I've never had the scope-fog problem because I don't use a scope on my shotguns, but I really have it with my glasses. I have used several types of facepaint, and have found all of them to be lacking in some regard.
The biggest problem though is that they are almost all 1) smear off when you don't want them to,...and 2) are hard to remove when you really do want to get them off.
...I think that has something to do with Murphy's Law,....
There used to be a product a few years back that was kind-of like a stick-on facepaint that you could stick on your face and then peel it off. I never used it so I don't know how well it worked....and I'm not sure it is even still around.
I have enough problems with the fogging eye-glasses when I wear a facemask, though, that I would like something better,...or some good suggestions on how to resolve that issue effectively.