The humidity of the static inside air is just what you have to guard against. Good air circulation is needed and so if you can store it on an upper level that will improve the matter. I have used borax powder dissolved in water and brushed on and also something else just bought ready made that gets absorbed more redily, also brushed on. My impression is both are equally effective, at least in my mind. With your old dry beams any solution will absorb well so water is as good as a petroleum extract as a vehicle. My sawyer sprays it on like you mention with a garden sprayer. An open sided shelter located to catch maximum wind with a good roof would be the ideal storage I would think.

Greetings,

Don Wagstaff