We use cast iron for finials to top off many of our handcrafted USA made birdhouses. Keep in mind that when exposed to the elements, cast iron will do what cast iron does – it will RUST! We love the organic nature of the iron and the aged look that the rust provides. However, if you prefer to keep your finial looking a bit more new, simply spray an aerosol clear coating (purchased at any hardware store) over it before mounting.