact resistance (it is used commercially to make bullet-proof vests) and Carbon Fibre, which gives greatly increased rigidity to the laminate. It is used for applications requiring a strong, rigid but lightweight laminate – eg. in racing car bodyworks or canoes and kayaks.


Used to thin resins in certain cases, eg. as an initial primer in boat sheathing. It is advised that not more than 5% be added to resin , gelcoat , poolcoat or flowcoat as the products are produced to the correct viscosity and should too much styrene be added it will loose all of its thixotropic characteristics , where drainage will become a problem and the products will not do what they were designed for. Styrene can also be used as a cleaning agent before fibreglassing on existing fiberglass , this is advised on all repairs and pool relines.

Wax Solution

Reduced tackiness in gelcoat resins – used in boat sheathing, making garden ponds, and other projects where gelcoat is applied as a surface coat. However, it is advisable to use only fully formulated Flowcoats, Topcoats or Poolcoats.

Foam Mix

Liquid polyurethane foam is used to fill hollow cavities, either to give increased rigidity (eg. in masts and booms) or usually to give added buoyancy to boat and canoe hulls. Urethane 2 part foam can also be used for structural strength.

Core Materials

These products are used to impart reduced mass and stiffness to GRP products and also assists with thickness build-up.

Tools and Accessories

Mixing Cups & Buckets

For mixing resins, pigments, fillers, etc. Plastic containers are ideal. Do not use polystyrene as it is dissolved by the resins.

Brushes and Rollers

For applying resin use the correct brushes and rollers. Use the correct rollers to remove all air bubbles from the fibreglass laminates. Be cautious of using plastic products as the resin may attack the plastic.

Metal Rollers

Needed to consolidate the resin / glassfibre laminate.

Catalyst Dispenser

A purpose-made safety dispenser is essential for measuring out liquid catalyst. For small amounts, a measuring syringe can be used.

» Back to Top

The information included in this document is given in good faith and is intended to assist you the customer in determining the suitability of this product for your application. Due to the diverse applications and conditions in which many of our products may be used, we request that you, the user, test and inspect our product to satisfy yourself of its contents and suitability for your specific need. This document does not constitute any guarantee or warranty expressed or implied. The exclusive remedy for all proven claims is replacement of our product and under no circumstances shall we be liable for any special, consequential or incidental damages.

Fibreglass How to Guides

Fibreglass How to Guide

Nivitex have various guides available for the DIY person that will help you with your next project from costings on projects to using various fibreglass products for mould making etc.

See all our DIY guides

Technical Information

Technical Information Guides

We have technical information available on various fibreglass products. These guides will help you with faults, how to apply products, grades of fibreglass and much more.

See all our Technical Guides

Need Help?

Vacuum Bagging Image

If you need assistance or you just need some reassurance that you are looking at the right products for your project.

Contact us we'll put you on the right track.

Request Information

Fibreglass Products

Associations image