This is how I prepared my used metal gas tank before welding on it. First I removed the gas tank from the vehicle. Second I removed the sending unit. 3rd I placed it in the bed of my other truck, strapped it down, filled it full of water and added a couple ounces of dawn dish washing detergent. Then I drove around town for an hour running errands, which allowed the water to slosh around then I returned home and filled the tank to the top with water again( I used the jig saw so it wouldn't create sparks instead of something that would make sparks). I used my jig saw to cut a 4.5" hole in the top so I could retro fit an EFI in tank pump. Once I had my hole cut I emptied the water out ( it had a very faint smell of gas fumes but didn't smell like raw gas) left the tank in my shop for 3 weeks until I could find time to work on it again. (Less time would probably be fine, this was just my situation.) 3 weeks later I placed the tank in a safe location, PROPPED UP MY IPHONE made a fuse out of old tshirt, placed the end in the tank and lit it and ran for cover. When the flames entered the tank nothing happened which proved that the tank was properly cleaned. USE EXTREME CAUTION!!! People have been killed when welding or cutting on USED GAS TANKS WHICH WERE NOT PROPERLY CLEANED. USE THIS METHOD AT YOUR OWN RISK, NOT RESPONSIBLE FOR ACCIDENTS!!! THIS VIDEO IS FOR ENTERTAINMENT ONLY. Thanks for watching.