We had old gasoline deposits, tar, varnish and the like but not so much rust, so we mixed up some LYE - sodium hydroxide with water and being careful not to spill any on the paint job, we filled the tank with about 5 gallons of the mixture. This is the same stuff used to open a clogged drain. It WILL remove paint if left long enough.
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We rocked the car vigorously several times over 2 days to slosh it around thoroughly. It was amazing at how well it cleaned the nasty goo left from old gasoline. After draining and flushing well with water, we did the same process with some phosphoric acid to "pickle" the inside surface of the tank. Drained all the phosphoric and let it air dry for a couple of days.
This process worked well for us to conquer the problems of crud left by old fuel deposits. If your tank has heavy rust, that requires a different approach depending on the severity of the rust.