This old, but very effective guide will help you to imitate damaged paint on any vehicle. Everything is extremely simole:
Apply any base colour looking like rust. Dubmull Brown from Citadel will do
Put some water on surface you want to have damaged. Be thoughtful and look through some photos.
Put some salt on thes places.
Leave it dry for a night
Put base armour colour above the salt.
Cerefully wash the salt away with water.
When the salt melts, there will be damage required.
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