What causes fog on the outside of the windshield that cannot be wiped off?

[DiDiana]

The fog on the windshield that cannot be blown away may be caused by a leak in the glass interlayer seal, allowing water vapor to enter the interlayer of the windshield. The solution is to locate the leak in the windshield seal, blow out the water vapor from the windshield, and reseal the windshield. There are three solutions to windshield fogging: Solution 1: Use the external air circulation + direct airflow to the windshield + increase temperature + increase fan speed + do not turn on the AC. This can solve over 90% of windshield fogging issues. The advantage is that it does not increase fuel consumption, but the downside is that it may cause discomfort. Solution 2: Turn on the AC, regardless of internal or external air circulation, adjust temperature and fan speed appropriately, and direct airflow as desired. This can solve almost all fogging issues. The advantage is comfort and simplicity, but the downside is increased fuel consumption. Solution 3: Apply a windshield defogging agent or detergent to the inner windshield, which can prevent fogging for a period of time. The advantage is that you do not need to use warm air for defogging, but the downside is that it requires proper application, may have odors, only lasts for a limited time, and needs to be reapplied.