This is the technique I use to mount/stretch watercolor paper to board, in this case, to be used in laser-etching and cutting (a process in a later instructable). You can also use this to mount watercolor paper to any other flat, relatively porous surfaces (books, sculptures, walls, etc).
It might help you to know a little bit about how watercolor paper is made and it's characteristics. I find it fascinating. Here are a few links about that:
Step 1: Gather Materials
I'm going to be mounting watercolor paper onto plywood to be laser cut. You can use this same technique to mount watercolor on any porous, solid surface.
140# watercolor paper (can be heavier or thinner) in this case I'm using Arches.
Plywood (i'm using 1/4")
Acrylic matte medium: the best glue for this job. Is water soluble when wet, water resistant when dry, acid free, etc. Modge Podge is the same stuff.
Large brush (or paper towels)
Step 2: Spray Down Your Watercolor Paper
Lay your watercolor paper on top of the plywood and, using your spray bottle, give the paper a good even spritz all over. You're looking for the point where the surface is evenly covered with beads of water without being sopping wet.
Using the brush or a paper towel, rub down the paper to distribute the water evenly.
Step 3: Evenly Distribute Matte Medium
Squeeze on matte medium over the watercolor paper. First drizzle it on over the whole paper, then use your brush or paper towel to distribute the medium to a nice, even coating.
Step 4: Attach Paper to Board
Flip the sheet over and, to the best of your ability, align it square to the wood at the edges before laying it down. It doesn't need to be AT the edge, just square to it- especially if you're going to be laser cutting. If you're not laser cutting, just lay it down wherever you want.
Smooth out LARGE wrinkles, but do not worry about even large looking air bubbles.
Watercolor paper will expand when wet, and then shrink when dry.The matte medium you applied will adhere the paper to the board.So even when it's got large-looking air bubbles, when it dries, it will shrink the paper, stretching it flat to the board. Look at the photo above- super large looking air bubbles. Looks like it's ruined! But no, it will stretch flat.
If the edges are not adhering to the board, apply more matte medium (mixed with a little water) just at the edges until they lay down flat.
Step 5: Dry!
leave your paper and board out overnight to dry, or in some flat files if you have them. Voila! Nice, dry, flat stretched paper. See how all the bubbles disappeared after it dried? Not unlike shrink wrap.
In this case, my plywood was nice and flat afterword. If your board is not flat, flip it over and use the spray bottle to spritz the back of the wood in the concave areas- this will stretch the wood out when it soaks in the water. You can then just let it dry or use weights to flatten.