You may already know this, but WD-40 was actually created as a water displacement/corrosion preventative for the skin of the stainless-steel “balloon tanks”* of Convair’s Atlas missile, our first ICBM.
Its use as a penetrating oil came later. (And as others have observed, there are far better penetrating oils available, if you go looking.)
* To save weight and cost, the Atlas tanks were stainless steel about the thickness of a dime, instead of the sturdier (and corrosion-resistant) but more expensive aluminum isogrid.
They had to be kept pressurized to be structurally rigid when upright. Painting the tanks would add unnecessary weight, but the salty, damp Florida air warranted something to displace moisture – hence, WD-40.
