An autobody friend of mine said that "Once bare metal is exposed to air, it'll start rusting in 20 seconds. You probably can't see it, and might be very insignificant, but it's there." Not sure if that's true or not, but I could imagine any moisture in the air will end up on it. If inner panels aren't protected, It could have started rusting before it leaves the sales lot when it's brand new! Probably insignificant at first, but as years go by, and depending on the atmosphere the vehicle is exposed to, will dictate the rate that rust accelerates.