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Lots of ideas here: https://www.thisoldtractor.com/moto_guzzi_loopframe_fuel_tank_cleaning_and_coating.htmlThe electrolysis method works well for tanks that have both rust and "varnish" inside.
I use Evaporust filled to the brim, leave it sit for a week or twoIf you add an aquarium heater every 10 Degrees C will reduce the reaction time in half. Heat and rust remover works great.
Hand full of drywall screws. Wrap it in a heavy movers blanket and lots of tape. Put it in the clothes dryer and turn it on air dry. Joking? Yeah, kind of, but it has been done Tank was sealed and is still going strong.