Way back when I first used Rain-X, One of the directions was not to get it on the paint.
Their FAQ has this:
Quote:
If you get Rain‑X® Original Glass Treatment on your paint, will it ruin the paint?
No, but you should rinse the product off with soap and water.
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So while it's completely awesome on your windshield, apparently it's not the best thing for your paint. I have no personal need to Rain-X my whole car, so the why doesn't concern me. I just don't use their washer fluid, doing it by hand every few months.