o.k. i have been through this a few times. i wouldnt waste any money on that kreem or an acid wash. i have done this numerous times, just take off the petcock run some water through the tank. fil the tank with sand or gravel about half way. and shake the hell out of it, then shake the hell out of it some more. then rinse it very thouroughly, and add an inline filter. then just keep it full of gas. i have had very rusty tanks with large amounts of flaking rust. i have used this method and have had success everytime.