COAST AIRBRUSH HAS SOME ALUMINUM PANELS JUST FOR THIS PURPOSE.
http://coastairbrush.com/I LEARNED BY STRIPING ANYTHING I COULD GET MY HANDS ON, MY GRANDMAS FRIDGE, MY SISTERS BARBIE DOLL OR WUTEVER. YOU DONT WANT TO PRACTICE STRIPING HORIZONTAL FLAT THINGS CAUSE THATS NOT WHAT YOU'LL BE STRIPING IN THE REAL WORLD. TO GET THE FEEL OF THE BRUSH YOU MIGHT WANT TO TRY OUTLINING THINGS BEFORE YOU TRY FREESTYLIN.GOOD LUCK