how do i back purge, run argon /co2 inside the tube while welding the outside? thx

Yes, tape the far end and run an argon purge in the open end, tape around the hose. Make sure you poke a hole in the far end so it pushes the oxygen out. We do it all the time while welding stainless, inconel, ect.

It has been my experence that the steel will melt before the ceramic in the coating. The best thing to do is go to a pool supply company and buy a gallon of muratic acid. Break thru the surface coat with your 80 grit sanding wheel, then dip the part to be welded in the acid for a few minutes. This will pull the coating out of the pores . Rinse the part off with water, blow dry with air .. and it will weld just as clean as new metal.
This also works well on getting the cad plate off of grade 8 bolts .
Be careful of the fumes ... wear a resperator

will the fumes make more hair grow on my chest? or kill me?

Not to jump in anybodys bowl of fruit loops of how to do this repair but this is what I do we build custom headers end exhaust so I have I bit of a clue of what im doing some days keep your comments to your self dave haha.
Grind were you are going to repair clean as good you can weather its ceramic or powder or paint it needs to be as clean as possible and if at all possible tig weld not mig you will get a nicer more controlled weld but if you mig remember thin tube it will burn away easy.
Remember the cleaner you get it and the better your fitups are the easyer the job will be have fun good luck.
Also the acid is a good idea we also do that sometimes if we are repairing old dirty stuff make it clean is the key and if you mig don't use flux core wire.

i have had my mig welder almost 30 years, so im much better with the mig than the tig machine i just bought. i have never needed to weld previously coated headers. the headers are in great condition, just gonna grind off the coating and have at it....thx for your input guys...rm

So they are ceramic coated..?? Not alumnized tube..? I'd be positively sure it got ground to base metal before welding. Regardless of the process.

Practice that tig, if I have a choice ill tig it every time.

Cardinal rule with welding, cleanliness is next to godliness.

i hads some local bozo come over a few days ago to give me some tig lessons... the bonehead could not weld one thing together with the tig... zero, zip... i threw him out in about 20 mins... looking for another guy that tigs everyday to come over and work with me a bit....

Cant really speak to your folical delema .. and I would imagine if you could stand the pain of inhaling hydrocloric acid fumes long enough .. then yes I would think it could kill you . But then you'd really have to be pretty brain dead to just stand there ...

Not cordinated enough to TIG weld ... maybe you been hittin the pool store already .. ;)

i dont think its a coordination thing with the tig, nothing wants to stick together, i cant get the alum to pool at all when i put some heat to it. i know some of the argon is contaminated here. one guy i know that tigs a lot, hook up every bottle when they deliver it and runs a bead to make sure its good gas or he refuses the tank...