Krylon makes paint specifically for plastic. I have used it and it works fine. I think that if it were mine, I'd try the rubbing compond approach "DavidG" suggested first.
I use black india ink on faded bakelite parts.
get some and try it on the back.
Make sure it is INDIA INK because only india ink will look black in the bright sunlight. Most other black inks will look purple or dark blue in the sun.
You can apply it with a Q-tip then wipe off the excess.
It will NOT have a painted look.
There is a product that I think Meguiars makes called Back To Black. It's similar to a polish that is used on black plastic bumpers or black surfaces to remove the chalky white residue. Worth a try...
Wash the part with soap and water, or alcohol using a old toothbrush to remove dirt and oil and you will have better coverage. If it is a hard shinny plastic it may not sink in very well.
What does the underside look like?
I am fairly sure even a light wet sand with 1500 paper then rubbing compound and a final glaze compound will bring it back to life.
They chrome plastic on new cars. Maybe you can research adding a black oxide (bluing) treatment? Maybe just a nice buffing might be good enough for application?
I thing you should sand it will 2000 wet and buff it out. I've attached a photo of two window frames where one was complete and the other not yet started.
Guys,
Thanks for all of the suggestions. I actually bought some of that Back to Black stuff and tried it on the back. It seemed to work OK, but then I read the fine print where it says 'Lasts for weeks'. NOT really what I was looking for.
So I decided to try just some polishing compound with a Dremel tool. You know those bars of stuff you can buy. A quick trip with the white compound and then the red rouge stuff seems to do the trick. Don't run the Dremel very fast though.
Here it is half done. Needs a bit more polishing but I like it. It doesn't look freshly painted if you know what I mean.
Nice work Heard. That’s what i thought would happen but had not been smart enough to think Dremel. No braid aid fixes of goop. Persistence will pay of in a very long term fix for you.