Now comes the tricky part, the part that will decide if your texture looks good or... not so good. We will go through the process of making scratches and adding color and dirt effects. The image I completed doesn't look very nice, because the scratches are very exaggerated, and the colour is simple, but as I say this will help to understand the basics of this process.
We will start by adding the scratches. In this step use once again the highlight tool, but with a brush that resembles scratches (you can search for grunge brushes on the net) and spread it out till you like it. BUT REMEMBER : A surface doesn't have to have many scratches to be realistic. Be moderate, not the way I did here!
Make some scratches with the burn tool, also not as exaggerated as this one, but with smaller intensity.
Now with the dodge tool, with a 1 pixel brush size, with some intensity paint in a subtle highlight below the biggest dark spots that you created. This way you will make a feeling of smaller supressions, and that this metal is kind of getting beaten.
This step is very important. You will have to pick a texture that you want and place it in the selection of the thing we have been doing so far, like you can see in the image.
Now set the blending mode to overlay. The blending modes are set in the place marked in the screenshot, there are several of them. Only Overlay/Multiply/Soft light/Hard light/Color have the desired effects, although the color one is something else.
To avoid exaggeration, we will erase some of the imag, but very subtly, with a mid opacity eraser brush.
Now also comes a tricky part, we will paint (yes paint, dodge and burn wont do this, since they shade according to the base texture only, so you will pick up a brush (could be any, but here's mine)
Anyhow, after you select your brush pick a dark brown color, and start painting in the direction of the gravity. In this case, below, so this will make that dirt is falling from the pieces, as well paint in some parts of the pieces, try variating colors, like adding blue in the highlights. REMEMBER: the opacity in this process for the brush is VERY LOW (around 5% or less), so it will resemble image below:
Now to finish it, just change the layer blending mode to multiply or overlay. I chose overlay, since these are dark tones, and here you go...you have finished! I hope that this tutorial was good to help someone out.