Is the firepan painted, or at one point painted?
If it is stainless, the rust is probably just OTHER particles that are rusting and should just be abraded off.
Is it just surface rust? If it is, you just need to clean the surface free of dirt and rust (wire brush, sandblasting, etc...) and then treat it afterwords. Paint doesn't actually bond well with metal so it must be etched/primed before paint if it is clean bare metal. After that paint it black, BUT I would look into paint to use. Some might off gas some nasty chemicals, which you may or may not want flavoring your food.
Here is a quick link to surface prep info RustOleum.com