Step 4: The Surcoat
Begin with a basecoat of approximately 2 parts Chaos Black to 1 part Skull White. Unfortunately, this is but an estimation as I mixed the paint colour many moons ago (about 8 months, I think, to use on my Imperial Guard, and lost the piece of paper I noted down the ratios on.) Leave Chaos Black in the deepest folds and creases on the model.
Next, mix an equal amount of Codex Grey into the original mix, and use this for the first highlight on the model, painting it onto almost every area you painted the basecoat, but leaving a small band of the original mix in the deeper creases.
Add in another equal amount of Codex Grey (making it 2 parts codex grey, 1 part original mix) and use this as the second highlight stage. Finally, using yet another equal amount, this time of Fortress Grey, lighten the colour and use this as the final highlight stage, painting it the highest ridges.