dave i think the best thing to do for these types of materials is to start with a photo. Of course it depends on what look you want in the end. If you want to do anything that looks real or almost real, i would suggest using a photo. Using photoshop to make rust seems pretty crazy to me. Its lots of hard work for a result -in most cases- that doesn't look as good as the real thing. I also disagree about the shadows. Ideally there shouldn't be any shadows on a texture.