My Saab 9-5 is about 6 years old and done 80K miles. The plastic headlights were getting dull and oxidized which is common for these kind of headlights. I used Plast-X before but never really got very good results with that. The 3M Headlight Restoration system turned out much better.
I can definitely recommend this kit to make your headlights look like new again. They are not 100% perfect but you'll only notice when you're up close.
Before pictures
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The 3M Headlight Lens Restoration System is a popular kit.
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This is included in the kit. I paid about US$ 18.
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These are a few things you need in addition to the kit.
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Make sure to tape off all areas around the headlight. Use a few layers so the sanding discs won't damage any paint.
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The first sanding (P500 grit) is the scariest part of the procedure. These headlights are about $750 each so it's not like you'll easily buy a new one if you mess it up. After I was done with the first sanding, I had a few doubts this would ever look good again.
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Second sanding with finer grit sandpaper (P800 grit).
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Third sanding with P3000 grit foam disc and water. Starting to look much better again.
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Final polishing with foam pad and rubbing compound. Done!
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And the before picture to compare.
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The kit comes with detailed instructions and there also is a video available.
http://www.youtube.com/watch?v=_t1RBw0IGXA