To view the animation again, please click on the replay button at the bottom of the YouTube screen Replay button
or the reload page button at the top left of your browser. refresh button

Glass Reinforced Plastic (GRP) / Fibre Reinforced Plastic (FRP)

Glass reinforced plastic (GRP) is also known as glass reinforced polymer (GRP), fibre reinforced plastic (FRP) and fibre reinforced polymer (FRP).

Glass reinforced plastic (GRP) is a composite consisting of a polymer resin, usually unsaturated polyester, vinyl ester or epoxy resin, mixed with catalyst and hardener and reinforcing fibres in chopped strand, rovings or mat form.

A one-off, original product may be made in GRP without using a mould, however the usual way of making GRP products is to lay the fibre reinforced plastic inside a mould.

Hand Lay-up of Glass Reinforced Plastic (GRP)

The easiest method of making a GRP product, e.g. a canoe is to use a mould. The mould may be made from GRP or it may be made from another structurally strong material.

The mould is first checked for defects, any scratches etc. are filled. The mould is then polished with a wax polish to a very smooth, high quality finish. Next, a very thin film of a liquid release agent is applied to the inside of the mould. The wax and the release agent are there to prevent the glass fibre product from sticking to the mould.

The gel coat is the first layer of resin to be applied to the mould. The gel coat is a mixture of polymer resin, hardener and a pigment mixed in the proportions recommended by the resin manufacturer. An exothermic reaction occurs as the hardener reacts with the resin and heat from the reaction causes the resin to cure and harden.

When the gel coat has cured, a layer of fibre reinforcing material such as strand glass fibre is laid inside the mould and a measured amount of polyester resin mixed with a hardener is stippled and rolled into the reinforcing layer. Once again an exothermic reaction occurs causing the resin to cure and harden and bond with the gel coat layer. When the first GRP/FRP layer has cured, another layer of fibre reinforcing material is laid in the mould and resin is stippled and rolled into it. As each layer cures, more layers may be applied until the required thickness is achieved.

When the product has fully cured, it may be removed from the mould. The coloured gel coat that was on the inside of the mould is now the exterior covering of the GRP product. 

Any blemishes in the mould will have been reproduced in the finished moulding. 


Please refer to CLEAPSS, Model Risk Assessments for Design and Technology in Secondary Schools and Colleges, Part 1.054 COSHH Regulations.

Click here to view the PDF version of this resource. GRP PDF GRP PDF
Copyright Laszlo Lipot.