In my last experiment I just used rusty objects laid on top of wet fabric. I kept the fabric wet but even after several weeks there was hardly any rust.
I used two different fabrics, both 100% cotton but one was a finer cotton weave and the other was muslin.
In the muslin I placed bigger items like a flattened tin can, a large round washer and larger hunks of metal. This one seemed to work much better and I'm pleased with the results.
I will continue experimenting because I love the red designs and the staining of the fabric that the rust gives.
The two examples above are the finer weave cotton and below is the muslin. I think I will try again with these same fabrics to see if I can fill the other areas with rust as well.