If you have a sizeable tractor available to you, you can add some 1/4" nuts and washers (easier to retrieve with a magnet after you are done) and strap the tank to the side of the tall tire and go for a spin until the crud is knocked loose. I have never sealed a tank before. If rust is so significant that removal of the rust and subsequent sealing is necessary, you would be money ahead to buy a new tank. If your tank isn't significantly rusty, you don't need sealant.
Carl
