hey man my dad told me a story once when i was younger about a friend he had who made thing in the process you are talking about. he was cuting a LP tank that he had gone threw with his normal procedure of washing it out 200 times, yes 200 time he was that carfull. as my dad was walking to his shop to deliver some welding rods or something along that line he heard an explossen and ran to the shop. what my dad found was not much, the largest piece of his friend left was hanging from the rafters. so this is very dangorus, so unless you want to die or have some one else die for you find another never filed tank of find a non flamabel way to do it. ( some one i know made a large pipe cutter from an old lathe took a week and a half but im glad he did, the tank still had a ounce of gas in it and he was going to tourch cut it.)