Scratched the glass on my right mirror

[Grumpy Goat]

Interesting problem ... and sorry hear of this scratch.

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If anybody is going to ask, I rinse the bike prior to soaping, use 3 different wash gloves, 2 buckets, change the water at least once during the wash and have a pickup plate in the bottom of my wash bucket to prevent dirt getting on my gloves. Has anybody come across a way to replace the glass on a mirror?
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While I am aware of the 2-bucket method, and with all the excellent steps you take for proper washing, I don't do any of that when I hand wash the bike. I use the same sponge/mitt that is kept clean, one bucket, one bucketful of wash water and no pickup plate. I also have no scratches from the washing operation and certainly none that would scratch glass.

That said I can't see how a bit of grit on the wash mitt would impart such scratches on the glass. Glass is very hard, and if it did not scratch the soft plastic parts of the bike then it was probably not a gritty wash mitt that scratched your mirror.

Any chance of a proper close-up photo to show this scratch on the mirror?

Is there a way to remove the scratches with 2000-3000 wet sandpaper (not sure)?

Doubt that would work since the grit in the sandpaper is probably similar hardness to the glass and with that size of grit you'd probably not make any headway. My suggestion is (if the scratch is worth US$215.94 / MAX BMW parts fiche) to replace the mirror, after giving the sandpaper a try (you never know).