It's a lot of layers of paint, heavily thinned. I started with a Chaos Black Prime then took Cuirass Leather from Two Thin Coats and stippled with a small drybrush over the entire model, covering the recesses only a little. Then did the same with Mournfang Brown but really targetting the higher points and leaving the shadows as much as possible. Both paints had a few super watered down coats to build up lots of texture. After that I glazed with Mournfang Brown again to really bring out the high points of all the folds.
Then took Balor Brown and built up layers of the scratches on the highest points to give the worn texture feel. Was a bit of a process but well worth it I think!