Your best bet is to have any bare steel covered up with HB primer. To shortcut it now with a crappy rattle can of so called primer would bite you in the arse. It'd be like painting your truck with a rattle can spray paint after putting all the work into it that you have... a complete failure.
The shiny steel spot is showing because it's just high enough to have the primer sanded off so there's two ways to address this, one right and one wrong. The wrong way would be to HB prime the area enough to bury the high spot. The correct way is to get the high spot lower than it is... a body hammer, perhaps some finishing putty, then HB primer. This is likely something you've already been doing anyway so should be fairly straight forward.
I am at a loss about what you would use as a "sealer" over HB primer. The only thing I know of that goes over HB primer is paint.
