Step-by-step: Applying rust to a surface
Before you add rust to any surface, you’ll need objects that are made of iron. Rather than painting an object shiny and new, it will be easier if the surface is already dull or matte with dust and dirt on it. That means there’s no need to spend time painting in specular highlights. I’m using textures from www.textures.com, which are free to download (with a premium option).
I start off by layering the textures as appropriate. I use the Liquify filter to warp the rust textures so they fit the contours of the car. To show the background, click Advanced Mode and Show Backdrop. You can also use Puppet Warp or Warp for simpler contours. Erase the edges of the texture so that it blends with the surface of the car, and set the layer to Overlay or Multiply.
Continue to layer textures on the car, while keeping in mind how real-life physics would affect the weathering. For instance, the constant daynight cycle would make the metals expand and contract at different rates, and therefore bend the radiator grille and even the body panels out of shape. Rust will be worse wherever the metal has been scratched or damaged.
The image is beginning to look a little flat, so I go over it with a simple grunge brush so that it doesn’t seem too clean. I add other details such as a layer of moss, broken glass and reflections from the environment as needed. Because any glass would have become grimy and dusty over time, I apply a blur to the reflections and give them a more matte appearance.