Painting grime and scratches on a RZ-1 A-wing interceptor 3d Model in Mudbox – the rule is – there is no rule – you paint and then you erase. Make sure to keep grime on a layer and paint away. I use a very simple brush and usually try to keep it very simple and straightforward. The dirty and random effect is achieved via applying and then erasing strokes.
Some people like the idea of layering grime textures – i personally like the hand painted look but both work just fine.