Seems to me that a few days of sitting open, in a hot shop, would evaporate everythin that could go "Boom"

With all due respect to that guy and his single match, Paladin, I would rather err on the side of caution.  I've spoken to several welders over the years on this very topic.  The consensus has been that a tank is never aired out enough.  One even told me of a barrel going poof after years of sitting with the bung open.

Everyone I have ever asked told me the only safe way to weld on a fuel container is to have it full of water while you are welding.  I realize this would make brazing difficult, but I'd hate to see Justin walking around with no eyebrows.  I would think if you are going to air out a tank for a few days, you might as well do it with the tank full of soapy water for a couple of those days.

Suit yourself, and let me know how it goes so I can tip my own mental scales one way or the other for the future.

One question I don't have an answer for:  Is it possible that the vapor doesn't exist until you heat the varnish that has accumulated on the inside of the tank?  Maybe a match won't do that, but prolonged heating with a torch would?

This company describes how they prepare a motorcycle fuel tank for paint, they actualy put it in an oven first: http://www.gas-tank.com/bike.htm. Not pro or against, just thought that it was interesting reading.