On the wheel well plastic liners I use that stuff Adams makes, it's a dressing I think it's the "Undercarriage Spray"? It's a dark colored liquid.
Mine look like they've been strafed by rocks. I used a strong solution of Adams general cleaner soap ( the green stuff ) with one of those sponges with the scrubbing pad on the back. After the wash & rinse, I spray the dressing and buff it with an old tee shirt. It kind of hides the whitish looking collection of rock chips. I suppose you could paint it after some sanding with chassis black. I never went that far with mine. The Adams stuff works well enough for me, they look black at least...but they don't look glass smooth if you look closely at them you can tell they have been around the block a few times.
Tom