If you have a screw-back watch that won’t open, get one of these plastic balls. They actually work, and they will not scratch the watch case.
I own the Pittsburgh watch case opener, and it will usually open watches. However, I have one favorite watch that defied the Pittsburgh tool. I read about the Watch Case Ball, and bought one online. Within five minutes of picking up the mail, the watch was open. (I had to inflate the ball; it’s shipped flat.)
The ball is made of plastic. It’s not actually sticky, but when you press it onto a watch back, it conforms to the surface and won’t readily slip on it. Now I go to this tool first, because there’s no chance of marring the watch case with it, and it works on all screw-back watches with no set-up at all.