Gel-coat is a relatively thick layer of resin with colorant added. In the manufacturing process it is the first layer of the part, sprayed against the inner surface of the mould. Its smooth shiny finish is an indicator of the mould finish. Any mould imperfections would be reflected in the gel-coat as well. Functionally, it serves as a protective finish to the structural portion of the parts composite lay-up, particularly against abrasion. The gel-coat also provides a barrier to moisture penetration into the weave of the part lay-up.
By applying a gel coat to the form, the surface of the gel coat does not completely harden in contact with the air, but remains slightly sticky to ensure adhesion to the next layers of the component, which is usually polyester, vinyl ester resin together with fiberglass, carbon fiber or kevlar reinforcement.