Introduction
Have you seen a Pinball game where the "paint" has peeled off the backglass and is missing or lifted away from the glass?  There are things you can do to repair this problem or at least minimize it so that the backglass doesn't continue to have paint flaking off and damaging the artwork.  The original artwork for the older games was applied using a 4 color silk screening process with an opaque area that would not let light come through from the backside of the glass, and other areas where the light could come through.  Typically, the area where light could pass through has a white ink or paint applied as a last step over a particular color.  This somewhat diffuses the light coming from the light bulb(s) and makes the light appear to be a little softer.

Light Bulbs Get Hot
The backglass has small light bulbs (number 44 or 47) that emit light to shine through from the backside so that the Pinball player can see scores, messages (such as "Last Ball In Play"), or scenes the artist thought would add to the beauty of the artwork.  Since these light bulbs get hot, then cool, then hot again, etc., the ink/paint would become somewhat baked and brittle due to the heating and cooling down process, especially since glass heats and contracts at one temperature, ink and paint at another.  Many times these older games would be left on site with the power turned on and the beautiful artwork lit up to attract a player.

Backglass Flaking Is Not Unusual
Many collectors know the characteristics of each game's backglass artwork problems, and come to expect the backglass to have some flaking since most games of a particular model will exhibit this flaking phenomena.  For example, Flash is known as a game that usually has flaking occurring around much of the red color found in the artwork (also the yellow and the black outline around the score windows).  The title of the game - Flash, uses red ink in the name and flaking often occurs in this area leaving small red ink fragments on the glass with much of it lifted and fallen off.  This means that a player, when looking at the backglass from the front of the game, can actually see the light bulb through the glass instead of the backlit red color.  You can also find many games where the score is displayed having the black border around the score window lifted or missing as well.

Minimizing Backglass Flaking
So what can you do to minimize this flaking problem?  Many collectors now use Krylon Triple Thick Crystal Clear to coat the backglass on the back side of the glass to keep any more artwork from lifting.  Remove the backglass carefully so that jiggling it around doesn't cause any new damage to the artwork (the backside of the glass is where the artwork is applied so be careful where you handle the glass), and lay it face down on some newspapers or cardboard for painting using the Triple Thick.  In the early days of my experimentation with Triple Thick, I tried to clean the dirt that may be found on the artwork as well as lifted artwork.  I found that this was not a good idea since all I did was make even more artwork lift or flake off.  Now what I do, is examine the artwork to determine if I can use some painter's masking tape (blue colored) to cover the clear parts of the backglass (such as where the score is displayed), and determine if a lot of lifting/flaking has occurred.  If not much flaking/lifting has occurred, then I use an old T-shirt as a rag and spray it with Windex to lightly clean the artwork.  Very lightly.  This will allow the Triple Thick to adhere better to the artwork.  If I'm afraid that I will make things worse by attempting to clean the artwork, I just spray the entire backglass with Triple Thick, even over the clear parts of the glass.  One other thing I tried in the past was to cut out small pieces of cardboard and lay them over the clear parts of the glass (such as where the score is displayed).  This backfired on me the first time since when I was spraying the Triple Thick, the spray blast caused the cardboard to shift and cover up some of the artwork.  I also ended up with Triple Thick sprayed on the clear glass.  My remedy to the shifting cardboard was to place a couple of dimes on the cardboard to hold it down.  That also gave me some trouble when the Triple Thick managed to get under the cardboard edges anyway.

Now I pretty much shoot Triple Thick over everything if I have a glass that's flaking too much.  That means over the dirt and anything else that's there since I am trying to minimize flaking, not create more.  I have used an Exacto knife in the past to score the Triple Thick so that I could use a single edge razor blade to remove the Triple Thick from the clear glass, but found that people don't really see the Triple Thick that was sprayed over the clear areas.  Unless there was dirt on the clear parts of the glass that I coated or it was noticeable.  In that case I go ahead and remove the Triple Thick from the clear glass areas using an Exacto knife  to trace around the border and a razor blade to remove inside the border.  In any case, the Triple Thick will seal the backglass and so far, this has not led to any problems (nor have I heard of anyone else having any problems with Triple Thick).  How many coats should be used?  I use two coats of Triple Thick when spraying the backglass.

