Hey mate,
I think that's coming along nicely. Maybe make your bump map a little smaller by upping the repeats to nail the main painted look.
To add some rust create a high-contrast fractal grunge map, mostly white with patches of black (e.g. amplitude = 1, Threshold 0.7, Ratio 0.6). Use this as your master rust map. Connect the rust map to a second Bump2D node with a bump value of about 0.1 and pipe it into your main Bump2D node's Normal Camera.
To the master rust map, add a remap ramp with two colors. Pipe in your master color into color 1, and make color 2 a rusty shade (I found that full saturation worked best). Pipe the remap ramp into the color channel of your shader.
Finally, you want to switch off the specularity for the rust, so add a contrast node to the master rust map (contrast = 0.1), and a multiply/divide node after that (multiplying by 0.65). Pipe that into your Specular Color channel.

Christian