Don't triple thick it at all. It's not likely to degrade if you have it in a stable environment.

If you really wanted to though you cut out pieces of cardboard and weigh them down with something on the areas you want masked (in games without the tint you could use actual masking tape, but that would pull up your tint). Then spray, and remove when dry.

I really wouldn't do anything to that glass though, too risky. There have been incidences of TT pulling the ink off the glass, ruining it.

Put LED's in your head instead of the heat producing lamps to keep that source of temperature change away as well.

If you BG is in good shape let it be. As long as the machine is in a stable environment, it will not degrade.

Alberto

I've had the TT curl the ink off an NOS 40 year old glass that was in perfect condition, didn't do it everywhere but in quite a few spots, not a happy camper.

Ya....leave it be is the consensus these days. As long as it isnt pealing or cracking like crazy, it isnt worth it.

It's usually best to leave it alone in that case.

Sealing it is a last resort when the backglass is falling apart.

Keep the game at a stable temperature and humidity, install LEDs, and the backglass should be fine.