I too weld as a hobby, autobody shop. I witnessed a hot tap on an 8" water pipe recently by the water company recently, they did spill some water, but not much and managed to graft a threaded valve onto it.
Interesting. I haven't attempted to weld real thin metal yet, but just pushed out a 2 inch deep dent in a Geo Metro by hand. Was afraid my hand was going to burst through the metal!