I used Andy Malton at Motorcycle Dent Repairs in Pickering for my Z1300 and GT750 tanks, as the amount of rust and shite inside them was beyond belief.....no amount of home potions worked.
He's not cheap but he's thoroughly professional, and can deal with any amount of internal corrosion or external dents without affecting the outside paint.
My GT750 tank turned out to be leaking like a sieve when leak tested, due to the amount of metal loss, so I okayed it for him to strip it and silver solder the leaks.
When stripped it was found to have been previously crash damaged and poorly repaired, but Andy did such a good silver soldering job and smooth down that it not only passed a pressure leak test, it also only needed basic priming in order to paint it.
Both tanks were zinc passivated inside after he'd removed all the rust and crap, so they were like new again.
I don't mind paying for a professional job like that, and if I'd bought any other tanks, they'd still be forty odd years old and in as dodgy a condition as mine were anyway.
However, although he doesn't like or normally advise to put any tank lining stuff in, due to his concern over the amount of metal loss from the GT750 tank, he recommended that I treat it with POR-15 tank sealer.
It's the only one known to be ethanol resistant, so I got some from Frost Automotive and put it in myself.
It's awesome stuff but very dangerous, so read and follow the instructions very carefully if you get some.