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. . . could you use a mount to CONTAIN the rust as opposed to protecting the stamp?
From what I have read on Stamp Community over the last couple years I get the impression that isolating any stamps with rust away from other non-affected stamps is the best course of action to take.
Possibly a mount could contain the fungus but I personally would not take a chance on it. The stuff does spread through a collection if not removed.
I like to think of rust as being like a toadstool I once encountered in a dark, slightly damp, dirt-floored area of an old house. I touched it accidentally and it went poof and all of a sudden the air was full of seeds floating around and following whatever air currents there happened to be, and unfortunately, me too when I scuttled away from it.
It can't get up and walk around like we can so must sporate (go poof) and hope that the seeds will land on something edible. All this happening in miniature of course on the stamp. The air currents would be the opening and closing of album pages.