Step 1: Supplies
Matte finish Brown spray paint
(optional) oil-based hobby paints in matte rusts, browns, reds, and yellows
Gloss spray paint in desired finish color
Gloss clear spray paint
One small detail paint brush
One soft painbrush (old cosmetic brushes work here too)
Step 2: Rusty Undercoat
If you wish to have a more detailed or old looking rust effect, you can accent it by mixing together various shades of brown with reds and yellows. Look at some photos of old rusted objects as a reference. Using a fairly dry brush, dab the shades of brown randomly in areas where you want the rust to show.
Step 3: Placing the Rust
Before it has a chance to dry, sprinkle salt in the wet locations, brushing away excess as soon as possible. Alternately, you can lay piles of salt in places where you want the rust to show and then use a brush, toothpick, or your finger to drop small amounts of water onto the salt. The wet salt will bind lightly to itself and mold itself along the surface and help it stay in place a little better. Let the water dry.