I've used a similar method to Stew.
We've made lost of things out of gas bottles (the local depot always has a few orphan empties with no local dealers to return them to, that they are keen to get rid of).
Usually we use an angle grinder and slitting disc. But always purge them with water first.
After leaving them for a few weeks with the valve open, I drill a small hole in the top (with a sharp drill, you don't want it getting hot), then poke the garden hose in to the valve, turned on a for a trickle. That fills the bottle, with the small hole allowing the displaced air to vent.
Then I start cutting whilst there is water still in.
The only issues I've had was when welding one up that we had turned into a water heater. The solvent from the paint inside caught. Went with quite a whistle and blew all the dust off the patio!
I have seen petrol tanks welded up, still with fuel in. In that case, the air inside was displaced with CO2 from the welding bottle, so that there was no chance of combustion inside the tank. Though fire risk from leaking fuel was serious. Not sure I'm brave enough to try that myself!
But, I do similar when welding coated metal sections, where you can't get/see inside. Such as car sills where they may have paint or waxoil in. I purge them with CO2 first.
Jules
