Get a quart of semi-gloss black paint with a hardener. It will be very durable, and a quart goes a long way. You can spray the rear axle, bumper brackets, and lots of other parts. I have used this, and other flats many times and have no complaints; plus you can order it online and have it in a few days.
Powder coat, epoxy, enamel semi-gloss. Aout anything you want to use. Is his a daily driver or show/weekend car ? That might make a difference for durability, touch -ups.