Sewing rusted fabrics is no different that stitching any other fabric except, and that's a big EXCEPT, when the fabrics are heavily rusted! Then there are all sorts of problems that arise.
For moderately rusted fabrics as seen in the above photo, a heavy cotton thread and a sewing machine needle with a large eye, like a leather needle, work well.
But what does one do when the fabric itself or the thread shreds during the stitching process???
This tutorial will help you stitch those fabrics and get the look you are wanting. I highly recommend having some extra rusted samples on hand and taking good notes on which processes worked for each sample, BEFORE you start the large commission piece. Doing so will guarantee fewer headaches in the future ;-)
iron on interfacing of some sort I use Mistyfuse TM
Sewer's Aid TM (Dritz) - liquid
Fray Check TM (Dritz) - liquid
Needles with large eye such as a leather needle
Heavy cotton or poly blend threads, rayon and silk thread will only shred to pieces in this process.
Nymo or Fireline for heavily rusted areas that you don't/didn't want to discharge.