As always on these forums, the safety police like to come out in their full riot gear whenever something even remotely dangerous is mentioned. Fill with an inert gas? Purge it with exhaust?? Geeze guys, why give such advise?
I've cut and welded a dozen tanks in my time and all I've ever done it is fill them with water until overflowing several times to wash anthing in there, out. Once you've done that nothing is going to ignite no howm no way, no time.
I also aint skeered to weld or gind near gas tanks either. I just finished rebuilding the frame and installing custom exhaust on my Galaxie with the tank half full (see my blog). Gas tanks don't just blow up everyday - or there'd be a LOT less of us doing this stuff....
Of course I'm not kicking safety to the curb, I'm just depositing a dose of reality into the conversation.