I used foam engine cleaner on my two knobs.  I rubbed the foam on the knobs with my fingers for a few minutes and it removed what ever causes the knobs to gets sticky.  I did this sometime ago and they still look nice and shiny, and are no longer sticky.  I might add the foam engine cleaner was the cheap stuff I used on my truck. It came from Wal Mart. 

  My recent edit,  I want to say,  I did remove the knobs to clean.