Rain Guard and its equivalent repellent coatings will do little if anything for external fogging. They will keep oily substances from adhering, making cleaning easier, and they tend to bead water into small droplets that tend to then run to the outside of the lens.
External fogging is an issue of either breathing on the lens, a result of great temperature differences, having the lens too close to your face when you are perspiring, or having a lot of humidity in the air. Use a lower power glass with longer eye relief that keeps the oculars away from you face a little more.
I have heard good things about Cat Crap (lord what a name!). But DO NOT USE RAIN-X. That will destroy optical coatings so fast you won't know what happened.
