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Lots of videos on utube of different techniques to clean and derust gas tanks.
(Search gas tank rust removal or similar)
I had good luck with a homemade electrolysis setup to clean both the varnish and the rust.
http://www.instructables.com/id/Elec...rcycle-Gas-Ta/
Another article - I use Borax as suggested.
http://www.practicalmachinist.com/vb...removal-75697/