One thing that always seems to throw off the suspension of disbelief in a 3d piece is it's prestineness. By adding noise, scratches and other real-life elements into the picture we can subtly enhance it. It looks dern cool too.
Here's where we are so far:
Adding noise is pretty much a guessing game.
You could just add a noise filter on a separate layer and lower that layer's opacity, but the result would be rather bland overall, and there are simple ways to vary the noise effects.
You could also use any number of grunge maps, noise maps or other templates (there are some nice ones on 3dTotal's texture CD!), but what if you don't have any available to you?
This tutorial will go through a simple way of creating a varied noise effect just by using Photoshop's native filters.
Start by creating a new layer and naming it "noise". Hit the "D" key to default your colors. Now apply a Filter>Render>Clouds filter to the new layer. Right away apply a difference clouds filter (Filters>Render>Difference Clouds) to the layer and keep applying it until you end up with something you like. Clouds and Difference Clouds create a random tileable pattern everytime, so keep trying it until you end up with a result that's high in contrast and has several obvious "veins", like below.
Now this part will be different for everyone, but open Image>Adjust>Levels. Drag the middle (gray) slider a bit to the left, making the veins a lot more pronounced.
Now you can apply the noise filter, I used the settings below:
Amount: 21.33, Uniform, Monochromatic
Now, here comes the trick. Lighten the layer opacity to about 10-15% and change the blending mode to screen You should end up with something like this:
You can sort of see the veins, but they don't detract from the image. You can further enhance or diminish the effect to taste by using image>adjust>brightness contrast on the layer. Here's how mine ended up. It looks a little too patchy, but that will be fixed up later.
Next: Color Adjustment and Final Touches!!