No no no look carefully my friend, there is dust, REAL dust on the roof grins.
I did this a while ago so ….A CITADEL (Warhammer) paint pot is in the picture typhus and I know I used several greens inc goblin green (laughs to self as I know you won’t have this lol) I love Citadel they have a section called technical that do ‘tricks’ I’ll look anyway paint aside
birch seeds, out on your trees right now free, you can colour soak them or use straight from the tree, crush sone use some
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whole added the vw for clarification then other leaf effects
Wood Green grass is cheap and basic but first layer on top of paint, the finer foam stuff is more expensive and you can rub it between your fingers making it finer and clumpier would look good on abandoned wagons,
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the glue is usually a type I can rub off if I over egg it (shhhh stop laughing)
I blacked the windows for depth, then choice a height for the sky behind and added blue/white/dirt then drew in a few branches maybe paint maybe sharpie
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the rust is from a kit on the vw, it is a chemical process and uses iron filings so real rust
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I often add a dull rust finish
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I am not above using a water effect (glue) and cleaning with real water for best shine before taking a picture
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oooh the deluxe rust
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and having an example a painted alternative trick is finding the colour, I like a dark brown and a orange brown for rust and an oil slick paint for dirt and oil
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a couple of brands for you to investigate, and my often discussed dirty water pot far left, I leave it until the water starts to stink literally because cleaning my brushes in it it makes it perfect for a dull dirty finish
…as more comes to mind I’ll do an update…hope it gives you ideas.