Here's a little movie on how to make a new "bristly" brush in the current Acorn 1.5 preview. I think this type of brush would be known as a spatter brush in other applications you might be familiar with. (Disclaimer 1: this is from an alpha of Acorn 1.5, and things will certainly change a bit before it ships. Disclaimer 2: I'm narrating the movie and it's not one of my greatest strengths. You've been warned!).

Watch how to create a brush in Acorn.