To get rid of residue on plexiglass panels or any other types of acrylics, it's recommended that you use warm water and dish soap with a soft brush. Also, if the surface has been left untouched for an extended period without being cleaned (more than two weeks), try not to splash soapy water on the material as this will create bubbles that may affect its appearance once dry. For stubborn stains, try using baking soda mixed in hot water. Rinse off the mixture after scrubbing with a wet cloth, and then air-dry your plexiglass panels.