http://www.autogeekonline.net/forum...eadlight-restoration-new-uv-sealant-idea.html
Check out that. I did the spar urethane on 3 vehicles and it is the best way to go. It definitely acts like a clear coat so they don't yellow or haze. I used 320, 600, 1000, 1500 and 2500 grit sandpaper. You can pick these up at a Pepboys or Autozone, or whatever auto parts store you have near you. Then, soak them in a soapy water mix, then splash some on the headlights and sand away. Make sure that there is enough water on the headlight to keep it wet.
The key is to remove any oxidation or yellowing from the light. After you went through these steps for the sanding, I took out the PC7424xp with some old Turtle Wax rubbing compound and a Lake Country CCS Orange pad. After compounding, clean the headlights with alcohol then use the Spar urethane/mineral spirits with the blue scott shop rags and just wipe it on. The lines you see will disappear when it dries.
Thats my method, yours can be different. Hope it helps!