Over time, the black plastic that is unpainted oxidise and assume an unpleasant gray complexion. So here are some suggestions that you can do something about it?
– Use solutions off-the-shelf
There are many products on the off-the-shelf specially designed to restore the black plastic trims. Give something like bumpers and moldings Autoglym Gel or – if you feel flush – expensive ‘Pneu’ Swissvax. Both will bring the black plastic liners back to its former glory, leaving a protective layer. Oh, and they will not damage the paint if there is a spill.
Raising the budget further, many are using Gtechniq G4 Permanent Trim Restorer, which – according to the manufacturer – will last two years.
If you are looking for a more permanent solution and you have enough time on your hands, you may want to respray the whole black trim. It is much easier if you can remove the trim piece and you need to do a damn good clean before you continue (here is the divider community manager John CT old MX-5 to get a good bath before spray).
Once clean and dry, it is ready for coating. Your local auto parts store sales paint specifically designed for use in decorative bumpers and other plastic parts.