Some time back the UP steam crew put up an update video stating they had sandblasted the air valves and coated the insides with some special paint. Now all I've ever been taught about them is that you do not put anything gritty on/in the innards and you clean them chemically. All work needs to be done in a clean-clean area, and best by experienced hands, or under the watchful eye of experienced hands. They're somewhat like automobile automatic transmissions--there are parts in there that do magic, and you can let the magic out if ya don't know what you're doing. Any coating applied (other than plating and that can be questionable) can flake and cause many, many problems; lots of little passages there for stuff to get stuck in/on.
Now before someone posts, "What do YOU know about it?" Well, I never rebuilt one on my own, but I did assist once a long time ago in a place far-far away and my conclusion was, "leave it to the experts!" Unless a dire emergency, and even then....