Chemical Fog
While I agree with the obvious towel contamination that has long been the cause of smeary window glass, there are several other causes to consider.
Vinyl-Vapor Residue often coats windows, especially interior windshield areas, with an oily residue resulting from gas generated from sunlight /heat on the plastic surfaces of interior materials. These windows require ample glass detergent to emulsify the residue. All too often, a quick window spray and a wipe of the towel will not remove the smear. In fact, the partially moistened glass actually causes the smear because inadequate chemical application stunts the cleaning ability, instead leaving a gooey mess that is smeared by the towel. Applying the glass detergent with a separate applicator sponge while "boxing the window" will break the surface tension of the residue... and the "boxing process" assures that the entire surface is treated with the detergent. Following up with a clean towel will collect and remove the emulsified scum. Flip the towel and buff to a streak-free shine.
If you use microfiber, be sure to segregate the microfiber from cotton toweling and wash separately. Also use a quality laundry detergent that is free-rinsing, thus eliminating any remaining residue in the rinse cycle. Some operators occasionally use vinegar in the washing cycle to aid cleaning and lint control.
Always segregate all your toweling when laundering and avoid cross use between glasswork and other detailing activities such as final wipedown. Smart operators recognize the importance of proper laundry practices and avoid cross contamination because it saves valuable time in the window-cleaning process. The number one complaint from after-care customers is poor window cleaning... and a carwash that focuses on being regarded as window-cleaning experts offer a valuable distinction that keeps customers coming back as well as telling their friends of that distinction.
