I must say, I really enjoyed that Truck. It looks very realistic, almost as if you took a photo of it and used a Smudge Tool across of it. The fact that it wasn't, is just astonishing, well done. :)
I use a lot of the smudge tool with some custom brushes I made but I just use a photo as reference. I also use the photo as a color palette ,selecting colors from the photo with the dropper tool. Having the colors directly from the picture make it a lot easier to create a realistic look. I really like the sketchy, visible brush strokes. Anyways here's another one I painted.