When painting on canvas, an artist will almost always have to stretch their canvas or purchase it pre-stretched on a wooden frame. However, for a watercolor artist painting on paper, there are choices.
Small size watercolor pads or “blocks” come with all four sides glued and only a small area unglued to facilitate removing a sheet (finished painting) from the block. In this case, one would not nor need to stretch the paper.
So why stretch our paper at all? With large sheets and large washes (passages of wet watercolor), un-stretched (un-restrained) paper has a tendency to warp or buckle, leaving hills and valleys which influence the flow of paint and eventually the appearance of your painting. Just as important is the impact water has on the paper itself. Watercolor paper is very fibrous. When the paper is web, the fibers in the paper swell up. But if we don't restrain the edges of the paper as it dries, the paper will want to shrink (good paper is 100% cotton). When the edges of the paper are restrained, the spaces between fibers (albeit molecular in size) will stretch and enlarge, making a more “porous” surface to receive your paints for richer smoother washes.
With individual larger format sheets, I always stretch my paper. Some artists, especially when working with the heavier 300 lb weight paper, feel they do not need to stretch the paper. But even with the heavier 300 lb paper, especially if you plan on a lot of large wet washes, stretching is advisable. I always stretch my large sheets, whether they be a full sheet (30 x 22) or a half sheet (22 x 15), cold pressed or hot pressed, 140 lb or 300 lb paper. You may see some artists using pre-glued tape like the brown paper packaging tape or masking tape. I have tried these methods and they can be cantankerous and unreliable at times. I use the sure method, tried and true: staples.
So here is the process:
First, I advise you start out with a product called “Gator Board” as the base for mounting your watercolor paper to. I started out my watercolor career with plywood but it became heavy and hard to manage and remove the staples from. Then I discovered “Gator Board”. It essentially is a specialized foam core board. It comes in two colors, brown or white, is 1/2” thick, and comes typically 31” x 23”, 1” larger than a full sheet of watercolor paper. The surface has been treated to resist the moisture from your wet paper.
The first step is of course to draw or layout your painting with a medium pencil like an H on your watercolor paper. Too hard (light) a lead and you will inscribe or engrave the paper, leaving a channel for the paint to settle in and show off your outlines. Too soft (dark) a lead and you will leave loose graphite on the paper to mix with paint and smear your paper.
Once you have your painting laid out on your watercolor paper, you now must soak it in clean water. Bathtubs work just fine for this. In my studio, I use the bottom half of one of those blue plastic storage bins just large enough to hold my 22” x 15” half sheets. For my full sheets, I use that large white plastic pan you can buy at Menards or Home Depot for putting under your clothes washing machine to catch overflows or leaks. Both of these I lay on top of a pair of saw bucks next to the basement slop sink and fill them from that sink (which is too small for even half sheets) with a sprayer hose. When using the large washing machine pan, it will be challenging to pick it up and empty it. I use a small tube and siphon it empty into the slop sink.
Let your paper soak for a good 30 minutes. Being such a high fiber content the paper is going to have a tendency to float. I use clean PVC pipe fittings placed strategically on the paper to weight it down to keep it submerged. DO NOT use hot or warm water. The paper has sizing in it to help hold the fibers together and hot water will start to dissolve the sizing, something you don’t want to happen.
Remove the paper from the soak, holding it by one corner to allow the remaining surface water to drain off. When that comes to a drip, hold the paper by the left and right edges in a “trough” shape, and carry it over to your waiting Gator Board, and lay it down on top, centering it on the board. Now you will see a lot of surface moisture still present, which we want to remove. So with a paper towel roll, pull off a strip of enough sheets more than the length of the board. You will need at least two lengths of this strip. Lay these paper towel strips over the top of your watercolor paper, overlapping the strips to completely cover your soaked paper. Now carefully with your hands, starting from the center, press on the paper and pull your hands across the paper towel, center to right and center to left to soak up all that extra moisture. Lift the paper towels away.
Now, using a heavy duty stapler with T50 1/2” staples, staple the watercolor paper to the Gator Board all around the edge, about 1/2” to 3/4” in from the edge and about every 2” to 3” on center. DO NOT use an ordinary office stapler. These staples are too light duty and the shrinking action of the paper will pull them right out of the board.
Allow the paper to dry, preferably over night. If the paper still feels cool to the BACK of your hand, then it still has moisture. Now if it is to the stage where it has stretched (the outer edges of the paper on the outside of the staples will be curled up) but still has some moisture (it’s cool to the back of your hand), and your initial under washes (or the entire painting for that matter) for your planned painting are to be wet-in-wet style, then now is a good time to deliver those washes. Otherwise, if your initial plan is for wet-on-dry, then be sure the paper has completely dried.
Have fun stretching!