I am not any kind of professional when it comes to any body work subject, but thought I would share some of my experience with a couple products.
I have used Rust-Mort, quite extensively actually. From my experience it does work. When I used it, I was skeptical to say the least, and after a few years, impressed. Keep in mind that it does say to leave some rust, scrape off most of it, but leave enough to chemically react. In other words do not sandblast it before applying the Rust Mort.
On the subject of covering bare metal, I am told use either self etching primer or epoxy primer. I used epoxy primer the first time around on my Galaxie. when I stripped a couple years later the epoxy primer stuck very well, actually hard to get off. This time I used self etching primer. I sprayed over some spots that needed to be filled yet just to cover it up until I could get to it. It comes off alot easier than the epoxy did.
Moral of the story is....I would use epoxy primer on bare metal even though it costs more.