One thing about the mentions of metal, there is very little steel in airsoft guns (as far as outer parts go), so therefore nothing to rust. BUT, because pretty much all metal outer parts are potmetal, you will get an oxide buildup on them which shows as greyish white splotches which are hard if not impossible to remove without affecting the black surface.
My recommendation, one past WD-40, buy gun oil and wipe/brush it on your metal parts regularly. It'll absorb into the metal and form a good barrier against moisture and water, and best part is, you can pinpoint where you want the protection, without the overspray onto plastic parts you'd get with WD-40.
I've used gun oil on my hand-forged high carbon steel axe head the past while, and not a spot of rust (it'll rust easily) on it yet.