This is an advice by a veteran car painter to us. Do not wax your car frequently because it is not easily visible but it ruins the paint somehow. If possible, wax your car twice a month and use water only. For grime and too much dirt, it is all right to use shampoo but again do not use it frequently. He said that there was an experiment involving 2 cars, 1 was waxed twice a week while the other was waxed twice a month. So there, use the car was sparingly to save on the car's paint and also on the cost of wax.