We used to get the worst of the rust off petrol tanks by putting a handful of nuts and bolts inside and shaking. Don’t use big ones or shake too vigorously or you’ll get dents from the inside.

Definitely petrol tank sealer and you even get a ethanol proof one now. I use the one thats white it makes it easy to see in the tank, dont ask me what the name is.

For a internally rusty Velocette petrol tank in a similar state to your saddle tank been stood for years with old stale fuel in it I bought a kit from Frost Auto Restoration containing degreaser something called ” metal ready” cleaner and primer and a lining sealant.  I did the nuts and bolts shaking thing first. Came out really well and nearly ten years later is still going strong with no sign of flaking or peeling. Not cheap if I remember, but does work, and it managed to do it without damaging the original paintwork.