I use a boat fiberglass hull cleaner.  It does a nice job getting the
surface clean and removing fine scratches.

OhioBeeFarmer
Getting Kids Involved in 4H Beekeeping
www.homestead.com/BeeKeepers/
www.homestead.com/BeeKeepers/BeesRUs.html

Subject: [BEE-L] cleaning observation hive

Something that won't damage the
plexi-glass or hurt the bees.