TUTORIAL 34:

This tool seems kinda handy, I really like the effect it has on the finished image. To make something look stained you gotta get the brush brusheezy, it was created particularly to create stain like smudges and what not. And you add a layer and paint an outline of where you want the stain to be and then you edit the type of brush you want and then continue making marks where you want the smudges to be.

Website: http://psd.tutsplus.com/tutorials/designing-tutorials/how-to-create-a-grunge-style-illustration-with-stains/

Link: How to create a grunge style illustration with stains