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If the rust is minor, I have used wd-40 and scotchbrite pads to bring the tables on my woodshop machines back to glory. works pretty quick and the wd is harmless. I came across a thread on here a while back about preventing rust on smokers, it said something along the lines of getting it hot, and rubbing vegtable/ maybe canola, or olive oil into the metal while hot. Try searching "how to season a new smoker"
How do you intend to finish your tank? If you plan on painting it I woulds avoid oils, as you will have to strip that off in order to paint. If it's the interior that you aren't going to paint then using a food grade oil would be the best. Flax seed would be the best choice if you can get the heat to it to harden it.