Modern Masters Rust Finish Kit

I've heard of these kits before, and I think I might try one out for this year's yard haunt.  It uses iron paint that is then coated with a rust activator to rapidly cause the paint to oxidize, thus creating real rust on any paintable surface (not just metal!)

@quiet_grave used it to make these awesome tin can jack-o'-lanterns:

Looks like you can pick this up at most hardware department stores.

Check out the Modern Masters website for a full product list of rust and patina effects.