Here is a quick example of how I start painting scratched metal, this process can be enhanced by more detail.
This 15 minute process gives you a good base to work from when doing concepts, as its super quick!
I will show you the process on a sphere, but you can use this for armour, environments, vehicles etc
I start by blocking out my basic form, in this case a sphere. I choose a mid value grey to fill it.
I also click the "lock transparent pixels" option at the top of the layers window. this means that I will only be able to paint over the area filled by the circle. I also add a shadow, but this is just for presentation.
Next I make a new layer, set it to Overlay. I take a textured brush and start laying it in with black and white, giving some idea of light direction and form but really just adding texture.
I now make another layer, set that to Overlay too. I now use a soft airbrush and paint the form of the ball using black and white, getting a clearer idea of the lighting.
I add a coloured reflected light to push the form more, also means I can show you another cool lighting trick later.
I don't go to light as I intend to keep the highlights on a separate layer, its way easier to edit them like that.
I make a new layer and keep it on Normal setting, using a soft airbrush and having white selected i do a quick atmosphere pass to help turn the form more at the edges, this is not a step you always have to do, just in this case it helps the ball read better.
Ok now for the fun stuff! this is a trick that works really well for reflective surfaces and I use it a lot.
Make a new layer, set it to Color Dodge. Fill the layer with black. You will notice that nothing has changed on the image, that because the black won't dodge anything, but what it does mean is that if you paint into the layer with white, and have a textured brush, you get really natural looking shiny lights, this is because your actually painting into a colour and not just painting white on a blank layer.
I use a textured brush to pick up the detail from my texture pass, I create annular highlights around the light most facing plane of the ball, something you can observe on textured metal.
Now here is a very similar trick but for coloured light, which will enable us to do the same with the coloured reflected light as we just did with the highlights.
Make a new layer, set it to Screen. fill it with black. Now go to Layer_New Adjustment Layer_Gradient Map, click ok.
Make a gradient that goes from black to a high chroma hue that matches your reflected light (in this case I go from black to orange).
By now some awful crazy shit has happened to your image, don't worry!
Hover your mouse cursor over the layer palette between the Screen layer and the Gradient Map layer you just made, hold down "Alt" and you will see the mouse cursor change to a box with an arrow facing down, click.
what you will see is that the awful coloured filter disappears and your layers will have been "Clipped" together :D!!
Now for the actual painting! click the screen layer, whatever value you paint into this layer from ALMOST black to complete white will be the colour of that particular value on that gradient map. So white will be the saturated orange, black will erase and any grey in between will scale in chroma the closer you get to white.
Sounds mad complicated, but once you see what its doing it makes lots of sense, it enable you to paint coloured light on your subject without messing with the rest of the painting, its a very clean way to add light to a surface, and completely reversible! honestly the applications for this are mental, I play with it all the time and its awesome!!
Finally, I add a scratch to the surface, just to show that up to this point we have been creating a base to work on, from here you can add patterned filigree, dents, scratches, blood, whatever, choices are endless.
This process can be adapted to paint a variety of surfaces not just textured metal, so play with it.
Final shot is of my layers just so it makes a bit more sense, I can also send you the file if you email me.
NOW GO PAINT.
