Hello, Is it ok to coat the firebrick arch with a layer of high temp mortar prior to insulating? Would it add more strength to the arch or be a waste of time and material?
Welcome back Darrell!
There’s nothing to say you couldn’t do this, but personally I wouldn’t.
The strength in your arch is partly geometric and partly mechanical. The arch is perpetually “falling down” toward the middle from each side, and holds itself up because it meets in the middle and the two sides press on each other to hold it up. (Thank the Greeks and Romans for discovering that, and @BrickWood for providing a form that follows their principles.)
Mechanically, the mortar between the joints fills in gaps to smooth out the arc and transfer load to the base that you built.
Coating it with high temp mortar won’t make it any stronger. And, high temp mortar outside the fire envelope of your oven won’t cure properly because it needs flame to make it rock-solid.
My advice is to go with choice “B” (waste of time and material) and proceed to insulating once you’ve finished the barrel per instructions.
Good luck and please keep us posted!