If they are black you could use some wd40 brings them up quite nicely. Though you will have to keep using it as it fades in the weather.
 
No they are not made out of rubber, they are made out of PVC.

During my 90 rebuild before I put them back on, I used oil and fine steel wool and cleaned them right up. They look brilliant now and super black. I also gave them a coat of some sort of plastic black but will use shoe polish in the future.
 

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