MB reccommends their own brand of sunroof rail grease. Use that ONLY. Any other grease or lube will attract dirt and only make things worse. The first step is to open the sunroof, and clean the rails with some WD-40. Spray the WD-40 on the rails, then wipe it off with a clean rag. Then, apply the grease on the rails, and open and close the sunroof a few times to ensure even distribution of the grease, and you should be set.[
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-Joe
-Joe