Your guess is right on the masking tape and I just make up a thin batch of Tamiya Buff or whatever color the dirt is meant to be and I AB it lightly holding the AB a far way away from the model.

Cheers
Cliff

Will it be a vehicle that has a window washer system? You don't indicate era, so I'm assuming it's a possibility.

I bring this up because all the the "dusty" wiper "wiped" windows that are portrayed never seem to have clear/clean/wet marks where the window washer left marks on it.

Think about driving your car on a wet dirty highway - you get behind a car and use your washer. What happens after you stop wiping? Invariably, there are "streaks" of water that follow down where the wipers wiped and at the bottom where the fluid was "pushed" down by the wiper.

The way i did mine was also using the masking tape but instead of paint, i use the matt clear coat and AB it. Once dry, just apply one layer of buff filter over it. When the filter dries, it would give an uneven look to it simulating the dust and dirt settling on it.

This was discussed before. The WW2 era US vehicles didn't have a liquid resevoir -- so the very stark clean/dirty crescent areas in some models seem unlikely to me. I like the idea of using just a flat coat, very lightly.

I think modellers like the clean crescent area because it looks "cool".

Jimbo I think Jeremy (Jazza) was refering to a super diluted mix of buff for the buff filter. I've actually tried something similar and it does give a more realistic dirty look as opposed to an even film of dirt you get when you use just A/B alone.