Hi
One of my mates used to connect the tank to car exhaust and run car engine push out the fumes and gases and old gases left in are neutral
another one says this :
, I wash the tanks out with parrafin to remove the petrol and keep it, then wash out with soap and water, now weld the tank, once welded fill the tank with your parrafin again to test it, don't use water as it won't search out any minute pin holes. I've been doing this for years without even the slightest pop or fizz, I hope this does help