Your bumper was properly prepped by being sandblasted prior to primer-
sealing, the primer/sealer is applied thru a spray gun, its not rattle can
and is a professional automotive product. The primer/sealer itself is a
strong defense against rust although farely UV sensative, so it fades
and looses characteristics if not top coated.
Use an oil based paint at the very least to top coat, apply additional
coats to the inside of the bumper if you feel your in a hard core rust
area......do the same to your lower rocker before install if you feel
its required.
After install, run a nice bead of black silicone accross the top of the
rocker protection area, you'll hardly notice the silicone. Do not seal
the bottom of the rocker area, you dont want to trap potential problems,
it best to let the area breath and allow particals to drop out rather than
lodge...
Proper prep is key.
Dont apply heavy bedliner to the inside of the rocker panel protection,
you'll never get the bumper in place, it fits 'that' snug.