To make styrene look like natural weathered wood I first paint it with Testors PLA Enamel wood color (the one in the little square bottle). While its still a bit sticky brush it with black and/or gray chalk. I like to combine the two. The effect you get depends on how "heavy" you are with the chalks. Used that "el cheapo" paint because it stays "tacky" for awhile and binds the chalks real good. See my post under scratching and bashing about the fireworks stand for a pic.