Wax additive is added to the final layer of gelcoat (without wax) or to the polyester laminating resin (without wax) on the final layer of a wet layup to produce a tack-free surface, and allow the material to cure to a hard finish. When you’re building up the layers in a wet layup application, you don’t want wax in the polyester resin because you want the layers to bond chemically, resulting in a stronger bond. It’s only the final layer that needs wax additive in order to cure. The wax creates an air barrier so the resin can cure to a tack-free state ready for sanding and finishing.