When using collage paper in your cold wax/oil paintings, use the thinnest paper possible. Examples are rice paper, magazine weight paper, tissue paper, and printer paper. I would not recommend card stock or any hefty paper; the only thing holding your collage paper in is the cold wax/medium and oils. When you do incorporate collage paper, keep an eye on any edges that may pop up the next day. Apply a bit more cold wax medium to make sure the piece of collage paper stays in place.