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I recently renewed the headlights on my 2009 Fit and wanted to share how I did it and why. Before doing this they were somewhat yellowed and frosted in appearance.
The basic strategy here is to sand away the top bit of plastic, then apply a UV-blocking clear coat on top.
Tape up the perimeter of each headlight to avoid accidentally sanding the painted area.
Sand away the outermost bit of yellowed plastic. I found a kit with a 2" hook and loop pad and an assortment of sandpaper:
That kit is OK but there may be others that have more grits between 320 and 2000. I used it with a variable speed drill. The 2" was a good size and I appreciated the drill's powered assistance.
To the 2" pad I added a soft foam pad to better conform to curves. I had a 5" pad that I cut but here are some 2" pads:
Starting with 320 grit I went over the headlights in a horizontal pattern, then vertical pattern. I used a spray bottle with distilled water and Dawn dish soap to keep the sanding area damp.
Once the slurry didn't have any yellow left, I went up in grits to 2000 (the maximum in the kit). I then sanded by hand using the 2000 and 2500 grit from an automotive sandpaper assortment:
One headlight did end up having minor circular scratches but at this point it's really hard to tell because the whole thing is frosted. Try heavily spraying with distilled water and looking for circular marks.
Next you need a UV blocking clear coat. I like this kit from Sylvania:
I previously used a Meguiar's product and it yellowed fairly quickly. Make sure you read the instructions before starting everything. It's important to not use any products with oils as it will prevent the clear coat from adhering.
The Sylvania kit includes a "surface activator" which sounds like nonsense but is actually really important. It allows chemical bonding to occur between the plastic and the clear coat.
In regular car painting the clear coat needs to either chemically or mechanically bond to the base coat. If you spray the clear ~20 mins after the base it can chemically bond. But if you wait much longer than that the only option is to enable mechanical bonding by sanding the surface with ~600 grit, giving enough texture for the clear to "key" into. Follow the instructions on your kit, but this is why some kits like Sylvania's tell you to sand to 2500+ grit where others only go to 600-800.
Follow the instructions to use the surface activator and then apply the clear coat. The instructions say a thicker layer is better, so re-soak the towel as you go over the headlight top to bottom. Once you get it applied don't mess with it.
