yep, tried a heat gun. It's stuck . When it comes off the glue is left, and some paper. Naptha is the best I can do, but there's lot of scraping, and gouging. And still remnants. I also have some lexan, same deal, if the paper is not removed in a reasonable amount of time and its exposed to sun or other elements, it may not come off. The plexi came from a car guy and he was storing it under his cars.. so oil and sun.
The lexan came from the shop of a deceased guy, and his widow said he died like 20 years ago, and the lexan was stored near the windows.