As you all know, it seems the BMW headlights take a beating from UV. The 335 I purchased had yellowed, cracked lenses. One of the many projects I currently have going with this car was to restore these while I wait for my VCG and Whitbread oil lines. The only true way to make this work permanently is to apply a 2k clearcoat. The kits out there that want you to just sand and polish, they'll yellow again because essentially you are sanding off what's left of the clear and UV protection. Even the wipe on UV coatings are a temp solution. I want to do things permanently.
I don't have a good detailed photo of them when I got it, and this one doesn't do it justice of how terrible they were. This was just after a wash, but these lenses were very yellow and looked like a topographical map.
Normally when I refinish headlights, I start with about 300 grit and work my way to 1200. These though, I actually had to start with 100 grit. I worked my way up to 1200 with a DA sander. Here's a few shots after I got them all sanded. The one thing to note is you need to sand until the lens is completely uniform. Ie, you either have to sand off ALL the existing clearcoat into the actual plastic, or if yours aren't in bad shape, just sand enough to get an even layer. If you see any "streaks" where the shade changes, it means you haven't gotten even material (ie, combo of clear and parent material):
I built myself a home made paint booth from a 10x10 pop up tent on the deck. I can't do it anywhere inside since my garage is full. Not a plug for these guys, but I've been using Spray Max2k for years with headlights. I used their headlight 2k 2-1 clear for this set. 2k is NASTY so make sure you have the correct PPE. But this stuff for headlights has the UV protectors/inhibitors built in. It's also got a bonding agent that flexes better with polycarbonate vs their standard 2k Glamour which is used for paint clearcoat.
Here's the result after 5 coats. I knew there would be a bit of orange peel so I wanted enough coats to wet sand after to get rid of the peel:
Brought them inside to dry:
Then I hit them with 2k, 4k, and 5k grit to knock off the orange. Need to wait at least 24 hours for the clear to cure before doing this:
