Masking slides

I have some slides that are used for research and talks. Sometimes there
is a distracting image to the side of what the slide is really about.
What is the best way to mask the slides?  If possible, I'd like to be
able to do the masking without changing the (cardboard) mounts.  But the
most important criterion is a material which won't hurt the slide.
These need to last a few years yet!

Please let me know of products which have worked for you. Thanks!

Cathy Atwood

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                  Conservation DistList Instance 6:44
                 Distributed: Friday, February 12, 1993
                        Message Id: cdl-6-44-007
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