I used a fist-full of 1/4" steel (slingshot) shot, capped the tank and freaked out the cats for a while shaking it up to clean it out.. had a fun time trying to get all of the shot back out of there, though, due to the shape of the filler neck on the inside of the tank. (There's a lip inside it liked to get caught on - a magnetic pick-up tool rounded up the last of it). Note though that it was a new tank, never had gas in it, so I didn't have to worry about sparks. (I also used the wife's fancy digital kitchen scale to make sure I got out the same amount that went in, no strays).