I know this has been covered a few times but thought I would through out something that has worked well for me.
I am trying to get off 20 yr old paper and had been making rather slow and difficult progress with Goo Gone and Simple Green.
Today I tried "Goof Off Citris Gel". Worked fabulously.
It goes on heavy (its a gel), then cover it with Saran Warp.
After doing this I came back 3 hours later and the old paper and glue came right off.
http://www.homedepot.com/p/Goof-Off-...86?N=5yc1vZ4jx
They do make a clear industrial clear liquid (in a can) that harms the plexi (dont ask).
The citus gel does not though.
Larry S.