Some of the art is positively amaznig! Take away some dirt with a brush, a pencil eraser or a cloth wrapped around a finger. The dark areas have the dirt fully removed, the lighter areas retain the dirt.
Really talented folks can use their fingers to get this done and even a bit of wet cloth.
Some make you chuckle -
I'm going to give this a try in a few days – on the back of my car. I think I'm going to use this photo for the concept. Since we will surely have no rain until after Halloween in the desert, this might be able to carry me through All Hallow's Eve!
I'll post a pic when I get this done!