Welding Tips and Tricks

Wondering if anyone knows the correct way to cut apart a used propane tank (gas grill size). The best I've heard so far is to vent it with the valve off, fill with water for a week, and then purge it with argon or helium.
Has anybody out there done this before? I don't want it to be my first & "last" time trying it!!
Rich

Iv done it with 2 bottles. Just left them upside down for a couple days (the gas is heavier that a ir so will drain out) and it was fine, cut it open and welded it...

if you are still concerned about the purge buy some cheap nitrogen-much cheaper than argon or helium i believe