see the topics on electrolysis for rust removal.
to do this on a tank you will have to have a sacrificial anode that can fit inside the tank ,as electrolysis works on line of site.
it works extremely well and will remove all traces of rust.
i would then go and follow the gravel and kero method inside to polish it all up.
wrap it up in bubble wrap and put inside your clothes dryer or similar and let it spin away
