<<A very interesting slide.>>
The weathering on the three roofs alone is enough to make it interesting:
1) The effect on the car on the far right can be recreated using the tried and true “splotches of rubber cement” technique,
2) The effect on the roof of the middle car can be replicated by a thin overspray of any appropriate dark color, but
3) The edges of discoloration on the left car are far more subtle – perhaps a bit of paint on a rag for the stronger weathering, followed by a light overspray of grime/dust. Anybody else have some suggestions?