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Do you know what the smudge is?
If it is related to iron and rust...
Bar Keepers Friend® Cleanser & Polish might help.
It has a small amount of oxalic acid that can bleach out iron.
Oxalic acid...

The first labs I worked in all had heavy bell jars and vacuum plates.
A decade later they all had protective covers.

Pumps from refrigerators can pull a serious vacuum...
At 14 pounds per...

Consider wrapping some tape on glass containers that you pull a vacuum on.
Perhaps window screen taped on. Or a cover made with screen or hardware cloth.
Implosions are notorious....

You are welcome...
Now I need to explore some of these bits in the specific context of making a great fabric strop.
I suspect the mix used by some is a company secret today and then now lost to...

I suspect the stuff in question is sizing.
As anyone that wears linen knows linen wrinkles shrinks sags and droops (lacks dimensional stability).
Canvas for painting is often treated with sizing...

--- some tickets need punching said a conductor once --- :popcorn: ---

Rust erasers are nice if you need them. They are not terribly fine
They do clean a hone of black swarff nicely without...