As I am in the process of sculpting the robot this week I have been experimenting with metal powders in resin to create different effects that I could use when casting him. As the picture of him shows the robot is quite rusty but in places looks quite metallic. The metal powders I have been using are iron and aluminium. The aluminium powder polishes up to a better shine than the iron, which remains quite dull. However aluminium doesn't rust. I am happy with the results I am getting from combining the aluminium and iron powders. I can create an effect that gives me the shine and rust that I need. I have also tried painting the rust effect by hand, but don't think this is as effective.
Creating rust: Leave the sample in a solution of bleach, water and salt for over 24hrs and longer depending on how set in you want the rust to be.