I stole this off the Internet many years ago. I used it on a couple of sets of tail lights for my old VWs. It works really well. Sorry I didn't copy over the photographs.
Refurbishing Your Tail Lights
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I don't believe there are any of us who have not been plagued with faded or crazed (small hairline cracks) tail light lenses. This project has the benefit of being relatively easy (2 wrenches out of 5 if you have some basic equipment). It adds a beautiful luster to the finished product, has UV Protection , and refurbishes your 30-40 year old, impossible to get, and ridiculously expensive lenses. Furthermore, following this method will give years of beauty to your lenses.
What you will need
• Air Compressor
• Paint gun, detail gun, or airbrush
• Automotive grade urethane clearcoat, activator, reducer, and fisheye reducer
• 800-1000 grit sandpaper
• Safety equipment for painting
Step 1
Remove the tail lights or headlights. This step is not 100%necessary. However, if you decide to paint with the lenses still on the car, understand that not all surfaces of the lenses will be refurbished. Also, you'll need to make sure that you mask the car very well or you WILL get overspray on your ride.
In the photos below you will see that I am refurbishing tail lights off an early model Opel that we customized into my sons car. These lenses were badly faded and crazed. Crazing occurs when the lens has been exposed to the sun, and over time they dry out and get small fractures in them.
Step 2
Thoroughly clean the lenses with a non greasy or waxy cleaner (don’t use furniture polish, oil soap, etc)
Step 3
Using 800 to 1000 grit sandpaper, sand the exterior (and if possible or necessary, the interior) of the lens. Most tail lights have a series of facets on the inside making sanding next to impossible. Once you have sanded to a satin finish, take a clean, lint free cloth, put a small amount of paint reducer or paint prep on the rag and thoroughly wipe down all surfaces on the lens. Use a tack rag to remove any lint or dust from the surface.
Step 4
Following the paint directions, mix enough of the clear coat paint, activator, and reducer to complete the project, then add a few drops of the fisheye eliminator to eliminate any wax or fingerprints you missed.
If your lenses are badly faded you can visit your local body shop and ask the painter to tint some (not all) of your clear coat paint (before mixing) to approximate the color of the lenses.
If your lenses are crazed, start with the inside. Paint a relatively heavy coat of paint on the inside of the lens. Place the lens face down on a clean surface to allow the paint to penetrate any cracks and crevices in the interior of the lens. Wait until the lens is flash dry (15-20 min) and paint another lighter glossy coat. When this coat has flash dried, hang the lens from a wire hanger and paint the exterior of the lens. If the lens is crazed, paint a relatively heavy coat on the outside first (not heavy enough to run) and follow up with another coat as you did on the interior. Allow to dry thoroughly and reassemble. If you do happen to get a run in the clear, allow it to dry thoroughly, wet sand the run out, and repaint.
Note: If you are using a tinted clear on the lens, add an additional plain clear over the tinted coat.
If your lenses are severely crazed as mine were you may not remove all of the crazing, but it will be drastically reduced. As with every painted surface on your vehicle, the lenses will need to be waxed on occasion and if you have painted your headlight lenses make sure you get the bugs off in a timely manner just as you would your front bumper to keep the acid in the bugs from eating through your paint.
