Well...you wouldn't have to oil it after each rain.. I was just pointing out that oil would be worn away by the elements.. sun/rain will start to degrade the oils protection... especially where it would catch and hold water... a small hole there would make a world of difference in keeping the rust at bay..
it is steel...and it will rust outside... the heavier the coating of protection.. the longer it will last.. but the further you get away from a natural metal look..
