Just read this posting on the main board and felt it would be good to post a link here as most of us are builders. http://www.jalopyjournal.com/forum/showthread.php?t=382835 It's a slow loading article that it links to, but worth the read if it prevents someone from getting hurt.
Definately a must read. Refrigerants do the same thing. NEVER weld on a charged A/C system (but we don't have those around here, do we!?!).