Years ago, you could take your plugs to any filling station, and use the official Champion plug cleaner. A blue/grey box with a rubber cover that had a hole in it to insert the plug.
 
You pushed a button and wiggled the plug around a bit, and the plug came out like new.
I don't know if it was done with glass beads, or walnut hulls, or sand.
 
Along side this box, was another that had a glass window in so when you screwed the cleaned plug in the box, you could see the end of it through the window.
 
You connected the high voltage lead and opened a valve to increase air pressure in the box.
The plug would keep arcing right up to full line pressure, where a dirty plug would not.
 
No longer available because "A" few people know what a spark plug is, and "B" Lawyers.
 
 
Lynn E. Hanover

Hey I used one of those too when cleaning the Cessna plugs, forgot about that step.

Hans