there are sites where you can download textures like rust, stone, tree bark, etc.,...
Also, if you have some photos of rust or magazine pics, just scan it in. You can use it in the "textures" filter or create a special brush by: 1)making sure it is a greyscale image and adjusting the contrast (higher contrast works well---not too high) 2)using the "object selection tool" or the "marquee" to select the area you want 3)go to the brush rollout menu and select "define brush" ...go nuts...make different sizes.
I''d do something more along the lines of using the "render clouds" on a new layer, up the contrast to give you white regions, invert, lighten, shadow the edges, set the blend mode to "darken". This''ll give you the chipping metal look. Then I''d make a new layer with some light noise, give is a vertical motion blur, then set this to darken. That''ll give you the detail to the rust. Then color in the area with a rust color.
I havn''t tried it, but I''d do something along those lines.