Cleaning plastic is something I have not tried in the past but I found it very similar to cleaning chrome. The steering damper knob was coated in gold paint overspray and oxidized. To remove the paint I soaked it in Coke for two weeks. When it came out of the Coke I scrubbed off the softened paint with an aluminum bristle brush. Next I put it in my polisher to clean the plastic. This process worked well. As for metals, there are buffing compounds for plastic. If I did not have a polishing machine I would try this method.
Heavy gold metal flake over spray & a dull finish
Finished and ready to assemble