Actually, the rust is left over from a project I did for chemistry last year. I had to design and make a battery using something other than zinc and copper. I extracted ferrous sulfate from fertilizer. The leftover was rust. Since it was already nicely powdered, I decided to use it for weathering, although I did just through a couple pounds of it away.
If any of y'all want some, I could mail it to you for free. No need to pay for something I have loads of.