Oh, and just FYI in case you aren't aware. Spray paint will eat styrofoam so you will need to use a non spray paint on your dome if you are doing the cinnamon method.
Ok, if the metal is steel or iron you can use this recipe to rust it:
1 c peroxide
1/2 Tbs salt
1 c vinegar
mix it all together and spray it on, soak it, whatever
If you need to rust wood, plastic or another type of metal such as aluminum you can use flat black and/or brown paint and sprinkle cinnamon over the paint.
