<<Forget the exhaust sealant, it will quickly blow out of the joint. The
flanges have to be faced. A good tool for this is a stationary disk
sander. If you know how to use a file, that will also serve the purpose.
I've had exhaust flange fasteners loosen, but I never had a warped
flange>>

I've found several exhaust flanges warped on a couple of Vanagons.

I used a palm DA with 40 grit on it and it got flat pretty quick.

I used a small piece of glass to check the work to be sure I was on the
money with the area in question.

Chimney glue isn't the answer for sealing the exhaust system up.
I agree with George.
It'll soon blow out from the pipe union.