Just to show I am not being idle over the holiday!
A few years ago I noticed some dust on the inside of the crystal on my CWC SBS. I had even less of a clue what I was doing then but I removed the movement and tried to clean the crystal with a lens cloth. I ended up leaving smudges and even more dust! I started to painic and hastily put it back together again.
I recently noticed that the battery was running low with the second hand moving every ten seconds or so.
So off came the back. Took me a few moments to remember how to remove the stem. The I realised there is an arrow pointing at the lever (not my red one!).

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Out came the movement:

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You can see the dust on the inside of the crystal:

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I carefully cleaned it with a zeiss lense wipe and blew all the dust away. Then refited movement and new battery:

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The caseback seal was fine so regreased it. And there you have it. One beautifuly clean crystal and movement working fine (any dust seen is on the outside!):

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Took me about half an hour and very satisfied with the result.
Merry Christmas everyone.