Can this be used between applying layers of makeup or just at the end?

I have used this both between layers and just on top and because you have to shake it, it tells me why it separates on my face, just like how it eventually separates on my face. I use it with their new foundation as well AND with their new Matte porefessional primer and sometimes it still separates. You'll need to touch up with some powder during the day if you sweat at all.