Polyester resin is a thick viscous liquid like syrup to which a catalyst (and sometimes an accelerator) is added.   However, polyesters, like most plastics, lack the inherent strength of metals and are very brittle.   In order to improve their tensile strength and allow them some flexibility, they are often reinforced by the addition of fibres of carbon, glass, sisal, cotton or other suitable materials.  Once reinforced with glass fibre, their strength can far exceed that of steel.

   There are two basic polyester resins used in the marine industry, orthothalic and isothalic. The ortho resins were the original group of polyesters and are still in widespread use. The iso resins have better mechanical properties and show better chemical resistance. Their increased resistance to water permeation has prompted many builders to switch to this resin in marine laminates.

   Curing of polyester is accomplished by adding catalyst and accelerator (usually fixed amounts of catalyst and variable amounts of accelerator) – although most resins are now pre-accelerated. Gel times can be controlled through resin formulation to suit the climatic conditions. The gel time of a resin is the time taken after the addition of catalyst and accelerator for it to set to a jelly like state.   Most modern resins are pre-accelerated, and therefore, we only have to add catalyst except when using pigment or fillers which may require an additional quantity of accelerator.

   The pot life of resin is the time taken for the mixed resin to gel in the mixing bowl.

When polyester resins harden after going through the gel stage, they produce their own internal heat which is called “exotherm”.   This exotherm is much greater when there is a large bulk of resin such as in the mixing bowl.   In thin layer form such as when it is spread out onto a mould with glass reinforcement, the heat escapes easily from the large surface area before it can build up to a very high temperature.   For this reason, the pot life of a resin is much shorter than the time taken for the resin to gel on the mould.

   When resin cures in a mixing bowl, the exothermic heat can be so violent, that the resin will smoke and crack and burn if touched.   Therefore, do not mix more resin than you can use in a reasonable time.

   It is not advisable to reduce the amount of catalyst to slow gel time because of the risk of under cure.   Sufficient gel time control can be had by varying accelerator content.   The disadvantage of pre-accelerated resin is that this control is removed.

   The shelf life of polyester resin is greatly improved if it is kept in a cool place away from light and it can vary from one week to three years depending upon storage conditions.

   Un-waxed polyester resin; where it is anticipated that a period of time will elapse between starting and completion of a particular section being laminated, an un-waxed resin should be used.   Un-waxed resins can take up to several days to achieve full cure thus facilitating the bonding of the subsequent layers. When cured, the surface, using un-waxed resin is difficult to sand so a coat or waxed resin or gelcoat will be required.