I'm someone who never puts any sort of product on my finger nails, and I can confirm that my nails work on my iPhone screen, both when I've lazily forgotten to cut them, letting them get annoyingly long, and if I turn my finger over so that my nail is just about flat on the screen - I checked while writing this, and confirmed the screen was responsive despite my skin definitely not touching it. (I'm a man who doesn't have any experience growing my nails to extend more than a few mm beyond my finger tips so I can't speak to that scenario.)