There is a product designed specifically for this purpose. It is called
DSR-5. It is a clear liquid, and it comes in pint spray bottles. It works.

I have a kit plane that sat outside in Florida for several years after the
original builder gave up on it. The lexan coating was rock hard and totally
impossible to get off. A few squirts of this stuff and it was coming off in
foot square sheets.

http://www.dsr5.com/





Now, after three weeks in the sun, the masking has deteriorated so
much that I can't get it off the lexan. It has perished into milions
of 1/4 mm square pieces that seems bonded to the lexan.