Either/or. It's personal preference but I personally put this on before my foundation and than use foundation on my face to cover up any fallout from my eyeshadow. So I usually do eyes first and than face
Personally, I would put it on after the foundation ! If you put the foundation on after the primer, the eyeshadow won't be sticking to the primer but it'll be sticking to the foundation, therefore it won't last as long as it really could
I personally put on my foundation first and then use the primer potion. I don't think you'd want to apply this to your eyelids and then put foundation on over it since it would kind of defeat the purpose as an eyeshadow primer.