Lifted Artwork Repairs
I have also tried another technique for handling lifted artwork that seems to have worked somewhat well.  Spray Triple Thick on the backside of the glass, then lay Cling Wrap (brand name is "Glad," similar to Saran Wrap) over the area where the artwork is lifted (but not fallen off).  Then I pressed down on the Cling Wrap to flatten the lifted artwork which worked somewhat well.  The only problem I had was that the artwork shifted and it kind of screwed things up anyway.  I let the Triple Thick dry overnight and the Cling Wrap pulls right off since it doesn't stick to the Triple Thick, then I shoot it with another coat of Triple Thick.  I probably need to perfect this Cling Wrap approach, but in some cases it did work out perfectly.

Beyond Protecting The Backglass
Now that the backglass has been protected from further flaking, what can be done?  If there isn't too much flaking, you could just leave it alone and reinstall the backglass knowing that no further flaking will occur.  Another trick after using Triple Thick is to remove the light bulb from behind an area that had flaking so that at least a player doesn't immediately notice backglass imperfections by viewing the bright light bulb through the glass where the artwork is missing.  You should replace the light bulbs with "cooler" bulbs that do not burn as bright (and hot) as the original bulbs.  This means substituting a number 44 with a number 47 light bulb.  Even if I suspect the game has the number 44 light bulbs in it, I go ahead and replace them anyway.  Bulbs don't cost that much and saves me time examining and wiping old bulbs.  You can also decide to match the colors of the artwork that have lifted and repaint those areas so that from the front of the game, flaking artwork will not be noticeable.  I haven't had good luck doing this over areas that are translucent since it seems the light will show every place painting occurred.  I have done it on occasion over the translucent area and removed the light bulb from behind the repaired area so that at least the colors look OK when viewing the backglass from a players position in front of the game.

Painting The Backglass
This is a tricky area that you may not want to do unless it looks like it will be easy.  By easy, I mean that you are good at matching colors and that the area being covered is opaque.  I have done areas that are translucent, but I think these kinds of repairs can be spotted unless all of the color is replaced.  Removing the light bulb from behind the translucent area can help since there won't be any backlighting of the repaired area.  Although you may be tempted to do small painting repairs to translucent areas, just remember that it will show up if the area is backlit.  It is usually quite noticeable too.

Matching The Color
I have used Acrylic paint that I found at a local hobby store (in particular, a Michael's arts and crafts store).  I have a set of Pantone Color Guides that I first use to match the color I want to repair.  Then I either go to the store and buy colors that are very close to the Pantone color, or search my existing acrylic color selection for one.  This saves me from guessing which color looks the closest.  Pantone Color Guides cost around 75 bucks, so they aren't cheap.  I bought mine on-line from Pantone and also use the color guides to match colors for decals that I make, so it was worth it to keep from doing all the trial and error color printing I was doing when making a decal.  Anyway, I get a color that is very close as well as black, white, silver, shades of the color I'm trying to reproduce, etc.  The small bottles are pretty cheap (around a dollar) so it isn't like the cost is high.  I use a trick that I learned on my own after mixing colors that I thought looked very close, applied the paint, only to discover the paint dried somewhat darker than I expected and had to remove it.  After a few of these episodes, I decided that there had to be a better way.  Now what I do is to mix the color on a palette and apply a sample to the FRONT of the backglass where the matching will occur.  Then I use a hairdryer to blow-dry the paint right on the front of the glass.  If it's too dark, I lighten it up, not dark enough, make it darker.  Then when I attempt to apply it to the BACK of the glass, I know that it will look right the first time I apply it.  After I'm through, I use a single edge razor blade to scrape off my samples from the FRONT of the backglass.  No guesswork involved when you do it this way.  I also use another trick on the FRONT of the backglass to show where the paint needs to go.  After removing the sample color from the front of the glass, I lay down strips of "magic mending tape" and draw directly on the tape where the borders of the color need to go.  I hold the glass upright and can see the border lines from the backside of the glass.  For me this is best done while sitting in a recliner and holding the glass upright for painting from the backside while using the TV as a backlight to see the border lines.  After the applied colors are dry, I use silver paint to cover the opaque areas to block light and it will also match the rest of the artwork so repairs may not be as noticeable from the backside of the glass.

That's about all there is to it.  The hardest part is matching the colors of course, but once matched using my "trick" of applying it first to the FRONT of the backglass and blow drying it to see the final color, I find that trial and error repainting on the backside of the glass is almost non-existent.

Hopefully you have found this informative and worded in such a way that the directions are clear.  I plan to put some before and after pictures up as soon as I get a chance.  At least this will get you going.  Good luck!