Those type of rust stains are sometimes easier than most rust. Keyword (SOMETIMES)
You can try a couple of different products you can get at a local hardware store. Make sure that you do a test patch in a hidden area, to see how it will react on the surface.
I would first try CLR, that is what it says on the bottle. The letters stand for Calcium, Lime and Rust. That product is specifically advertised to remove rust from porcelin and stucco. I would say that it is the milder of the two.
The other product is Iron Out. This chemical is pretty strong and has a pretty strong vaper when your mixing it. So be carful.
Good luck and let us know how it went.
Make sure you test, you don't want to leave a discoloration spot on the chimney if it is to strong
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Dan Flynn
Flynn Service Pro
www.powerwash.net
708 715-4205