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A Guide to Glass Reinforced Plastic – GRP

Glass Reinforced Plastic is commonly referred to as GRP. Structured from composite materials, it is well known for its durability, strength and ease of maintenance. It is also widely recognised for its corrosion and rust resistant properties alongside its ability to adapt to almost any size.

It is widely used across a variety of industries to make an extensive range of products such as flat roofs, water storage tanks, garden ponds, boats, jewellery and car exteriors.  

GRP is essentially constructed from two layers – a type of resin and reinforced with a glass fibre material e.g. chopped strand matting or glass fabric.

Polyester Resin

Polyester Resin is the most common type of resin used in GRP. It comes in a variety of chemical combinations which determine its suitability for specific fibreglass applications (e.g. RoofingCastingLloyd’s Approved Projects, Surf Board Builds / Repairs).

Most Polyester Resins on the market today are ‘pre-accelerated’ which means the resin is already combined with an accelerant. Catalyst will still need to be added to the resin to allow the curing (hardening) process to start. Generally, 2% catalyst addition is sufficient for the resin to cure at room temperature. More or less (min 1%- max 3/4%) may need to be added to the resin depending on the temperatures of the working conditions. (More in cold weather, less in warmer temperatures). All polyester resins sold at FibreGlassDirect are supplied with the catalyst.

Once the catalyst has been mixed through the resin, the mixture will start to generate internal heat, also referred to as an ‘exotherm reaction’. Great care should be taken when working with the catalyst as it is extremely flammable and corrosive. Ensure that the appropriate personal protective clothing and eye protection is worn (e.g. gloves, safety goggles and overalls). Always store in a cool, safe place.

The resin can be applied alone or mixed with other materials such as talc powder, silica or silica sand to create finer mouldings or repairs. It can also be mixed with metal powders to mimic metal castings if required.

Other GRP Resins

Other resins used within the GRP Industry include Epoxy Resins, Vinylester Resins and Fire-Retardant Resin.

GRP Glass Fibre Reinforcements

Chopped Strand Matting

The most popular type of glass fibre reinforcement for GRP is chopped strand matting. Chopped strand matting is available in a number of weights depending on the strength required. FibreGlassDirect supply chopped strand matting in 300g, 450g, 600g and 900g. As the thickness of the matting increases, more resin will be needed to ensure the mat is fully ‘wet-out’.  Once enough resin has been applied, the chopped strand mat will become transparent.

To ensure you have enough resin for your fibreglass project, use our handy fibreglass materials calculator.

Woven Roving and Glass Fabric

Woven Roving and Glass Fabric are used to increase the strength and stiffness of fibreglass laminates. Similar to Chopped Strand Matting, it is also available in a variety of weights. It is normally used in between layers of fibreglass mat and not used as the external later. If you would like a smoother layer for finishing, we would recommend using surface tissue on top of the chopped strand matting.

Alternative GRP Reinforcements

FibreGlassDirect offers an extensive range of tapes such as Glass Fibre, Kevlar and Carbon Fibre. Fibreglass and Carbon Fibre Rods are also available to buy online.

GRP Tools

One of the benefits of working with GRP is that very little specialist equipment is required. Most GRP laminates will require a brush for applying the resin, a paddle roller for working the resin into the chopped strand mat, mixing buckets, mixing sticks, sandpaper and a catalyst measurement dispenser.

Further information on GRP

FibreGlassDirect has put together a list of comprehensive guides to help our customers through a variety of GRP applications.

Our technical team can also be contacted on +44 (0) 28 41753738 or reached via email sales@fibreglassdirect.co.uk