Threw in two double hand full of really sharp an new "Spacks" screws and tumbled it by hand several times over a week. Removed 98% of the rust... Rest was done with a liquid rost converter. Afterwards washed out several times with a soapy water, then with clear water, let it dry 2 weeks in the room where my house heater was (roughly 28°C all day...) and afterwards coated the inside with Kreem White. That was 20 years ago, still no sign of rust.
The device above is a great thing. However, how many tanks do you do? Do you need a device to do the tumbling? Not sure, my tank would fit into my clothes dryer, but for sure my wife would kill me if I tried...