I would have suggested a good metal conditioner had I seen the thread earlier. A scuff pad and Dupont'* metal conditioner will easily remove any surface rust like that. This is basically phosphoric acid if I remember correctly. I use it on my Mustang all the time. It works to etch/prep the bare metal in the area for paint too. BTW, POR15 is not my favorite product. I had trouble with it peeling off in large sheets in my engine bay in the '66. I ended up removing it all, re-prepping it and will now be applying epoxy primer and paint.
Black 2000 SLE