Hard to tell exactly without seeing it in person. But sanding and several applications of Picklex will remove and neutralize the rust.

The final surface will likely be a mix of brown and black, but you should be able to rid of a lot of the rust and what's left will be neutralized, based on what I've seen Picklex do and what the maker of Picklex says it does.

I would hand sand during some of the applications to see if you can release as much of the surface rust as possible, then do any filler work (if deeply pitted) and then prime. If pitting is light, apply heavy coats of filler and sand.