Making plastic or craft foam look like metal isn’t the easiest trick in the world, but it can be done. This is a great skill to learn to help you with cosplay props and weapons and also armor. DeviantArt user Risachantag came up with it. First, the plastic shuriken were coated with a base coat of chrome spray paint. To make it look more like worn and used metal, she used a dry brush first. After you dip your brush in black paint, do the following:
“Scrub the brush on newspaper for a bit until there’s only a little bit of paint on the brush. Putting pressure on the brush until it splays out, scratch the brush around so that the paint comes off the brush unevenly to give a rough texture.”
See the full tutorial at DeviantArt.