MILITARY MEMORABILIA

How do I clean the inside of a sword sheath?

After purchasing my new katana, and cleaning it several times, I have deduced that the reason it continues to get smudged and marked is that the inside of the sheath leaves dirty marks on the blade. How might I fix this problem?

Would a slender bottle brush work?

EDIT
Forget the brush. Go with the answer below.
I gave him a thumbs up. I wonder who found it necessary to give me a thumbs down.

Antique American Victorian Civil War sword with metal sheath

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