[1978 T/A]

I don't see why you couldn't use a surface primer. Epoxy primer/sealer lays out a lot thinner than a surface primer and is really intended for filling light sand scratches before you shoot a base coat.

What kind of sanding sponges are you using?

I would reprime the whole bumper with at least 3 coats and sand from there. If you're not going to apply a final coat of sealer before paint you need to wet sand the bumper with 600.

This is the process I generally go through in repair:
Filler if needed.
3 coats of surface primer.
Guide coat and sand with either dry 320 DA paper or wet 600.
Lay down 2 coats of sealer.
Lay down base coat & clear.