A dark color in rust or charcoal, salt and your top color. I used regular salt for this, but you could use rock salt or epsom salt as well.
Spray your darker base color and let dry. (I didn’t do it for the sake of this tutorial, but I would recommend using a combo of rust with a bit of charcoal here and there for the base coat…)
Sprinkle the top with salt in the random places you want to create the chipped and weathered look.
Carefully, carefully spray short bursts or your top coat. You have to use a light hand for this or you might blow the salt away with the force of the spray.
Let the paint dry, then knock/brush off the salt with a rag or a dry paintbrush.
You can leave it as is or you can lightly sand it.
This technique has lots of possibilities… making wooden signs look like they are rusted tin… adding to wooden frames to make them look like they’re iron…
I plan to take a basic terra-cotta flower pot and make it look like a chippy iron pot.
There you go. Easy peasy, shabby chic.