Watercolor paper that's 140 pounds or heavier is an excellent substrate for collages, but it does have a tendency to buckle or curl when subjected to multiple layers of paper and medium. This can make matting a finished piece a bit challenging. Fortunately, there's a simple way to overcome this problem. Here's the method I use to get my non-dimensional collages to lay perfectly flat:
1. Lay a piece of freezer paper down on a clean, flat surface, with the waxy or shiny side facing up.
2. Place your collage face down on the freezer paper.
3. Using a spray bottle, lightly spritz the back of the collage with clean water. (Do not soak your collage!)
4. Cover the collage with a second sheet of freezer paper, with the waxy or shiny side facing down.
5. Place a sheet of plywood on top of the freezer paper, making sure all parts of the collage are covered by the plywood.
6. Set a heavy weight on top of the plywood. (I use a stack of books, but you could also use bricks or a bucket of water.)
7. Leave the whole thing undisturbed overnight, then remove the collage. It will now be nice and flat, and ready to be matted.
For very small collages, 5x7 inches in size or less, it's not necessary to use the plywood. Simply place the stack of books directly on top of the freezer paper.