Triple Thick is unpopular with a small and vocal crowd. But it is used with success by many more collectors.

Poly-Crylic is nice, water based and easy to apply. But would you touch a backglass, especially a damaged one with a brush or a foam applicator?

I use Polycrylic for the cabinet of my machines, but for the backglass, it is Triple-Thick. Very happy with it, for the past 20 years.

Yves

As you can see, there are many and varied opinions. A few things things that you must do:

1) Hide with tape (Scotch Magic tape) the openings for the scores. Carefully, cut the tape with the exact dimensions of the openings.
2) First coat must be very light, almost dusting. Let it dry for a few minutes, while monitoring the reaction. Then apply a couple of thicker coats.
3) While still tacky, using tweezers, remove the Scotch tape that you placed in the openings. Do not wait for the glass to dry, before removing the tape....
4) Let the glass dry fully. It will dry perfectly clear.

In this post #117, I am describing the process of Triple-Thicking a Mata Hari damaged and dirty backglass:
https://pinside.com/pinball/forum/topic/ycv-bally-mata-hari-the-eye-of-the-day/page/3#post-4559079

Yves