I'm currently detailing my Mom's car. Eventually her car isn't one of the most well kept cars. Eventually after detailing her car, I noticed that i couldn't get the front windshield super clean. It seems to have like oxidzation or crud on it. You can notice it when you sprinkle water on it, its not smooth. I tried using 20/20 glass cleaner on it and using claybar on it. It still seems to leave those stain. Tried adding some Rainx to see if it will help out still same results.