The cloisonné didn't need restoring. A good cleaning was all that was needed.
I did however fill in the gap around the metal Healey insert, with baking powder and superglue. Kept building it up then sanded it smooth.
Now that the wood is flush with the perimeter of the metal insert, I sprayed the the entire knob, including the cloisonné with Varathane Spar Urethane.
Spray, sand, etc...10 coats to build up the layers. Now the top of the knob is as smooth as the rest of the knob. You don't feel the cloisonné.
Time consuming. Reward, the entire knob feels as smooth as glass.