The most common scratches on a vehicle are wiper arcs on the windshield or back glass.
These are caused by sand and debris trapped in the wiper blade while it is wiping the windshield. This damage appears as a white or frosty arc in the shape of the wiper arc.
SIDE TEMPERED GLASS
Scratches on the side tempered glass are usually caused by sand or debris in the seal around the window in the door. The scratches are usually vertical from the end up and down motion of the window opening and closing.
Other scratches may be caused by scraping ice with a metal object, valdalism or contact with an abrasive object against the glass.
HARD WATER STAINS
Hard water, when left to dry on the glass, can leave behind mineral residue as the water evaporates. The buildup of mineral deposits eventually becomes visible and will inhibit safe driving vision. The hard water stains appear as white or opaque circles on the glass.
Many vehicles are washed with a soap and acid mixture to more effectively remove grime and debris. When the acid mixture is allowed to dry on the surface of the glass it will lightly etch the glass which interferes with the clarity of the glass. The acid staining appears to be dried soap suds on the glass that won't rinse off.
After a number of years, plastic dashboards emit vapors from the vinyl on to the inside of the windshield glass. After years of accumulation, the surface becomes very difficult to clean. The smear of the vinyl residue can make light appear diffused and inhibit safe driving vision.
The surface of glass is rough and uneven when viewed under a microscope. These irregularities trap dirt and other debris that adds up to inhibit clear viewing through the glass. The glass appears dingy and less than clear and sparkling.