Sometimes the restoration is "just" removing varnish.
https://www.youtube.com/watch?v=t64g4XvSApc
https://www.liveleak.com/view?i=2c5_1510167630
Or for a better video
https://www.youtube.com/watch?v=mGE2mag-Vr8
The varnish protects, but it also yellows and retains "stuff" that's in the air.