Epoxy

From CosplayWiki

Epoxy is a coating used to cover and seal the fiberglass cloth. It comes in two parts: a resin, and a hardener. The mix ration depends on what brand/type you use, but is often 2:1. Depending on what sort of fiberglass cloth you use may determine what type of epoxy to use. You can probably get suggestions from the retailer. System Three makes a general purpose epoxy that they say works in most applications.

You can also buy smaller containers of 2 part epoxy that are used in smaller projects at almost any store.