After putting a body-kit on my car i bought new sill plates which had a protective plastic on the top, so i would suggest peeling the plastic off to see what they look like.
I have peeled them off a bit and i think after a clean they will look ok. The bit with the plastic on does feel plasticy (surprisingly) and the other bit feels like metal.
But i do know that often metal parts have a plastic cover to stop corrosion.
The short answer is it should of been removed years ago. It's just the plastic protective layer they put on to stop the metal getting scratched in transit.
Had you removed it before you would have noticed that the finish is shinier with the plastic removed.
Take it off and use a decent metal polish to bring them up as good as new. I recommend Meguiars Metal Polysh (their spelling not mine). Available from most Halfrauds stores.
The AutoGuide.com network consists of the largest network of enthusiast-owned enthusiast-operated automotive communities.
AutoGuide.com provides the latest car reviews, auto show coverage, new car prices, and automotive news. The AutoGuide network operates more than 100 automotive forums where our users consult peers for shopping information and advice, and share opinions as a community.