We cut tanks every day, first and for most is the tank off gassed and de-valved? if not let someone qualified do that. depending on if the tank has been sitting a while or not sets the steps for what you do for clean out. if it's already cut and relatively dry inside just wire wheel and acetone to clean up residue and your first couple seasoning fires will take care of the rest. if the walls are pretty well oil coated and tank is open. Hit it with a pressure washer and dawn followed by wire wheel and acetone. The acetone will evaporate in seconds without leaving a residue. DO NOT USE BREAK CLEANER SPRAY!!!! it will leave a residue and when welded will create a poisonous gas!! DO NOT UNDER AND CERCOMSTANCE CUT A TANK THAT STILL HAS VALVES IN IT !!!!!!! The oil used is combustible and not flammable it does not off gas vapors that are easily ignited like gasoline, it will flare up a little but nothing to serious, it normally takes direct flame to set it off but rarely will sustain a flame. of coarse this is at your own risk .. :)
Jason