Yeah, I thought about this. If you apply smithing, technically you will also be removing the oil coating to get to the rust. This is something TB will have to decide about. Edit - If smithing already doesn't remove the rustproof status from an item, then he has maybe already decided. :P
Either way, I don't mind if a potion of oil also magically removes the rust, but then we have two "potions" that do the same thing. Or rather, then a potion of oil is better than a potion of rust removal.