I would be apprehensive to use any sealant just for the fact that the fumes may be harmful. I would almost guarantee that if you screw the fitting into the barrel and it tightens up there will be very minimal air leakage to affect the temp control. Keep it simple and remember that the fire needs air so there will never be a time where you completely shut it off unless your trying to smother it out.
Its Si-intistic, Jack!
One lovely UDS
Shanaynay--Custom Home Built Smoker on Wheels-SOLD!!