Tip for watercolour, acrylic or gouache: STRETCHING

The idea is to prepare the sheet or paper so that it cockles as little as possible during paint application and once dry returns to its original planeness.
Papers absorb humidity to varying degrees. It depends on the nature of the fibres, on their makeup, their thickness and the surface treatment.
The stretching technique used will depend on the following: a thin, poorly sized paper will deform rapidly while a well sized card will react inversely.
Hence the importance of learning to recognise the different papers and when they are ready for use.
There is no real rule of thumb and the best way to proceed will become evident with experience. It changes depending on the paper, the climate and even the seasons.
Before beginning, take a thick wooden board and 3cm-wide bands of adhesive paper.
It is best to use adhesive paper which should be lightly dampened on both sides rather than glue which sticks less well to damp paper.
Several methods can be used for wetting the paper. It can be totally plunged into a container, dampened with a sponge or a spray gun.
There is no universal method: there is no point in totally immersing very lightweight paper as it will only weaken it and make it difficult to manipulate. This technique should be reserved for heavier papers.
The easiest way to master the humidification process is to work with a large soft sponge which will not alter the paper’s surface.
Wet each side of the paper taking care to spread the water out evenly.
Some patience is required and a few seconds should be allowed before wetting the other side. After a few moments you will see that the paper is starting to absorb the water – do not wet it too much –as soon as the water shines on the surface, it has had enough.

After a few minutes you will notice that the paper has softened. Ensure that this is uniform and that no corner, for example, has stayed dry. If the paper is soft but not too wet, you will notice that it is matte in certain places and that it shines in other places where the water has not been absorbed. If this is the case, delicately wipe the paper’s surface to get rid of the surplus water.
The paper is wet, flat and plane. The next stage is to attach the four sides using the adhesive bands which cover the edges by ½ cm. The must be stuck carefully because the tension of the water will be extremely strong.
The sheet should be totally dry before starting to paint.
Once the work is completed it must be totally dry before removing the adhesive paper.
Note: Sometimes this is incorrectly called priming. True priming uses glue which is completely inappropriate for watercolour as the glue alters the chemical composition of the paper and would ruin the watercolour.