Just to clarify, I mean surface roughness like what is associated with copper head gaskets and a freshly decked block half or milled head. These surfaces may feel smooth to the touch like what you mean, but actuallly have a roughness to them even though it is invisible to the naked eye. If the roughness is not within a certain range, you run a higher chance of blowing the head gasket.
By the way, do you have pictures? I definitely would like to see the flanges. After welding the flange to the exhaust pipe on the outside, did you finish weld the inside lip of where the flange and exhaust pipe meet? That's what I ended up doing to ensure no changing of the relationship of the flange to the exhaust pipe when things get hot.
The gaskets I made are still holding up and I saw no carbon traces at the flange ends of the mid pipe.
I did however see where the metal hanger which holds the midpipe to the transmission mounted exhaust bracket has been cracking. This bracket is roughly 18 gage in thickness and welded in such a way as to prevent the appropriate direction of movement due to thermal expansion.
The end of the bracket at one of the weld locations is completely severed and the bracket at the second location is severed roughly 1/4 of the way through. Its only gonna get worse:
Either way the bracket needs to be replaced so I will either canibalize the hanger from the stock exhaust and weld it on in this area or I will fabricate one that has the same shape and dimensions.
If anyone was curious the downpipe I have is the MadDad V2 2 piece catted.