Chuck wrote:

...
Out of curiosity, I immersed a 37 uH inductor wound on an FT37-43 core
in tap water. Using a (1 kHz) inductance meter, I saw no change in
inductance. Unfortunately my bench has been co-opted by other projects
and a measurement of Q at RF is in its own queue.

Also, from the Micrometals website:

"Micrometals recommends that all uncoated cores should be sheltered from
high humidity or rain since they will eventually form surface rust."
...
Chuck
...

Chuck:

Yes, that is what mainly worries me, oxidizing of the iron core (or,
rust.) I am not even sure how the rust would degrade the cores
performance; since, the sheer mental picture of the physical look of a
corroded balun is enough to make me attempt to escape this condition. I
have "revived" old oxidized cores with tripoli and a buffing wheel, then
a protective coating.

I always attempt to waterproof any balun I make. Sometimes this takes
form as an old plastic container with holes in the bottom for any water
which leaks in to escape, but keeps the rain off ...

Regards,
JS