Most defog is just a soap film, and many divers including myself just use baby shampoo diluted in water. Store-bought defog is expensive and doesn't really work any better.

However, when diving you leave your defog wet (hence the baby shampoo so your eyes don't get irritated) after a quick dunk in water to rise off the excess. A coating that dries will be very different, although I am sure they sell it somewhere!

I've heard of people using Rain-X on their masks, but never tried that before. Maybe it'll work on glasses too!