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The one thing I found was the clear put over the lens from the factory was a real bear to sand off.
Rick........ ya there? Huh?
I had good results with Rust-Oleum 2X ultra cover gloss clear for plastic. Bought at Wal-Mart. The lens has to be grease/oil/wax free and dry. Spray very light coats. Do one lens and let dry before doing the other to be sure your technique is right. Has a tendency to "alligator" if you do it wrong.
Before you buy a lot of polish and sandpaper -- try polishing with toothpaste. It works great on our Subaru. It's cheap and easy enough I don't mind doing it every spring.
I am in the replacement camp. $78 for a set for my old Hyundai, 20 minutes a side to install, with new bulbs. I never thought about coating them. How about Coppertone?
Does it whiten and reduce sensitivity of the lights as well?