One tip, prebake the epoxy for 30 minutes, then see if it has raised and needs to be sanded back down. Also, you might need to spray some GK over the epoxy area only, bake and sand it smooth like a primer. I have had JBW appear more porous and therefore obvious after a single application of GK.
10-4 .. Since I hot blue most of the time (non-AK) I have to tig weld using the 3.5% nickle steel filler rods Brownells sells for deep pitting & damage. When the bluing is finished you cant see the repair...But I see now how the JB stuff would work under gun-kote or paint . If it was used on a barrel would it hold up after a few mag dumps?
I've never used it on guns, just on carburetors I was modifying for racing. If you get it really hot, it will break-down, smoke, and just crumble away. I don't think I would use it on anything that would get very hot, at least hotter than baking GK at 300* or so. Even then I am only using it like Bondo, as a filler.