I had a 1975 International 1/2 ton pickup a few years back with cruddy, rusted up tanks. A mechanic friend of mine put a couple of big scoops of rough edged gravel in each one, fastened them to the front of an old cement mixer, and just let them rotate for several hours. Worked like one of those small rock polishing machines, and the tanks cleaned up really well. Might not make you too popular with your neighbors though, if they live close by.
