Preparing watercolour paper by pre-stretching it is vital element of painting in watercolours and learning how to do this will provide you an absolutely flat surface to create your painting on. With untreated watercolour paper, the surface can become uneven as it becomes saturated with water. Warping can occur with any watercolour paper which has a mass lower than 300gsm, dependent upon the amount water which is absorbed by the paper when the painting is being created. With the technique of stretching the watercolour paper, an application of watercolour paint will dry with the treated paper remaining flat and even. Additionally there is much less chance of wet watercolour paint moving onto unwanted areas of the paper.

There are several methods of stretching watercolour paper, but the least complicated one involves immersing the paper in a sink which has been filled with lukewarm water. The paper can be submerged - being careful not to crease it - by dipping it in the water and letting it soak for approximately ten minutes. To remove the paper by hold it by one of the corners and support the rest of it while the water drips off. Then lay paper on flat a surface, making sure it is perfectly smooth and use a sponge to soak up any excess water. The paper can then be gently stretched by flipping it over a few times on to the surface.

Another more advanced method involves the use of stretcher bars which are more commonly used for stretching canvas. This method is far more efficient but is also more complicated and time consuming. The stretcher bars should be the correct size to be able to create a frame which is two or three inches smaller than the watercolour paper on all sides and the paper should be 4 to 6 inches larger than the area you wish to paint on.

First of all, connect the stretcher bars together to make a frame and make sure it is perfectly square. Soak the paper in a sink of warm water for about ten minutes and then blot off the excess water with a sponge. Place the damp paper over the stretcher bar with all 4 sides overlapping evenly. The next step is to staple the paper to one of the edges of the stretcher bars. Once fixed to one side, the paper can be stapled to the other sides of the stretcher bars. Lay the stretcher frame flat as the paper dries and when it has dried fully, the staples can be taken out and the paper removed from stretcher frame. The dry paper will then have to be trimmed as it will be creased where it wrapped around the frame.

One further technique is to use water soluble glue which is gum, starch or wheat based. This method consists of soaking the watercolour paper and blotting the excess water as before. After the paper has been placed on a flat surface the glue can be applied along the inside edge of a mounting board approximately 3 inches wide. Mark some borders on the paper which are also 3 inches from the edge. Next place the paper on the surface of the mounting board, being careful to keep it within the marked borders. Then allow the paper to become dry whilst the board is laid flat. The watercolour painting can then be done on the paper - within the borders - while it is attached to the board, but the area which is attached with the glue should not be painted on. After the painting is finished and dried it can be cut off of the board.

Another similar technique than above can be achieved with the used of 'butchers tape' and a flat mounting board. The damp paper is attached to the board with the tape instead of glue.

Preparing your watercolour paper before painting can help you create artwork which has a professional appearance and create a pleasing final painting. There are simple and more complicated techniques which can be used to prepare the paper and different techniques will produce different end results. If you are not sure which method to choose why not try more than one and select the one which suites you the best.