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When I redo plastic head and tail light lens, I use 2000-3000 grit wet-dry sandpaper. Then, while using a garden hose, I wet sand the lens. It usually takes about a minute per lens and will easily remove heavy oxidation. I then use a polymer based wax like Nu finish to buff and seal it. Since this method lasts several months I haven't seen the need to clear coat the lens, as it would make the process of re furbishing the lens the next time more difficult.
However, sometimes it is easier to replace the lens assemble on some vehicles, especially older models.
I've found good prices here.
https://www.rockauto.com/catalog/catalog.php
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