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Chris, I use an acrylic paint I found at Wal Mart. The brand name is Apple Barrel, #20405 Antique White. It comes in an 8 oz. bottle (enough to last a year!!). Found it in the crafts section.
I thin it down with Windex, 20 to 1. It doesn't take much, and it can be wiped right off with alcohol and a paper towel.
I may just be the photo but the rust does look too much like paint rather than a silver roof with rust showing through. I'm not sure if you were working from another prototype picture but the one at the top of the thread doesn't have all the rust at the bottom either.