After giving the model a white undercoat, I wanted to give the main tracks a black undercoat.
I made a mistake. I used a can of Citadel black spray paint to paint the majority of the model except the flatbed. As you can see the results were less than satisfactory. The paint had dried and almost had a dusty look to it.
In the end I decided to respray with a different brand of matt black spray paint.
This seems to have worked better.