We seal windows with a hydrophobic sealant that chemically bonds to the surface. Glass is actually uneven and full of peaks and valleys. Dirt and hard water get stuck in the uneven surface.
The sealant fills the gaps and keeps the contaminants from adhering to the surface. This makes routine cleaning much easier. The process is expensive due to the cost of the product. The glass also has to be cleaned to perfection before applying the product as well. This includes removing any staining or hard water damage.
Rain-ex and waxes will work for a week or two, but don't last very long. Our product last anywhere from 1-3 years.