I have a toy that I'm customizing and wanted it to be real rusty.
This is the results with a lot of trial and error. I first used pastel washes. I mixed the pastel dust with the alcohol but it dried really light and was very chalky white. I toned it down when I added a dulcote matte spray but it still didn't look right.
After that I added some rust color paint but it was very monotone and looked like paint. I read about using real rust, so I have some old lawnmower blades that are encrusted with rust and used them. I sanded the rust and got all the dust and tried dry brushing. It was to faint so I added a little water and mixed up a paste and crammed it in all the crevases, then did some drybrush strokes.
I finished it off with by dabbing some OD green and Brown krylon paint with a sponge. Most people who see this in person think it's metal so I'm happy with the results even though it's a little over the top.