Seems like everything these days is made of plastic, and headlight lenses (capsules) are no exception. Although these plastics have a special UV resistant coating, your headlight lens can become dull, yellow and hazy from exposure to pollution, sunlight, harsh chemicals and road salt. This damage reduces the effectiveness of your headlights, making driving at night less safe.
Rather than spend up to $500 (plus labor) to replace fogged headlamp lenses, you can restore your vehicle's yellowed, cloudy headlamps in 30 to 45 minutes without any special tools or skills. And the results are fantastic. You can buy the supplies you need at a well-stocked auto parts store, or you can get a kit online (one is made by Permatex). The kit contains four grades of wet and dry sandpaper (1,000 to 2,500 grit), plastic polishing compound, latex gloves and a flannel polishing cloth.
Note: The following procedure will not repair damage to the inside lens of the headlamp. You should replace a headlamp if the reflective coating is peeling or corroded.
Wash the headlamp with warm soapy water. Rinse with plenty of clear water, then dry. Then mask the area around the lens so you don't scratch the paint. Soak the sheets of sandpaper in water, then start with the grit that suits your situation (Photo 1). If the headlamp is just dull or yellowed, start with the 1,500 grit and work up to the 2,500 grit. If there are light scratches, start with the 1,000-grit paper. With sandpaper, the higher the number, the finer the grit. Sand one direction with the first grit, then rinse and change direction with the next (Photo 1A). Keep doing this until you're finished with the 2,500-grit paper.
Wash the headlamp with plenty of clear, cool water and dry. Wet one corner of the flannel cloth with the polishing compound. Using firm pressure, polish the headlamp in a circular pattern until it becomes smooth and clear (Photo 2). Allow the polish to dry, then use the clean end of the cloth to buff off any polish residue (Photo 3). Repeat the polishing process. Depending on the damage to the headlamp, small areas may still appear foggy. Try repolishing only those spots with polishing compound. Buff the area again and inspect. The lens should look perfectly clear!
To keep your headlamps clear, wash often with a mild detergent and a soft-bristle brush or sponge, flush with plenty of water and dry. Never clean clear plastic with wax, polish or any chemical that's not formulated for the task.