One of the surprises I found when I received the motorcycle at home was to see that the fuel tank was rusted inside, it even had a pore through which it was leaking gasoline.
The same thing happened to me years ago with an old Montesa that I have and that I solved in an effective, economical and easy way.
I bought EPOXY RESIN treatment, cost 20 euros, it is used to repair boats on any surface, steel, wood, polyester, etc.
It has two components, resin and hardener, the mixture is 3/1.
You simply have to wash the inside of the tank with grease solvent and let it dry well, NO NEED TO USE RUST REMOVER, prepare a resin mixture of approximately 750cc. and pour it inside the tank, we have previously covered the holes of the gauge and the fuel valve with a ball of paper.
We turn the tank horizontally and vertically, in this case being careful not to cover the vaporizer tube, remember that the most rusty part is usually the base with the union of the sides.
After a few hours we repeat the turning, we remove the cap from the flume to eliminate the excess resin that we will store in a container.
There is no need to be in a hurry, the mixture takes a couple of weeks to dry completely, the longer the better, we will control the drying with the excess that we pour into the container, if you can't stick a finger nail in it, it is ready.
The result is a clean, lifelong repair.