Speedometer Repair

[KUMARB]

The speedometer glass on my 280SL is smudged from inside. I am planning to take the bezel out and clean the glass. Now, several people on this forum have indicated that they had experienced fogging of the speedometer glass at times. It appears that the dirty glass could have been from fogging.

1) What can I do to prevent future fogging inside the speedometer?  If I seal the dust cover, will that help?

2)  While I have the speedometer out, does any thing need lubricating or attending to?

Thanks for sharing your thoughts.