I've used clear finger nail polish in the past to seal my eyes.
@Shawn, I made a mistake and used clear polish on a foam golf ball. I spray painted it white, painted on the pupil and then covered it in clear nail polish. It cracked! It would be a good effect on a goulish character but I was going for a little boy. I don't know if it would crack on a regular ping-pong ball or not, or maybe it was just the brand, or the combo of the foam/spray paint. Anyway, one must experiment to find what works. That's half the fun!