Sounds good. Just follow the instructions on the bottle. If there is a lot of surface rust, it will take multiple brushings of Picklex and maybe some sanding during the process to help remove the surface rust.

If the goal is to remove as much surface rust as possible and then neutralize any remaining surface rust, then Picklex will work.

Even if the surface doesn't become spotless, the remaining brown/black stuff left on the surface is neutralized. I've found that real old surface rust doesn't always clean up to like new. It will always be stained. But the goal is remove/neutralize and get ready for the next prep steps.

If you have pitting in the metal, and depending how deep it is, you'll need to apply some good quality filler over the freshly sanded and cleaned picklex surface and prime with epoxy primer. If pitting is light, you can spray the epoxy primer on heavily in the pitted areas and sand from there.