[OKC_Kent]

It's best to use something other than Muriatic acid.
I use any product containing Phosphoric Acid, like Naval Jelly, or certain Tile & Grout cleaners. Check your hardware store or Home Depot. Seal the openings with duct tape, pour in the bottle (you do not need to fill the whole tank) and drop in a length of chain, or a handful of nuts and bolts to break up the rust. Seal the top and shake it around checking until the tank is clean. Rinse it out good with water. Use a hair dryer in the top to dry it out and don't worry about the little flash rust that will happen. Swish it with light oil or fill with gas and it's good to go.