I remember years ago there was a paint-like product that you could pour inside a cleaned tank, shake it about for a bit and then drain out leaving the tank with a pristine epoxy (?) coating inside. It was meant for tanks which had light porosity and were otherwise structurally sound, so a split seam for example wasn't the typical market.
For some reason the name "slosh tank sealant" came to mind, I googled it and amazingly came up with something that very much resembles what I was thinking of
http://www.rust.co.uk/0002-slosh-ethanol-resistant-petrol-tank-seal/p405062/Personally I'm not a big fan of such things although I agree they can work. It's basically a thin coating and all coatings rely on good preparation, the thinner the coating the better your prep must be. I can see how you can swill the tanks with acid to get rid of loose and surface rust then follow up with neutralising and the sealant coat, but as before, how long it lasts is down to getting the surface perfect.
I'd use it if I was on a budget (I did something very similar on my Elan tank when that started leaking in the 70s) but for a long term car I think I'd just replace the tank. The problem goes away for ever and you can move on to the next "feature"... you'll find plenty - it's a Lotus
Brian