On several occasions I’ve noticed that my windshield appeared slightly opaque, so I cleaned the exterior surface. When I noticed that the windshield was still discolored, I checked the interior surface and found that it was covered by a layer of crud, despite the fact that it had not made physical contact with anything since it was last cleaned. This residue has a waxy consistency, making the interior surfaces of the glass much more difficult to clean than the exterior surfaces.
What is this substance that builds up on the interior of my automotive glass? How do I prevent it from accumulating there?
I can’t find a cite to back this up, but the plastics used in the manufacture of automotive interior trim emit vapors which cause a greasy film buildup on the inside of the window glass.
Are you a smoker? Smoke from cigarettes will cause a hazy buildup over time on the inside of the windshield, and it’s very hard to remove.
Does this happen more often in winter, when your heater is running? I’ve had a buildup like that once when I had a pinhole leak in a coolant line; it made its way into the airstream and from there onto my windshield without noticeably affecting my coolant levels for quite some time.
If no to either of these, then I’d go with the fumes from the plastics and vinyl in your car. I don’t know of any way to prevent it, honestly. Perhaps a damp paper towel over the dash when you exit?
It is the outgassing of the various materials used in the interior of the car. If you use Armor-All or some such stuff, it will be worse.
Some cars use interior materials that are more or less hypoallergenic and these fog the windshield much less than cars that aren’t.
For example the interiors used in Volvos meet the Okotex 100 standard
Not all cars meet this standard. Older cars were much worse. Early 70’s cars from Japan would fog the inside of the windows is a week or two.
You best bet to keep your windows sparkling clear is to clean them often either go to a full service car wash, or do them yourself every week or two.