If it were me, i would not mess with epoxy or such compounds unless i have to. Normally when you buy secondhand there is always a "matte stain" in the middle where most of the drawing happens. The 21UX-es come with a plastic film coating on the glass in order to simulate a "paper like feel" and it gets worn out pretty quickly. But, with some patience it can be peeled off completely - after that you just need to place a screen protector over the bare glass and it should look as good as new. Here's a link for reference.