Actually, I tried the stuff on a piece of thick exhaust tubing with some surface rust and after about an hour the rust wiped off clean. Then it had no effect on the tank.
My sand blasting method was kind of creative. I put a rubber hose on the end of the sandblaster to get at the back of the tank. Then, I stuck a bolt in the end and drilled a few holes around the side of the hose, to make it spray at a right angle to the hose to get the sides of the tank. Kind of like a sprinkler head.