I wouldn't strip it yet Desmosn!
A thinned wash of Badab Black over him now. Then shade in the chaos brand, nurgle wounds, paint in the eyes, bridle, hooves & hair. For the shading use small amounts of wash & put it where you want it rather than flooding areas, don't be afraid to use a mix of colours

Then see how he looks. The rider is the main focus anyway so getting the mount done at all is cause for congratulation, let alone painting it brilliantly
If you're getting stuck this tutorial might provide some inspiration, it's nurgle & a chaos chariot...
http://www.coolminiornot.com/article/aid/849/page/1
...by the rather talented AlienGod3.
Cheers, B.