Until the old fuel varnish and rust is gone?

And this could take a while. I use boiling CLR bathroom cleanser rather than vinegar and I finish it off with a POR 15 kit urethane liner. In the finest tradition of Louisiana redneck technology, I duct taped my Aermacchi tank to the jacked up rear wheel of my grandson's four wheeler and tumbled it with nuts, bolts and washers inside. It made a LOT of noise and I had to put a window box fan on the quad's motor to keep it cool but it did speed up the removal of a lot of rust.
Seriously, I have seen some of my worst tanks take a week of tumbling and using fresh reheated cleansers to get it clean.
Rick