On the ones I have put a backer on, so far I have laid the picture down, face down, sprayed the back of the picture with 3M 77 adhesive and laid the backer on. I use flat black poster board. So far it has worked just fine, however a person could get spray adhesive in the cutouts. I have not noticed it so far, but I've been watching for it. I keep a jar with mineral spirits in it, some pipe cleaners close by to clean it up if needed. With poster board it would have to be cleaned before the poster board is put on, I would think. Not sure what the MS would do to poster board. One of these days I will test it.