I took the tank from.my Porsche 914 to Gas Tank Renu. They actually cut a section out to blast the inside, they weld it back up, pressure test, then coat inside and outside of tank. Inside looks like a white epoxy, outside is a black undercoat type material I had them just paint the visible portions of tank.
I would have done that had i seen rust or metal for sure - but it the case of my tank, it had some sort of tan/reddish coating inside that had started to crack and lift away from the metal - must have been some kind of pour-in & tilt back and forth coating from a previous owner? Maybe later tanks had this interior coating from the factory? It was very hard and solid where it was still adhering strongly, but it definitely was visibly lifting and cracked in some areas. So with that kind of texture to deal with, physical-impacting was really needed to "break-free" all the lifting scales and loose edges. Once that was done (lotsa shaking taking turns), we gave it a real good inside washing (yes, with water and soap) - and rinsing with water - then a hot air drying with a hair dryer for an hour. THEN, before any rust could begin to form, i >immediately< poured in a quart of Masterseries Silver, and rotated the shit out of it, to 100% coat and encapsulate everything inside - smooth and sealed now, and harder than rock.