Hmmm, I guess I need to work on my building skills and knowledge....when I fuel-proofed the firewall and fuel tank compartment on my last model, I just mixed up epoxy normally (without thinning) and slopped it on with a scrap of ply.
Actualy you do not. You are the only one here that did it right. Epoxy resin is easily brushable, how else could we lay any thing up with it? Whereas it might not make a lot of difference sealing a firewall, thinning epoxy is bad practice. Thinned epoxy does not have the same integrity as the full strength. Some of them do not cure completely when cthinned.
Ed S