Its will pull any contaminants off of the top coat. Even if your going to buff it, clay it first you will get a much better shine and that real smooth finish. You also dont need to use the spay that comes with the kits. You can use soapy water
We wash the cars then with fresh soapy water clay one panel at a time. Lots of suds and water will keep it well lubricated and keep you from marring the finish.