"Gelcoats are special resins that are designed to form the first surface of a composite mould or part. Gelcoats are generally applied as a thickness of 0.3-0.7mm and are specially formulated to provide the part or mould surface with properties such as resistance to UV (ultraviolet) degradation, hydrolysis or osmosis (where water is absorbed into a composite over considerable time). Gelcoats also include thixotropic additives to make them thicker and more able to stick to inclined surfaces of moulds...

Tooling gel coats are specially designed to provide a highly polish-able and durable surface to epoxy based moulds. They are formulated to create a surface capable of yielding hundreds of releases, resist surface damage and work well with a wide range of release agents"...

...quoted from the EC page on Gelcoats...

Cheers,

Kev.