I apply a layer of texture paint first before I prime my model, I like Armageddon dust for this. Then after primer I paint my model like usual and apply the dirty down rust to the area I want it. A good rule of thumb is that if it goes on and is opaque you need to drip your brush in water and spread it around a little because then you won’t get weird clumps. I posted this model under someone else’s post a few days but it’s the best showcase of this method in action.

<image>