Bad move!
In front of witnesses!
Not only will you be charged with a Battery, you'll need to hire a lawyer to defend you.
Besides that, the jerk can pick up a phone book, and get a lawer for free to sue you in civil court! Even if he doesn't win, you'll have to hire a lawyer again to defend yourself.
The satisfaction of cleaning the car with the jerk just isn't worth the consequences.
Just my 2 cents worth.