I believe most steel conduit is galvanized. If your's is , be careful welding it, as the burning zinc can give you zinc poisoning. The galvanization (zinc) could also be effecting/contaminating the quality of your welds.
that all depends upon whether you have sufficient bead upon the outside, there is no real strain in the joint, your not grinding the outside flat again.
Why I am going through this is I would like to weld up the exhaust header and tailpipe/muffler on my 66 midget myself instead of taking it to a muffler shop. The outside will be ground for appearance on the header but not on the tailpipe/muffler. I will be welding 1 3/8 OD .065 tubing and 1 7/8 OD .065 tubing. This will be a mostly stock engine and not making any attempt to tune the exhaust piping other than the OD. I will most likely use one of these two designs. Which one will depend on clearance between the #4 tube and the foot well.
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I won't get to this project until next spring and plan to spend lots of time practicing this winter. Depending on how the practicing goes will determine if I have to go to a muffler shop or do it my self.