I've been reading this thread with interest and am in much the same boat as you wonder . I recently got a NOS ( an original NOT a repro ) backglass for my Williams Lucky 7 . I was going to seal it with the Markseal product mentioned earlier . I have actually seen the FirePower backglass that Skybeaux done with Markseal in person and it is VERY , VERY nice . I was going to put all 47 globes in the headbox to cut the heat down etc. but still wouldn't mind sealing the back to protect it from scratches etc. that can happen when you have the glass out of the machine .
  After reading this thread though I'm in 2 minds whether to do it or not ...