WD-40
Spent a few years in the Marine Corps reserves and after weapons were cleaned at the end of drill weekend we'd spray them down with WD-40 and check them back into the armory. Next month when they were checked back out they'd be covered with surface rust. After a couple of months of this we finally decided that the WD-40 was causing the problem.
It might be good for freeing rusty bolts, but using it for rust prevention it's worthless.
A thin coat of machine oil will serve you better.