I picked a space marine and gave it a yellow powerfist. This is just a marine done for this tutorial but you can translate this technique to any color you want. So Blood Angels, Ultramarines, Goff's, the technique is the same.
These are the colors I used:
Castellan green - Elysian green - Yriel yellow - Flash Gitz yellow - Warplock bronze - Gehenna's gold - Auric armour gold - Leadbelcher - Runefang steel - Abaddon black - Administratum grey
Step 1. Basepainting
This is what I call Basepainting, I paint the model in the basic colors of my chapter. If you take your time you can paint up the model nice and smooth. Make sure your paint is watered down a bit because it paints nicer and easier and looks a lot better. No one likes a lumpy model unless its nurgle.
The base colors I use are
Fingers and arm: Castellan green
Hand : Yriel yellow
Skull: Warplock bronze
Hose: Administratum grey
We will just concentrate on the powerfist so I only describe what I do Step by Step on that leaving out the rest of the model.
Step 2. EdgingWhen you want to give your model a bit of depth the easiest way to do so is by edging it. Just paint really fine lines along the edges in a lighter color then the original base color.
Fingers: Elysian green
Hand: Flash Gitz yellow
Skull and hoseconnectors Warplock bronze
Thin down your paints again and lightly edge the model. The skull was done with very watered down Gehenna's gold on the raised parts only.
Step 3. Washing
Next I washed the complete Powerfist with watered down Abaddon black (1:1). This will give the model even more depth and creates the illusion of shadow. There’s nothing to it, just wash it and make sure it doesn’t pool. Don’t over do it and you’re good.
Step 4. first scratch layer
Now we are getting to do some battle damage! The first layer that is… I used Flash Gitz yellow to make the first scratches. This color is lighter then the washed Yriel yellow on the rest of the powerfist so it will stick out a bit. We are creating the look of the Powerfist very light scratches as if only the very top layer of paint has come off as a result of battle damage. You now have to imagine where you want your battle damage to appear. Again around the edges and raised areas is a good idea because that’s where the chips of paint come off the easiest. Because this is a Powerfist its not hard to imagine where the fist will be most scratched when punching down xenos. I made some small narrow strokes along the line of movement of the fist as this looks the most realistic. Also a few small scratches on the middle of the hand. This will become more clear in the next few steps.
Step 5. Second scratch layer.
This step is basically the same as the last step but now we’ll use Abaddon black and stay within the area that we just painted with Flash Gitz yellow. Leaving just a fine line of Flash Gitz yellow showing it will create the illusion that this layer is scratched deeper then the last step. As if the model is scratched to the basecoat of the model. The pictures should make this step more clear.
Step 6. Last scratch layer.
It begins to feel like rinse and repeat but this is the step that brings it all together. When you take Leadbelcher and do the same as step 4 and step 5, leaving a fine line of the Abaddon black showing it will seem as if this is the deepest part of the scratch. I also touched up the skull with some Auric armour gold. Look at the picture of step 6 and it should be clear what I mean here.
Step 7. Touching up
In the last step I use Runefang steel to do some touch ups. I only put very tiny painttips in some places to make it seem these scratches are fresh or recent or that they are scratched more often over and over again. Again it also gives a bit more depth.
I also touched up the skull with a 1:1 mix of Runefang steel and Auric armour gold.
The result looks quite satisfying to me so I’ll leave it as it is. You might want to experiment with adding a step or two yourself. I sometimes do a layer of very very watered down wash between step 6 and 7.
That's it! I hope you enjoyed this short tutorial.