I think I posted on this a couple years ago, but it's well worth
mentioning again.  Our Aussie friend Bob Klepner was right on target
when he asked whether Pinball machine collectors have a way to deal
with preserving artwork painted or printed on the back side of glass.

We pin-game collectors use a product called Cover-Your-Glass ("CYG"),
available only by mail order from a pinball supply house:

  The Pinball Resource (Steve Young)
  8 Commerce St., Poughkeepsie, NY 12603
  (914) 473-7114

Steve sells "CYG" in two forms: Regular (very thick and syrupy) and
Lite, which I recommend.  You put it on the glass (lying perfectly flat
and level) with an eyedropper.  After a day it dries to a clear acrylic
coating that does not yellow or crack over time, and which preserves
your artwork forever.  I've used it on radio dial glasses as well as
several pin games.  Just don't let the temperature get below 10 degrees
Fahrenheit (about -11 Celsius).

CYG will not restore peeling or flaking that has already occurred,
but it will "stabilize the site" and prevent any further deterioration.
You can apply touch-up paint over the dried CYG coating (very thin
if you use the Lite), if you dare!

Great stuff.

Mike Knudsen