When I purchase a coating agent, there is a kind of the hydrophilicity and hydrophobicity. Water repellency is good or may cause troubles. These are classified by how long of water droplets will stay.
The hydrophilicity has a short duration, it is easy to stream down to the surface, and water flows through. The advantage is tolerant for the dirt which is the best point of the hydrophilic glass by its property. It is the function that becomes easy to remove the dirt because it is hydrophilic. It is easily washed away with water even there is a dirt attached.
Transparent glass which is used for the house window is a typical example. A hydrophilic coating agent is recommended for the person who is not put under much trouble to a car. However, you must not continue leaving it unattended because it is hydrophilic, and a dirt is easy to fall.
Dust, iron powder, sand, pollen, oil and chemicals in the atmosphere can be attached to the car. The function as the hydrophilicity is lost and changed into hydrophobicity. The surface is uneven by these small materials. It is because the water cannot get in between the dirt. As the result, I change to a property like resisting water. When it sticks too long, you have to wash it off roughly and the precious coating comes off little by little each time. After all it can be said that periodical maintenance and car washing are important. I stopped at the environment, but let's keep the car washing in mind once a month.