Bolt Gun Metal, Badab Black Wash,Bestial Brown, Vermin Brown, Chain Mail. Mythril Silver(Optional)
Start by painting the metal areas of the model with boltgun metal and then give it a coat of black wash.
Once it is dry grab your bestial brown paint just on the tip of a dry brush or old brush directly from the pot, and start stippling it in random places over the metal area. If you are unfamiliar with stippling , it is basically quickly stabbing the area with the tip of your brush leaving a blotchy pattern. You can also buy stippling paint brushes but an old drybrush will work fine. You still want to remove the paint from the tip of the brush in the same way you would if you were going to overbrush the area before you start stippling.
Go over the bestial brown areas you just put down and stipple vermin brown on the same area. Try to leave a little bit of the bestial brown showing to simulate deep rust and to give it some texture.
*Note* You could also take it one step higher by applying a "highlight" stippling of Blazing Orange paint on the brown areas, but I don't like the brightness it creates in this instance. I was going for a slightly rusted look rather than a corrosive rust look. If you are doing rust effect on a piece that would be sitting in the elements for a long period of time like a wrecked vehicle, junk yard metals of barrels, fences, or wreck markers you should definitely add a layer of orange to your rust effect.
Next were going to lightly drybrush the metal area with boltgun metal. Try to only dry brush some of the edges and any of the areas that you feel need to come down in shade. We're then going to highlight the edges without rust with chainmail and mythril silver.
It doesn't look as sharp as the hairspray salt technique, but it still does the trick.
See you next time!