I bought a TIG unit last year and it is a whole other world from MIG or ARC welding. Its so quiet and peaceful. It is like using a magic wand on metal that slowly liquifies and blends with the other metal. Good gas welders can do the same thing but with a lot more heat. The problem I ran into with the bung was that the bung was very easy to melt but the old SS exhaust pipe, although much thinner, was very hard to melt, probably due to the heat cycles it has been through. You are correct about penetration. I make sure that both pieces turn to liquid before I add filler. I still don't have a good feel with foot the foot control yet. There is a thumb wheel that fits on the gun and I may try that.