Choosing which areas of a vehicle to weather is fairly straight forward. I generally go for areas that would normally wear on the real thing. Areas such as foot platforms, bumpers, grilles, panel corners, and “high points” such as bolts, hinges, hatches, etc. I also like to add paint to unpainted sculpt details like tools, etc. See the front of this stinger and the Night Ops Hummer;

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"The future's uncertain and the end is always near." - The Doors "Roadhouse Blues".