look at rustoleum black stainless lacquer, it looks like a gunmetal bare steel look, no metal flake. I cant find it online on their website, but at autozone they have it or did.
I did one coat etch primer, two coats black sand primer, about 6 coats lacquer, looks great. Cant find my pic to show
You can polish bare steel all you want, and it will still rust. If you've ever polished bare steel, doing an entire bumper would require a lot of work.
Aluminum would be easier to polish, but will oxidize quickly to a dull silver.
This is the best stuff to use for that bare metal look. It's easy to apply, and looks better than polishing or powder coating.