With mine, I painted the whole thing gunmetal silver, then dry brushed the whole thing satin black. The butt stock is painted a reddish brown, then I rubbed in black with a sponge, pulling it all in one direction so it looks kindaile wood grain
This should be useful too...
http://www.starwarshelmets.com/original-stormtrooper-stop-that-ship-helmet.htm
You want to nail all those drips & imperfections. The Stop That Ship trooper is the same armour as Cantina Cap