So in my opinion the best way to repair this to paint the entire bumper.
I am going to leave out a bunch of steps, I will assume you have some experience and know to wash, degrease, wipe, tack, how to sand level, etc.?
The bumper is aluminum so the rust isn't an issue it is stuck on the paint from the bumperette. Simply sand it off.
Sand the entire bumper with 80, then 150.
Fill and fair any imperfections in the bumper. Sand to 220.
Find a paint jobber near you that sells 2K paints in a rattle can. Spray the bumper with a coat of 2K high build primer. You can also buy online. Cost is about $17.00 per can.
Sand to 600.
Have your jobber mix your paint code in the 2K can as well. Spray the bumper with two light coats, let flash fifteen to twenty minutes at 70 degrees, you can also force flash using a heat gun. Cost is about $25.00 per can
If you are going to use a single stage (no clear coat) follow with four or five wet coats. Again let flash between coats. If you are going to use a clear coat paint with your base just enough to completely cover the bumper with color. Let flash and follow with clear. Four or five wet coats flashing between each coat. Clear coat cost about $20.00 per can.
