For best results......
Of all the products I've used, without doubt, clay has had the biggest impact on your finish. I tried clay initialyy when I got tired of lacklustre results having used a quality polsh on my wifes neglected Micra. The Meguiars kit was easy to use. The results were unbelievable. Having only clayed the bonnet, water began running off it. A couple of coats of polish and a wax later, the car was transformed. You wouldn't eat off a dirty plate, so why polish on top of dirty paint?