Weathering and depicting surfaces
I try to introduce imperfections into most of the edges and surfaces. I do this with Photoshop’s Scatter and Smudge brushes. For the wheels I use mud textures with blending modes, before masking them out with more texture brushes. Don’t spread out your imperfections evenly. If you look at real-world objects you’ll often find where there’s the possibility of contact with other objects.