It is so important to stretch your watercolor paper so it will lay flat when the piece is finished - unless you are using the thickest grade of paper. Water has a tendency to expand paper. When you add a layer of paint to waterlogged paper, the paint will prevent it from returning to its original size and it will buckle or ripple the surface unless it has already been stretched.
The "Wrapped Board" Method: There are many ways to stretch watercolor paper but I have for several years used a method which is an adaptation of Gennady Spirin's technique (my favorite illustrator.) This is different from the standard stretch and tape method, which I think produces mixed results.
You will need: Watercolor paper, backing boards, metal clips, large tub of warm water
- Identify the right side of your paper. Mark the right side with an X or, even better, lightly write on the edge what brand and type of paper it is. (Such as Lana Cold Press.) You can tell which side is the right one by looking at the manufacturer's watermark. You should be able to read the watermark from the right side. Also, if you like to tear your paper into halves or quarters, mark each corner before you tear. It will be difficult to tell later just by looking which is the right side.
- Soak the new sheet of paper in a tub of clean, warm water for ten minutes. Make sure the tub is clean and big enough so your paper doesn't fold or crinkle. I use the bathtub with 2 inches of water in it. Stretch several pieces at one time so there is always working paper available.
- When you lift the waterlogged paper out, hold it by one or two adjacent corners to let excess water drip off it. (Don't touch the surface with your hands or try to brush the water off. The oil from your fingers will inhibit paint from adhering later.) Hold it up until the water isn't dripping any more.
- Take your backing board and lay your paper onto it, right side up. (For backing boards, I use plates of heavyweight - 1/4 inch- plexiglass, smoothed around the edges. Water doesn't soak into it like wood and you can hold it up to the light like a lightbox) Your paper should hang over the edges at least 1/2 inch on each side. You can pull the paper slightly at the edges to position it but, again, don't rub or smooth it with your hands.
- Using small .75 inch clips, fold the paper around the backing board and clip it in place every two to three inches. This will prevent the paper from moving as it dries and shrinks.
- Let it dry completely for several hours. Overnight or for a full day is best. When it is completely dry, you can remove the clips if you wish. The paper will remain curled around the backing board by itself. Then you can paint on it as is.
- Protect the paper. When your stretched paper is completely dry, slide both board and paper into a plastic sleeve or clip a sheet of clean paper over the front of the wrapped board to protect it when not in use. If you are wrapping several boards for later use, it is important to protect them from dust and dirt. Warning: the paper must be completely dry before covering or, if you've been painting on it, make sure the paint is dry.
- When you are done with your painting, remove the finished piece from the backing board by slicing it around the front edge with an exacto-knife. You will see marks where the clips have pressed into the paper around the edges. This is not a problem if you will be matting your finished piece. If you don't want to see these impressions, cut them off. Take this waste into account when calculating the size of paper to stretch. You will have to have a large border around your painting with this method.