Hi fellows. May I know if the same type of chalk paint (I assume it ain't just ordinary black paint) that is used to paint the walls (to make it a chalk-"writable" surface) can be used to paint bomb shelter doors? I've heard that due to the fact that the bomb shelter door is made of metal, the chalk paint can't adhere to it assuming it is to be painted directly without making a false (wooden) surface on top of the door?