I saw spray paint at Lowe's that makes it look rusty, old, and hammered. Don't remember the brand, but it was around $6.00 a can, and came in rust, black, red and a few other colors.

Jan

Hgtv.com (crafts message boards) has had some
postings in the last month or so
about using brown or black spraypaint and sprinkling
cinnamon on it while wet. In the pictures they posted,
it does look rusty. I don't know how the cinnamon
thing would hold up outside though. There was a
spray on the market, don't know if it is still out
there, that would make things rust. Check everywhere
they sell spray paints.
That is the only help I can give you.

Go to a stain glass store. They will have the products that you can just wipe on to age metal.

One simple and inexpensive way to make metal rusty and old looking is to simply spray vinigar on it. Do not spray it very heavy, just mist it with a common spray bottle. Do not delute the vinigar.Wait a couple of hours between applications, the more you apply, the more it will rust.