Quite frankly, I would be
much more worried about the condition of my basement and its contents because of the high RH.
I would focus on solving
that asap.
Just thought that I would follow-up on this aspect ...
Awhile back I noticed that the basement of my house, as years passed, was slowly getting more humid during the warm/hot months.
At first, just developing an "air-flow" plan mitigated the issue ... but I finally decided that I needed a dehumidifier for seasonal use.
I got a "70 Pint" Frigidaire and ran it to-full overnight, that first night. Next morning as I entered the basement I stopped ... something was different ... after that one night the air smelled & felt different. Fresh.
Since then I have developed a routine. I constantly monitor the RH down there and when it starts to creep up towards 60%, I will turn on the DH prior to going to bed. There are rare, terribly-humid periods when I find I need to double-up on that pattern, running the DH to-full during the day, also.
The DH in conjunction with the air-flow plan does a
great job in my basement.
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Relative to propellant storage in a humid area ...
I have some propellants that have been survived for decades while spending long periods in HUMID areas.
So long as they are in properly-sealed, good-condition containers, any degradation that occurs will not be due to the humidity levels outside of the container.