Over time, a plastic fluorescent light cover begins to turn yellow from a combination of factors including reactivity to chemicals and/or heat as well as build-up of dust, dirt and oils from insufficient cleaning. Although you can't reverse plastic breakdown from chemicals and heat, cleaning the yellowing plastic can brighten it, remove surface stains and prevent some forms of permanent dark staining caused by heat from light bulbs and/or sunlight baking surface particles and debris into the plastic.
Remove the plastic fluorescent light cover from the fixture for cleaning.
Wash the cover's interior and exterior surfaces with water, mild detergent and a lint-free microfiber cloth to remove loose dust, dirt and other debris.
Pour isopropyl alcohol on a sponge and rub it over the cover's surfaces to break up any oils clinging to the plastic trapping dirt and debris. If the alcohol doesn't break up any oils completely, combine three parts baking soda to one part white vinegar and rub as a paste over the surfaces of the cover. Wait at least 10 minutes and then rinse the paste away with warm water. Wash the cover again with a mild detergent and rinse to remove any remaining residue. If you're cover still has dirt, oils, stains or dark yellowing unrelated to plastic damage, go to Step 4.
Rub a wet, scratch-resistant scrubbing pad or wet, melamine resin foam eraser gently across the cover's surfaces. Rub in slow, even sweeps across the surface so that any scratching that may occur appears uniform and in a single direction. Apply light pressure to help reduce any possible scratching. If necessary, use baking soda or toothpaste in combination with either tool to break up/wear away tough stains.
Rinse the cover again and air dry before returning to the rest of the light fixture.
To protect against future yellowing, paint your plastic fluorescent light cover with a paint that sticks to plastics---a paint that doesn't peel or change colour if exposed to heat or moisture.
Always use a scratch-resistant scrubbing pad or melamine resin foam eraser as the "last" option when cleaning plastic, as "scratch-resistant" pads can still scratch certain plastics and melamine resin foam erasers have surfaces that act like sandpaper. Never use bleach or harsh chemicals to clean your plastic fluorescent light cover. Although certain products can whiten the plastic, many cleaning supplies can damage it---melt the plastic, cause faster plastic deterioration or increase the speed at which the plastic yellows after the brief whitening period. Never smoke around your plastic fluorescent light cover as the smoke can stain the cover.