cars

So, from a deep dive into headlight restoration I've done, this is what I got.

First, the Suns UV rays damages Polycarbonate plastic and must be removed via sanding, with going down to Polishing than Buffing producing the best results
https://www.youtube.com/watch?v=E7Maoj8X8mo
https://www.youtube.com/watch?v=NuAiUu9KAew

Second, a sealant/coating is applied to increase the longevity, ordered by about how long they last outdoors:

A season (3~6 months)

• McKee's 37 - Headlight Protectant Spray https://www.amazon.com/McKees-37-MK37-130-Headlight-Protectant/dp/B01IB15TZ0

A year

• Meguiar's Keep Clear Headlight Coating https://www.amazon.com/Meguiars-G17804-Clear-Headlight-Coating/dp/B01M4RVVX6
  All top-rated kits
  See https://www.youtube.com/watch?v=kyVCEbfrU-c & https://www.youtube.com/watch?v=zfPSd_0aQLY
  • https://www.amazon.com/SYLVANIA-Headlight-Restoration-Headlights-Protection/dp/B01KIVYE6G
  • https://www.amazon.com/CERAKOTE-Ceramic-Headlight-Restoration-Kit/dp/B084RQKLV8
  • https://www.amazon.com/3M-39195-Headlight-Restoration-Heavy-Duty/dp/B08745K56G

2~3 years

• (One part) 2K Glamour Automotive High Gloss Clear Coat Spray Paint
  See https://www.youtube.com/watch?v=UEJbKLZ7RmM & https://www.youtube.com/watch?v=EJvr5YhKMT4 (starts to peel & yellow)

3~5 years

• (Two part) 2K 2in1 Headlight Clear Coat https://www.amazon.com/SMT-3684068-SprayMax-Headlight-Clear/dp/B09642P1XQ
  See https://www.youtube.com/watch?v=UQs8MJH2qwY

5 years

• Clear UV film (PPF) like "3M Scotchgard Clear Paint Protection Film"
• UV cured hard coat. Requires a UV heat lamp & air paint sprayer

Best video overview https://www.youtube.com/watch?v=UG54xt56uZo

Side note: Tail lights will not yellow since they are made from acrylic. Acrylic is brittle and does not have the impact resistance for headlight applications.