500 PSI Mask Defog is without question the clearest, longest acting and most effective defog on the market. 

One application works for at least three dives in one day. 

Cleans your mask each time you use it. 

The bluish gel is similar to toothpaste and is designed to be applied either wet or dry.

It's thick stuff and takes some rubbing to work it in, so you have to be liberal in how much you use. 

The bottle has a rocker-type cap—you won't have to unscrew it, only to lose it overboard. Apply with fingers only, then give it a good rinse. No eye sting.