Most cars today have headlight bulbs covered by a large polycarbonate lens. The polycarbonate is a sturdy plastic that holds up against road conditions better than glass. Over time, the polycarbonate lens may become cracked or pitted, causing your headlights to dim. Replacing the lens can be costly, but a visit to an auto supply shop for a few inexpensive items can help you to smooth the cracks out of the headlight lens in a relatively short amount of time.
Wash the headlight lens and a foot around the lens with an auto detergent. Any loose dirt and grit on the surface of the car leads to even more scratches as you work through the repair process. Wax in a one-foot radius around the headlight lens, but not the lens itself, in order to protect your paint in case any compound drips onto it.
Protect the area around the headlight. If you have any chrome trim around the headlight, remove it since polishing compound may harm it. Use blue painter's tape around the headlight. This type of tape pulls off easily without damaging your car's paint.
Begin the wet sanding process. You will use a series of wet sandpapers, working your way from most coarse to finest. Keep in mind that the higher the grit count, the finer the paper. Start out with a 1000 grit paper and keep a container of water handy to dip the paper in as you work. The water makes the paper glide over the lens' surface easier. Work in side-to-side motions until you have covered the entire lens.
Use increasingly fine sandpaper. Change out your sandpaper to a finer 1500 grit, then go to 2000 and 2500 and, finally, 3000. Each time you change the sandpaper, work at right angles to your last sanding. When you switch from 1000 grit to 1500, use up-and-down motions instead of side-to-side. Change directions again when you go to the 2000 grit paper. Use an ample supply of water with all of the sanding applications.
Polish the headlight lens. Use a microfiber cloth to rub a polishing compound onto the headlight. This smooths out all of the scratches on the surface of the lens and restores the original shine. When you have rubbed long enough for the polish to dry, use a terrycloth towel to remove all compound from the surface.
Wax the lens. Use an auto wax with a microfiber cloth for this step. The wax helps to protect the lens from the elements.