I need to fuel proof my cub's firewall. It looks like the ARF factory applied something, but I'd rather make sure its done well. I've heard that thinned out epoxy works well for this. I don't need any color - in fact, it'll all be under a cowling anyway.

What is the best way to fuel proof engine area? If the epoxy route is best, how to I thin epoxy? How far do I thin it? What else am I not thinking of?