I did buy the Ork Battlewagon Upgrade Pack and decided to add the Deff Rolla to my Ork Battlewagon. Having given it a black undercoat, I drybrushed it with Tin Bitz.
The next stage was then to drybrush with Boltgun Metal.
See the full workbench feature on the Ork Battlewagon.
See photographs of completed Ork Battlewagons from various shows.