Painting scenery, painting in bulk, and getting a good end result can seem mutually exclusive.
Here is how we did a bulk batch of heavily weathered shipping containers for the “container city” board.
The containers were spray-painted black and then dabbed with brown (Tamiya flat brown) with a large brush. This was only done to areas that might have had the paint flake off.
Then we added a light stippling of Citadel Jokaero orange
After that, we painted on splotches of water and sprinkled salt. This can be left overnight to dry but I put it in the oven at 70 C for half an hour.
Once the salt was dry, they got a quick spray of white spray paint from the top to set up a gradient
Then they were sprayed with an airbrush (but if I could have found spray paints in the right colours, I would have used them.
These were left to dry overnight.