If your tank is in good shape, this is my approach to it. Drain the tank completly out. Make sure its dry. Run your hand along inside feeling for any bubbling, unsound surfaces, ect. Take a standard issue wire brush and clean all of the loose metal free. Then get a bottle of CLR($2-$5) and coat the entire inside of your tank. After about a 1/2 hour, all of the surface rust should be clean. Take a standard issue cleaning brush, and scrub the sides just to be sure. Pour the clr into a disposal container, flip the tank over to get the little loose pieces out, take a Damp, not soaking cloth to get the residue off the walls, dry with either paper towels or heat, and your tank should be rust free. Total cost $5 if you have the towels and brush, $20 if you dont.