This is a Forgeworld Ork Gunwagon from their Display Cabinets at GamesDay 2005.
This is a well painted model and if you look at this close-up it will give you some ideas on how to weather your Ork models.
I use to not like this model, the fact it was based on that awful plastic four wheeled Ork Battlewagon (from GW a few years back) was probably the reason.
However I think because I got some of the Epic Ork Gunwagons I have grown to like it. I did try and buy the basic Gunwagon at GamesDay, but they had none in stock.