Windscreen
Laminated glass with a wiper dragging across it. Most windscreen smearing is a film the wiper is spreading rather than a wiper that has worn out.
Start here
What this is, what tends to go wrong with it, and what to do about it.
Made from
What tends to go wrong
What hard water leaves behind when it evaporates instead of being wiped away. A mineral film, not dirt, and the fix is at the drying stage.
Why it is difficult: Water hardness, Evaporative deposition
A resin that hardens on exposure and bonds tightly to what it lands on. Left on paintwork in sun, it etches rather than merely sticking.
Why it is difficult: Solubility and polarity
Mostly textile fibres and skin, held by static as much as by gravity — which is why a dry cloth on a plastic surface makes it worse.
Why it is difficult: Static and dust, Mechanical action and abrasion, Why different surfaces get different dirt, Surface roughness
Fats and oils, which water alone cannot lift. Removed by surfactants, by heat, by solvents, or by alkalis that convert the fat into soap.
Why it is difficult: Why oil resists water, Emulsification
A seasonal particulate that is part of ordinary dust for most of the year and overwhelms it for a few weeks. Dry removal first, because wet pollen stains.
Why it is difficult: Why different surfaces get different dirt
- Insect residue· no established method
Protein, and therefore a stain that heat sets. A summer bonnet is a protein deposit baked onto a clear coat by the engine and the sun.
Why it is difficult: Heat-set stains
- Bird droppings· no established method
Acidic, alkaline in places, and gritty. On vehicle paint in sun it etches within hours, and what remains after washing is a mark in the finish rather than on it.
Why it is difficult: Why old stains are harder, Coating thickness
Pressure-sensitive adhesive left behind when the paper tears away. It stays tacky by design, which is why it collects dirt and turns grey.
Why it is difficult: Adhesion and cohesion, Surface roughness
The science behind this: Leaching and clouded glass
Part of Car exterior.
In detail
A windscreen is laminated safety glass, chemically tough and mechanically vulnerable in the way all glass is: the scratching comes from grit trapped between the surface and whatever moves across it rather than from the cleaner. That is the glass industry's own account and it explains the wiper. A blade dragging a piece of grit across the screen cuts an arc-shaped scratch, and a blade with hardened rubber chatters and smears. Most persistent smearing, though, is a contamination film — traffic film, exhaust residue, road salt, silicone from polishes and the oily haze that outgasses from a hot dashboard onto the inside of the glass. Cleaning the outside and not the inside is why a screen looks worse in low sun after being washed. One caution: the glass trade is explicit that razor blades and metal scrapers should not be part of routine cleaning and that metal contact with glass is to be avoided; ice is scraped with a plastic edge for the same reason.
Related
- Windscreen is made from Glass
- Windscreen is commonly affected by Water marks
- Windscreen is commonly affected by Tree sap
- Windscreen is commonly affected by Dust
- Windscreen is commonly affected by Grease
- Windscreen is commonly affected by Insect residue
- Windscreen is commonly affected by Bird droppings
- Windscreen is commonly affected by Pollen
Sources for this page
Sources
Glass Technical Paper FB01-00: Proper Procedures for Cleaning Architectural Glass Products
National Glass Association (NGA) with the International Window Cleaning Association (IWCA), 2023 · Trade or professional body
Relied on for: That scratching is caused by abrasive particles trapped between the glass and the tool rather than by the cleaner, and that razor blades and metal scrapers should not be part of routine cleaning with metal contact avoided. (states this directly)
Glass Technical Paper FB06-05: Proper Procedures for Cleaning Flat Glass Mirrors
National Glass Association (NGA), Mirror Division, 2019 · Trade or professional body
Relied on for: That ammonia-containing cleaners attack coatings and backings applied to glass. (inferred from the source; not stated directly)