Catalyst allows polyester, gelcoat and vinyl ester resins to cure by reacting with the promoter in the resin, or with heat. This starts the chemical reaction of the resin with the styrene monomer (present in the resin) allowing cross-links to form between them.
These cross-links act as braces, joining the components in the liquid resin together. When some of the cross-links have formed, the resin forms a gel and is said to be “gelled”. When most of the cross-links have formed, the resin forms a solid and is said to be “cured”.