it seems some kind of hardner
Fiberglass resin is an epoxy resin. You usually mix it with equal parts of hardener (always follow the instructions). Without the resin, it might take a year for it to harden. The more hardener you use, the faster it hardens. Temperature has an effect on hardening speed. The colder it is, the slower it hardens. The hardener should be in the box with the resin, when you purchased it.
It will harden the foam without the cloth. When mixed, the resin gets real hot as it cures, hot enough to melt some types of foam. I would try it on an extra piece first and see if it works good.
Using Bondo Fiberglass Resin
