that happened with my bike a long time ago at that time i had a kawasaki 1980 model, and about 7, 8 years ago its fuel tank got rust, mechanic took the tank off emptied that, and then filled it with handful of concrete (BAJRI) and shaked it for a good ten twenty mins, then he washed the tank with water dried it and it was clear of rust for a good two three years. I also asked him the same thing some sort of a paint or something which could stop the rust inside tank, but he just said no nothing can stay inside. However there are some points to be noted,
That fuel tank was from a kawasaki 1980 model and it could take the inside banging of concrete without any harm to its outside paint.
Secondly its fuel cock was also taken off before cleaning process.
And third is that you know that two stroke bikes use mobil oil with petrol in fuel tank mostly, despite that kawasaki got rusted, it means that you cant stop the rusting process inside that tank.
Cars are a little bit different from bikes since their fuel tanks are bigger and have less curving they can be cut a adjusted later, but i dont think a bike's fuel tanks can be cut and welded to its old shape.
Funny part is that this cleaning is so simple you can give it a try at home.
Han another thing normally we push dusters under the fuel tanks usually it doesnt make any difference but when the duster is wet it helps rusting under the fuel tank from outside also, happened with my CD 70. so next time when your bike is parked in the rain or your duster has got wet first of all dry that before pushing it under the tank
Sorry for such a lengthy reply but couldnt help it,