The problem is crazing, microscopic cracks in the plastic. The application of the product fills in the cracks temoprarily, but eventually the plastic dries out again and the cracks reappear. I've had some success with plastic polish and lots of elbow grease, but if the crazing is too deep, it won't work. I believe the crazing is mostly caused by long exposure to sunlight.

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2001 X5 4.4 100% options
2004 M3 cabriolet SMG