How to stretch watercolour paper

Have you ever tried laying a background watercolour wash only to find that the paper buckles up with the water, creating pools of water and pigment in some areas, and peaks in others? Do you find that your paper doesn't sit flat even once all the paint has dried? Paper expands when it gets wet, and shrinks as it dries, but stretching watercolour paper prior to use can fix this problem.

Have you tried stretching watercolour paper in the past, only to find that your stretching method (or the one they tell you in the books) didn't work? Perhaps try the method I describe here - it's worked for me!

It's high time I got back to doing some watercolour paintings of wildlife, and this time around I'd like my paper to be completely predictable and stay flat when I lay a background wash! The watercolour medium is random and unpredictable enough without adding warped paper into the mix. Framers don't like warped paper either - it makes their job a lot more difficult!

So I decided to start from scratch and create three new watercolour backing boards (these are wooden boards that I stretch my sheets of watercolour paper on, and that act as a back support when I'm painting). I cut three boards 54 x 40 cm (approx. 21 x 16 inches) from 12 mm plywood.

It was important that the dimensions of the boards were at least 5-10 cm (2-4 inches) wider and longer than the dimensions of the watercolour paper that I wished to stretch upon them (in this case, A3 sized Arches hot-pressed smooth 100% cotton rag watercolour paper, which is my favourite watercolour paper).

After cutting the boards I carefully sanded the edges and corners to round them, and then added three coats of oil-based polyurethane varnish (waiting approximately 12 hours between each coat, and sanding in between coats as well; see Fig. 1). The varnish seals and protects the wood and prevents surface water from raising the grain (and eventually ruining the board), it also helps prevent the board from bowing with time.

Now that the boards were dry, it was time to stretch the watercolour paper. I marked the back of each sheet with a pencil (lightly inscribing 'wrong side') so it would be immediately clear which side to face down onto the boards even when the paper was wet.

I use my bathtub to wet the paper (see Fig. 2), because I have no other containers of sufficient size to fit one or more sheets of A3 paper. It is important to carefully clean the tub or container, and rinse it thoroughly to ensure no soap residue remains, otherwise it may affect paint uptake when painting down the track. I just use normal tap water, and set a timer for 15 minutes once the paper goes in. The paper should be completely covered by the water, however it will likely float, and that is okay so long as it has been completely immersed. I also place the paper into the tub wrong side up just to ensure no dust falls on the painting surface during immersion.

While I'm waiting for the paper to dampen in the tub, I collect the materials I will need for stretching the paper. In particular, since I varnished my plywood boards, it was important to then 'scuff' the edges using very rough sandpaper, in order to create a rough surface that the gummed tape can stick to. Without scuffing these varnished edges, the tape would likely peel off the board surface, providing no strength for stretching the paper. In Fig. 3 you can see the scuffed edge.

The materials I use for stretching include the backing boards, a roll of paper kitchen towel (for wiping my hands to rid them of water and glue), wide brown gummed watercolour stretching tape (that can be picked up from any art store), a pair of scissors and a bowl of clean water. I then cut the gummed tape to fit the length and width of my boards (making sure there is no overlap from too long tape curving around the board edges as this can cause the tape to peel off prematurely as well). You can see the materials I use including the cut pieces of gummed tape in Fig. 4.

Once 15 minutes has elapsed since I began to immerse my watercolour paper in the bath tub, I remove the paper from the water. It is important not to leave the paper in the water for too long, because that will lead to the sizing binder (used to 'glue' the paper cotton fibres together) dissolving in the water, and the paper itself becoming too absorbent when you go to paint on it later after it has been stretched and dried. I drain the paper by holding it by one corner, and waiting until the steady stream of water falling from the opposite becomes a slow drip (see Fig. 5).

I then take the paper carefully out to where I've set up the boards and tape, and lay the paper gently on the center of the board, attempting to line up the edges of the paper to be equidistant from the edges of the board and sitting square the whole way around (see Fig. 6). At this stage you should also check to ensure there are no air bubbles or pockets between the paper and the board.

Now it's time to apply the gummed tape. This tape has a water-activated glue (or 'gum') applied to one side of brown paper, so I use the bowl of water to pass the tape through, pulling the tape through the water along its length (one hand pulls the tape, the other pushes it into the water; Fig. 7). The tape is too waterlogged at this stage to apply to the paper, however, so to remove some of the water, I run my fingers close together one either side down the length of the tape. I only do this once for each piece of tape (more than that and you will remove too much of the glue).

Then I lay the tape (glue side down) on the edge of the watercolour paper, sticking it to the board, with approximately 1-2 cm (1/2 to 1 inch) overlap over the paper itself (see Fig. 8). I apply the tape to all four sides of the paper in quick succession, ensuring the paper and tape haven't dried in the meantime. Note how none of the tape curves around the edges of the board - this is important! Curves in the tape lead to the tape not sticking, and/or peeling off down the track.

Be careful to dry your hands in between applying each piece of tape and ensure that no drips of water from the tape fall on the watercolour paper - these drips contain the dissolved glue, and will prevent paint adhering to the paper properly. Also be careful not to get any drips of water on your roll of gummed tape - since the glue is activated by water, any drips can stick the tape together in the roll, rendering the whole roll useless. This is also why I always store my tape in a zip-lock bag (to prevent humidity from ruining it by activating the glue).

Next it is important to ensure that the tape dries as fast, if not faster than the paper being stretched itself. This is crucial because wet tape doesn't provide any resistance against the pull of the drying paper. For this reason I use a piece of dry paper kitchen towel to gently dab away excess water from the back of the tape (see Fig. 9). Do not use the same piece of towel to dab the watercolour paper dry (the glue will ruin your paper surface).

Once any surface water is removed, I place the board with the paper and gummed tape on a slight vertical slope to allow it to dry (usually outside is best). I place it paper-side down to prevent dust, insects and stray animals from touching the paper surface inadvertently (see Fig. 10). Depending on the humidity and temperature of the environment, I rotate the board (turn it from resting on one edge to the adjacent edge) once every 20 minutes to allow for even drying (water tends to move down the paper, and the lowest piece of gummed tape may not dry before the paper if you're not careful). Once the paper is dry, it is ready to be used for watercolour painting!