The cape was painted using airbrush over the zenith primed mini. The airbrushing work was done before any other areas were painted.
About 4 successive layers of grey going from dark grey towards light grey/Bleached Bone. The final two layers or so layers were sprayed on at an angle straight from above to get the zenith effect.
Over this I worked in some shades (using various browns, greens, reds) now and then as I was working on the other areas.
When most of the model was done I sponged on some almost pure black with a bit of blister foam. And over this I sponged a light colour - Bleached bone. I also did few "scratch"-highlights on some of the black spots using bleached bone. "Scratch"-highlights meaning that the black sponge marks were highlighted illusionistic by adding a highlight to the grey cape just under the black spot. This makes the black spots looks like tine holes in the fabric.
Finally, I worked in some more washes in certain areas - mainly to emphasize the folds and areas that are in the shadows.
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I hope you can use the information provided above. I plan to bring this technique to good use on some Astartes within the year...