What exactly is rust?
Rust & Fabric
Rust dyeing does damage the integrity of fabrics slightly; so if you are looking for an archival process, this is not it. Items with very strong or dark rust-dye will be more difficult to sew as the rust dulls the needle quite quickly.
The bottom line? Rust happens. Even if you do nothing to facilitate it. Throw some fabric out in the elements with steel, and it will happen.
OK, enough of the technical stuff. I hope your eyes didn't glaze over too bad. Come back Tuesday and we will start getting down to specifics.