Plastic fluorescent light covers take all sorts of silent and often unseen abuse. One contaminant that causes yellowing light covers is dust accumulation. Yellowing is a result of exposure to heat from fluorescent bulbs and ultraviolet light. Most plastic covers can't be kept from natural light, and, for obvious reasons, the cover can't be kept from fluorescent bulbs. Once a plastic light cover is yellowed, it may take a lot of effort to whiten it again. You may decide the work required isn't worth the trouble, but try cleaning the yellowing plastic light cover first.
Remove the plastic fluorescent light cover and shake it over a trash can to remove dead bugs and loose debris. Carry it to the bathtub.
Fill the tub ¼ full of hot water and add enough liquid dish detergent with an anti-grease formula to create suds. Use a sponge to thoroughly clean the yellowing plastic light cover by scrubbing the light cover with firm back and forth movements. The anti-grease formula will cut through the oily residue created by dust accumulation.
Remove the light cover, drain the tub, and dry the cover with a towel. If the plastic is very yellow, proceed to the next step.
Place the yellowing plastic fluorescent light cover outside on a patio or old towels.
Add to a blender 1 pt. of 10 percent hydrogen peroxide and 2 tbsp. of xanthan gum. You can buy xanthan gum at drugstores. For five seconds, mix the ingredients on high. Place 1 tsp. of glycerine into the blender and select mix on high for another five seconds. Allow the ingredients to sit for one minute in the blender and then mix on high for another five seconds. Put ¼ tsp. of OxiClean into the blender and stir it until dissolved with the plastic stirring spoon. Carry the blender with the gel outside to the plastic light cover.
Pour the mixture over the yellowing plastic fluorescent light cover, ensuring that the entire plastic has been covered with the gel. Leave the plastic cover in the sun for at least a day.
Wash the gel remnants from the light cover with water from the garden hose and towel dry. Replace the plastic cover in its position.