Stretching one's own canvas is common for the creation process of oil and acrylic art. Most watercolorists stretch their paper before painting, also. The process makes the fibers of the paper expand, helping to avoid buckling and warping. It's much more enjoyable to paint on a flat surface and be able to use as much water as you want. Also, it's easier to paint a flat wash if the paper is stretched
Watercolor paper comes in three different weights: 90lb, 140lb and 300lb. This number actually corresponds to the weight of 500 sheets of paper. The higher the number, the thicker the paper. Watercolor paper with a 300lb weight is almost like cardboard, so it doesn't require stretching before painting, but it's also more expensive and will take longer to dry. Paper that's 90lb or 140lb will need to be stretched.
How to stretch watercolor paper:
The easiest way to stretch watercolor paper is to soak it in a clean bathtub. To begin, place the paper in cold water for 5 to 10 minutes.
Once the paper has been submerged for a suitable length of time, take it out of the water.
You ideally want your paper softened but not totally floppy. A good way to determine if your paper has stayed in the water long enough is to gently bend a corner -- if it falls down, the paper stayed a bit too long in the water, if it comes back to its position, the paper has not been in the water long enough, and if it stays where it is, as seen above, you've reached the perfect soaking time.
Note: If the paper stayed too long in the water, you can still stretch it and it will eventually dry, but you might have lost most of the sizing and the paint may get absorbed by the paper very quickly. This is the same reason why it's better not to use hot water, as it will also dissolve the sizing.
A stretcher board like this one will hold your paper on the board; you can keep your paper on the board during the painting process.
Once the paper is soaked, hold the paper by two corners, lift it out of the water and onto your stretching support. The most common supports are stretching boards (seen above), gator boards (which are kind of a thicker foam board), and medium-density fibreboard (MDF). This support needs to be acid-free and solid enough to stay flat when the paper dries and shrinks.
The paper should cling to the stretching support. Remove any excess water by smoothing it with your hands.
Stapling the paper to the board starting by the center of each side.
Some boards have a system to hold the paper. On others, you may have to staple or tape the paper so it stays in place.
Personally, I never had much luck with the tape method, so I prefer to use a stapler. Start stapling the paper to the board, starting at the by the center of each side and working toward the corners, separating the staples by a few inches.
Note: You will loose some space on the sides of the paper, so when planning your painting, allow for a few extra inches around it.
Let you paper dry in an horizontal position so the water is equally distributed. After a few hours, it should be drum tight and ready for your next painting.
Stretched paper won't buckle even if lots of water is used
You might also enjoy our posts Painting Effortless Watercolor Washes. See also our list of 6 supplies you will need to start Watercolor painting