So, for my high school shop class, I need to effectively dirty some cars (with pretty bad paint jobs) so that the kids can wash them. It's hard to judge their performance if the cars are clean to begin with. I was thinking if I just has some fine powdered dirt I could mist the cars with water and then toss the dirt on the day before. I was just curious if anyone had any better ideas? This won't be on any cars with a decent paint job. Mostly 20 year old donors.