Glass Reinforced Polyester (GRP) is, as the name suggests, a polyester material reinforced with the addition of glass fibre (also commonly known as fibreglass). The material is produced by combining thermosetting polyester resin and glass fibres under strictly controlled factory conditions. Glass fibres are applied into the resin with each layer of fibres facing in different directions, enabling the stiffness and strength of the finished material to be controlled. The effect is a strong, lightweight material that is naturally watertight.