It depends on what exactly is causing those shiny spots. If it is just a build-up of oils/grease from the skin of the finger tips, then it should be able to be carefully cleaned off. But if it is a result of the fingertips slowly buffing the factory finish of the plastic as the knobs are used and the fingertips repeatedly brush against the plastic over time, then that is permanent. I guess it could be theoretically possible to carefully and precisely sand a rough finish back into those patches of smooth plastic, while avoiding damaging the nearby plastic, but then the same problem would likely happen again in the future as you continue to use those knobs. And good luck sanding in a new rough finish the matches the original finish and the nearby unaffected plastic!