I've had great success using POR-15 motorcycle tank cleaner and sealer. The kit comes with three components: a magic stuff that totally dissolved a very thick layer of black tarry residue from inside the tank of my 1936 AJS. I think it is caustic soda (NaOH) based, but am not sure. They say in the package that it will also remove previous tank coatings; a phosphoric acid-based rust remover/metal conditioner, and a POR-15 tank liner which hardens on contact with moisture in the air. I believe others in the group have some expererience with POR-15 coatings.
I highly recommend it if you can get it.
David A.