Yesterday I was about to toss all the Plexi scraps I have because the paper backing is stuck on so tight it can't be peeled off. I decided to try laquer thinner, assuming it was going to melt the plexiglass since they were headed for the trash anyway.
I moistened a paper towel with it, place it on the paper backing and a few minutes later it had softened the backing enough so that I could peel it free (with a bit of work - the scraps are probably 25 years old).
The Plexiglas appears to be completely unharmed by the solvent, which really surprised me. I thought would cloud it or melt it. I am now going to try to figure out the best way to scale up the process to get the backing off some very large sheets of 1/4" stuff. If anyone's interested I'll post the brand of laquer thinner and the type of Plexiglas it's working on. I know I have some Lexan scraps mixed in, so it may only work on certain types of Plexi.
-- Bobby G.