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.
Read here, starting at about the 5th post. I know the thread isnt about tank-sealing per se, but Masterseries is a well recognized tank sealer as well - and an
incredible, thick, sealing, rust-arresting, will never-let-go Amphicar primer, particularly for floor & seams!
http://forums.vintage-mustang.com/vintage-mustang-forum/811826-master-series-silver-question.html