Haze can develop on a car windshield for a variety of reasons. Some causes are nuisances that can be easily fixed, while others are more severe issues that require professional help.
Hard water residue can leave behind a thin coating of minerals, usually from limestone or calcium in the water supply. The culprit is usually tap water from washing the car. If the water is allowed to dry on the windshield instead of being wiped off, the minerals in the water leave behind spots or a haze. Repair is relatively easy. A window cleaner that contains a mild acid is used to break down mineralization. The windshield is then buffed to high clarity.
2. Vinyl Off-Gassing
Haze on the inside of a windshield can be due to the off-gassing of the vinyl and plastic inside the car, which occurs in hot weather. The issue usually affects newer cars since they are still actively off-gassing. A dull, smoky haze develops on the glass, which can be difficult to remove. Parking in the shade reduces off-gassing. The good news is this haze will easily wipe off with standard window cleaners.
3. Acid Etching
Acid etching is a more severe cause of windshield haze. It can be a result of acidic rainfall, pollution, or bird droppings that are left on the windshield without being promptly cleaned up. The acids eat into the glass coatings, resulting in hazing and sometimes minor pitting. If the damage is caught early, a repair service can buff the window to smooth out the etching and then polish it to renew its clarity.
Sandblasting damage can be quite severe, as it often results in a multitude of pits in the glass, which gives it a hazy appearance and low clarity. Exposure to high winds and driving on dirt roads are common causes of sandblasting. Shallow pitting can often be fixed using the same methods as that for repairing acid etching. Deeper pits from more severe sandblasting may not be repairable, so a new windshield will be necessary.
5. Wiper Damage
Using old or damaged wipers, or regularly turning on the wipers when the windshield is dry, can result in etched arches in the glass. If shallow, your repair tech can fill in the etches with an acrylic product and then buff the window to full clarity. Deep etches or a large amount of etching is more difficult to repair, so it's usually advised to replace the windshield.
Contact an auto glass repair service if you have further questions.