Another idea you can try is to create some "scratch" brushes.  Draw a few jagged lines using a 1 pixel brush, in an area of 32 x 32 pixels.  Grab that and use "Define Brush", then save it as Scratch 1.  Repeat the process with different patterns of lines, saving them all under different names.

Create a new layer and select one of your brushes.  Switch on Brush Dynamics, and vary Roundness and Angle.  This will ensure your brush changes orientation and shape as you paint with it.  Select black as your Foreground colour and use a low opacity.

Paint on the new blank layer with this brush.  Change brushes after a while and repeat the process.  With care, you'll have a layer full of fine scratches which you can then blend with the painted textures.  Duplicating the layer and changing the colours can also give the effect of scratches through to bare metal, or just into an undercoat or primer.

If you create a layer of scratches only you can also use them to make a bump/displacement map. 

Paint over some areas in plain grey, as if the paint has worn off.  Add brown and orange speckles for rust. 

A low opacity Clouds layer in browns can also give the appearance of varying levels of dirt across otherwise plain areas. 

Coppula eam se non posit acceptera jocularum.

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