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A mix_shader is used to layer two different standard_surface shaders. We will use one standard_surface shader for the top green paint layer (green) and another standard_surface shader to represent the exposed metallic surface underneath.

• Create a mix_shader and assign it to the object.
• Create a standard_surface shader and rename it 'Green Paint'.
• Create another standard_surface shader and rename it 'Metal'. 
• Connect the Green Paint to Layer1 of the mix_shader and the Metal material to Layer2 of the mix_shader.

You could also use the layer shader.

• Connect a noise shader to the radius of a curvature shader (bias and multiply will work too). Experiment with different noise settings to get the look that you want. Don't forget that you can connect a color_correct shader to the noise shader to further refine the appearance of the 'worn' look.
• Connect the curvature curvature shader to the transparency attribute of the layered shader. This will control the blending between the two standard_surface shaders.