Headlight 'glass'

My ZS headlight 'glasses' (for wont of a better term) have a coating over about 50% of them that is less than clear. Is that an original coating, part of which has come of, and the remainder going opaque? Or something that is developing?



Some time ago a product called Plexus was recommended, which brought up the uncoated parts well but made no impression on the coated area, even with a cotton mop wheel in a drill. That has got quite bad now, and I can see the day when it will fail the MOT through an indistinct beam pattern (I've had that on another car).



Any ideas?

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