Hm. What about adding a lightweight protective coating to the stuff, like a layer of paint or something? Obviously that wouldn't be as nice as an innately waterproof version, but it should still be very useful... Most every version of the Elevator I've heard proposed required constructing the actual cable in space and then lowering it into position - if that was done, then most of the cable's volume could be load-bearing coils of this stuff with only a thin protective outer layer along the lower end - ie, the part actually in the atmosphere and exposed to water vapor.
Ja, -n
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"I'm terribly sorry, but I have to kill you quite horribly now."
Ja, -n
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"I'm terribly sorry, but I have to kill you quite horribly now."