There are countless videos online showing people restoring oxidised black trim to great success using peanut butter. Want to give it a try? Don’t. The reason it works is because of the oils in peanut butter, so by using oil on its own, you can get the same effect without making a colossal mess and making your car stink.
Boiled linseed oil is a popular choice: simply rub on, clean off the excess and allow to dry.
Despite looking like a drastically different approach, this is still all about oil. By gently heating the plastic, you’re bringing out the oils within the material, restoring the colour.
It’s always good to start on an inconspicuous area first, and make sure you don’t stay in one place too long: you could end up warping the plastic. Also, anecdotal evidence suggests that while this is a relatively long-lasting solution, plastic trim pieces generally only last a handful of treatments before going permanently grey. This makes sense; after all, there’s only so much oil to be drawn out.
If you’re not into weird and wonderful hacks, there are plenty of off-the-shelf products specifically designed to restore black plastic trims. Give something like Autoglym’s Bumper and Trim Gel or - if you feeling flush - Swissvax’s more expensive ‘Pneu’. Both will restore black plastic trim pieces to their former glory while leaving a protective layer. Oh, and they won’t damage paintwork if there’s any overspill.
Upping the budget further, many swear by Gtechniq G4 Permanent Trim Restorer, which - according to the manufacturer - lasts as long as two years.
If you’re after a more permanent solution and have enough time on your hands, you might want to respray the whole piece of trim. It’s far easier if you can remove the trim piece in question, and it’ll need a damn good clean before you go any further (here’s the splitter from CT community manager John’s old MX-5 getting a nice bath before being sprayed).
Once it’s clean and dry, it’s ready for a coating. Your local car parts store will sell paints specifically intended for use on bumpers and other plastic trim pieces.