This article is a guide to stretching paper for watercolour painting, using the traditional 'gummed tape method'. If you are using paper 356 gsm (260 lb) or less, it is normally considered essential to do this, otherwise the paper is likely to cockle after you paint on it. As an alternative to this method, you can buy commercial paper stretchers, that work in much the same way. However, two advantages the commercial paper stretchers have, is that they don't take as much getting used to as the gummed tape method, and they allow you to dry the paper with a hair drier, meaning the whole stretching and drying process could take minutes instead of hours. But for this article, we will use the gummed tape method.
Leave the paper to soak in a tray of cold water for one minute. This causes the paper fibres to expand. Don't use hot water or soak the paper for too long, as this can cause the paper to lose some of its sizing. It is important that the paper is equally wet, front and back, and is evenly soaked throughout.
Lift the paper out the water, shaking off any excess, and place on a towel. Gently pat the paper with a towel to finish removing excess water. Don't over dry it. We need to keep the paper damp for now.
Place the damp paper on a clean wooden board, at least 12mm thick. Thin board, such as hard-board, may be too weak to keep straight during the stretching process.
Gently rub a sponge over the paper to smooth it over, and remove further excess patches of water.
Stick a moistened strip of gummed tape along each edge of the paper. Try to use tape at least a couple of inches wide, so that it can withstand the stretching process. For very large sheets of paper, over 18 inches square, at least 3 inches wide would be better. Another thing to note: Keep the reel of tape dry. If the reel gets wet, it becomes useless, and has to be thrown away. Use a different sponge for moistening the strips of tape, to the one you use to smooth the paper over. You don't want to be smoothing over sheets of watercolour paper with a sponge that has glue in it. Also, don't over moisten the tape, or you could remove the gum.
Have roughly a third of the tapes width on the paper, and two thirds holding it to the board. Each of the four strips should be at least a couple of inches longer than the edge of the paper they are holding. Wipe off any excess water and gum from the edges of the paper, and run your finger along the edge, to ensure the paper is properly held down along each edge, and that any air bubbles are squashed flat.
As an additional measure, you may want to stick some drawing pins along the edge of the paper, where the tape overlaps the papers edge. This helps the tape hold on to the paper, and prevents the paper pulling away from the tape as it dries.
Leave in normal room conditions to dry overnight, keeping the board lying down flat, and away from sunlight and heat sources such as radiators. As the water evaporates, the paper will be unable to shrink due to the tape holding it in place, and therefore the paper fibres will stretch, meaning that when you start painting on it with watery watercolours, the paper should retain its flatness.
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