Danny: hard to explain in text, but hear goes.
When you do a portrait cutting it's full of holes (yeh you knew that), then you paint a backer board.
You don't want to spray glue all over the painted backer, cause in the place's where there is cut out you wood sea the paint plus the glue. (that make sense).
So instead I spray the back of the portrait, that way you place the portrait on the backer that is painted the glue is no going to show on the backer only on the parts that are left on the portrait.
(Hell I think I may have over complicated that).
The same goes for when place coloured paper behind the portrait.
***Merlin***