Take the sugestion to search the forum using words like tank, acid, rust.
I have used the gravel and solvent method, the Kreem kit, muriatic and phosphoric acid, you will find details on all of these if you search..
My favorite is phosphoric acid. I have had no problems with re-rusting, especially if you use pre-mix. The oil seems to be a good enough protector. The in-line fuel filter is also a good idea.