The Crossroads of the Aether
The easiest way to simulate rust is with artist chalk. Wander down to your local art source and pick up a pack of “earth tone” chalks, not chalk pastels they are not the same thing. There will generally be 6 rust gradients in a pack. These simulate rust from 24 hours old (orangish) to 3+ years old (dark brown).
To apply use wet water (2-4 drops of detergent in a quart of water). Select the shade you want to use first and place a few drops of wet water in the chalk stick. With a not overly good brush stir the water while pressing down on the chalk. This makes a slurry of chalk that can be applied to the item to be rusted. Apply until the desired effect is achieved in the wet state. Then let dry. The chalk will become substantially more visible when dry. DO NOT PANIC. When a clear fixative is applied to the object the chalk will appear as it did in the wet state.