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Technically any time anything comes into contact with your paint it will scratch the paint. Including washing, drying, and buffing off wax. The goal is to minimize the damage done after every wash. Just an fyi if anyone wasn't aware. As far as clay goes, it's an absolute necessary tool for detailing. The problem is that the clay will only remove recent spots. Once the hard water etches itself into the paint or glass it will require a buff (for paint), or polish (for glass). Rainx is a good "protective barrier" in-between washes, but it's basically rubbing alcohol. Glass polish will remove contaminants clay can only dream of removing

 

Just to clarify, Rainx is silicone. It works by getting into the microscopic imperfections of the glass to make it "smooth as glass." That makes the water bead as it has no tiny cracks to cling on to.