elenasgrumpy,

I go along with Ken on the heat thinned epoxy. I started doing this years ago when trying to get the fiberglass reinforcement fabric as smooth as possible on wing center joints. A really experienced builder put me on to the thinning with heat scheme. Now when I'm using epoxy for what you are talking about or for wing joints, I mix the epoxy and set the mixing container in a shallow pan of hot water. If it takes too long of course you have to keep replacing the hot water but this works like a charm. Once applied, specifically to the fiberglass joint, I squeege the excess off with a putty knife and let it set up. As for a firewall and fuse interior, this works well with a brush too.

Cheers,
Andy