What is the best way to make a snow covered roof on a box car? In the past I have used Chalk powder after I have applied a heavy amount of dull coat. But I takes a lot of spray and chalk and more spray so it won't come off. I looks great on small areas like steps and dosen't get that yellow look. So much chalk is lost is absorbing the spray. But for a large area like a roof top of a car I need something thats not so time consuming. Any ideas?