dude ! two words , fingernail polish,/remover... ok maybe three,, DO NOT sand,grind or refinish tanks AT ALL !! they need to be inspected INSIDE AND OUT !! federal law as well as common sense will tell you that you need to SEE what is going on with the structural integrity of your tank,,, do you REALLY feel comfortable with 3200 lbs of compressed ANYTHING on your back ??? in this case I would suggest JUST SAY NO !!!