Nice work BTW OP.
You know I am new to the tig process and I love it BTW. But I havent taken the time to look into back purging yet. From what you just said, I assume it is shielding gas pumped inside the work, to keep the penetration inside the projects welds from contaminating? I noticed that when I was welding up my charge pipes for the jetta and the wastegate/exhaust work I did too. The welds outside were butter, but the inside was all dumpy looking.
please enlighten me, not to steal the thread.