hmmm, i think that anything is possible, and that a vechile covered totally covered with dirt (and i mean totally) would be a bit too dirty for QEW. however, i feel that QEW is strong enough to take care of a majority of dirty vechiles, and that if needed, use QEW x 2.
also, microfibers should not get dirty. the only reason they should get dirty would be becuase you either missed a spot, or maybe a light dust settling on the surface of a car. just remember to dry QEW before it dries on your car, or youre gonna have to do it all over again.