My mate that does film props does some carbo stuff, has a vacuum over etc, if stuck I can ask him, he's currently doing props for three major films, inc batman.
He's just done a load of poly HANDGRENADES(200) but would not let me have a single one, looked absolutely real, even had rust on them, rolled them in iron filings that have been left out in rain a few days while poly still soft so it sticks!!
tabetha