there are several ways to make rust. one is to take steel wool #oooo, put it in a glass jar, (mason), fill the jar with vinegar. leave the lid loose, as this mixture will give off gas as it rots the steel wool. when it is evaporated, youcan brush it on anything and it will give a rusty haze. this works good for diaramas, because it gives wood a weathered look. another is a product called ROTTENSTONE. it makes anything look old and weathered. i used it years ago on my railroad with excellent results. i got it at a craft store, i believe. its been years since i have used it, but i believe it is still available