I set up this file as a way to work on procedural brushes for my own purpose, but I feel like someone might be able to use these few stencil brushes in their own work. There are four brushes that are best used painting on the Stencil slot of the material on the included helmet object that I built for a demo back when.
Breaking Clouds - this is pressure sensitive, and if you release pressure as you drag downward, you can get some interesting effect like leak stain or erosion.
Scraping is just a really simple breaking/scraping brush, allowing to reveal the primer/metal below the paint layer.
Scratch Random is a generic procedural blend that uses a color ramp to form a thin line, and the brush mask is set to rake and random so that you get some quick effect.
Sun Bleach can be used as a way to lighten an area and softly reveal the metal, or color pick from the diffuse colors in the paint layer and use it to darken areas.
I hope you find this useful.