I have to admit that I let this go far too long before I addressed it. The black paint on the front urethane bumper was bad when I got the car in that the black had been slowly chalking off, and you could see the primer in large sections of it. It wasn't down to the plastic yet, but several years of neglect took care of that.
This is a year ago, before I started on it:
The yellow area is where the paint and primer had completely failed, and weathered away. The darker yellow has been exposed to the elements the longest, and has gotten oxidized and rough.
Fast forward to a couple of days ago....
I sanded the entire bumper to remove the worst of the oxidized, weathered yellow tint, and to remove the paint where it was severely weathered. In places where it was solid, but dull, I just scuffed it up good to promote adhesion of the new paint.
This is after the first sanding. The black splotches are where I've applied the special "Flexible Bumper Repair" filler material. It's a two-part system used to fill in pits, dings, and small divots, and can also be used to glue split and torn bumper covers back together. It cures to a very flexible material that sands easily and accepts primer and paint.
One of the tricks to use it for filling small cracks is to grind out the crack (or divot) so you're down to good, solid material, like I did here:
This gives the compound something clean to bond to, and usually stops the crack from propagating any further. I used a carbide ball-nosed bit in my Dremel, and it went through this stuff like warm butter.
After it cured overnight, I sanded it all down again, cleaned it, and inspected it. Some of the pits and voids hadn't filled in all the way, so they were given another coat of the filler.
The object is to get the cracks and divots filled in so that once sanded, you can't feel them with your finger tips. The "lightning bolt" in dark grey on the left side is the desired result, and the spot to the right of it needed a bit more filler.
This is just about the final result:
As you can see on a closer level, I had a whole lot of little cracks and divots!
I'll let this sit overnight, and then clean it again tomorrow morning. If it passes the "Finger Tip Test", which I doubt, I'll mask it off and prime it. Primer usually reveals more flaws, so I envision at least one more round of sand/fill/sand/prime/sand before it gets the topcoat of satin black "Flexible Bumper Paint", some kind of weird stuff with enough warnings on the can to scare DuPont.....