Patience needed. When I started painting in gouache about two years ago i had so much problems with stretching the paper. I tried my best but most of the times it looked like this (picture above) when the paper started to dry. This results in crappy looking artwork when the paper turns all bubbly and ugly. At the same time I saw an exhibition with original Tove Jansson illustrations, and was secretly happy to notice that SHE HAD THE SAME PROBLEM. Ha! She was a genius, but still she couldn´t make the paper obey her every time.
I found different tutorials in books and on the internet, and by combining these, asking in the art shop and trying out different methods, I can finally stretch a paper (almost every time). There´s no fast track, I´m sorry to report. And even if you do exactly as described below, sometimes it just doesn´t work anyway. It must have something to do with the percentage of moisture in the air when the paper dries, or on the evil bubbly paper-god who plays tricks on us. So don´t blame me. Here we go:
Step 1. Be neat. Clean hands, clean table... no chocolate crumbs or pizza stains anywhere in your workspace! If someone walks past your table with a doughnut, there will be little doughnut-shaped grease marks on your paper, I promise. Cut your paper in the size you want. Draw in pencil the outlines of your painting, I use a 4 cm marigin which is more than enough for the tape and a white marigin around the painting. (I used to paint to the borders of the tape but I noticed that then the tape gets soaked again which makes it loosen easily). I sketch on cheap sketching paper, and then draw very thin outlines on this final one before I soak the paper. I try not to make mistakes because every eraser movement makes the surface of the paper a little broken.
Step 2: Soak the paper in water. Use clean, new water in a basin that is big enough for your paper. Make sure there´s water on all sides (air pockets under the paper disappear if you lift it and soak it again). Don´t use hot water. I soak this bamboo/rag paper for 20 minutes (different time for different papers). Sometimes it´s difficult to wait that time, but if you don´t, the paper behaves strangely later. Soaking makes the fibers in the paper expand, meaning the paper is a little bit bigger now.
Step 3. Lift the paper, let the excess water drop into the basin. Place it on a strong drawing board. It has to be thick enough so the drying paper doesn´t make it curl up. Paper is strong, believe me. The bigger the paper the stronger the pressure on the tape.
Step 4. Smooth the paper gently with your hands or with a special watercolor sponge, it has to be perfectly flat on the board. Use a CLEAN cloth or rag (or like here, toilet paper...) to dry off all extra water. If you look at the paper from the side you see the wet spots. Now let the paper be for a few minutes, while you cut the gummed brown tape. Cut it slightly longer than the sides. Take your time, read a newspaper or work on something else for at least fifteen minutes. Letting the paper rest is important, because it shrinks a bit, making the job a bit easier for the tape.
Step 5. Use an old brush (not the one you use for painting) to moisten the glue side of the tape with water. Don´t use too much water, it will make the glue less strong. DON`T SOAK THE TAPE, if you do that, the tape will be useless.
Step 6. Glue the paper to the board with the tape, one side at a time. Glue opposite sides first. (Here: first front, then back, then right, then left.) Smooth if bubbles appear. One third of the tape should be on the paper, the rest on the board.
Step 7. Now you are done, and should leave the paper to dry for several hours. But, you can also tint the painting area while the paper is still wet, this way you get an even surface. Mix a light color containing two or three different colors but not black or white. Here I have mixed some ochre, green and some sepia, and lots of water. Mix enough color for the whole surface at once. Hold the board a bit tilted and start painting from the top, from side to side in even strokes. An hour later or so, when the surface has dried a little, you can add colour to the parts where you don´t mind if it blends a bit. DON´T lift the board up for the night or something else stupid, it has to dry flat to dry evenly.
There, done. This is the tutorial I tried to find some time ago, so I hope it helps someone!
Edit: An easy way that works. Take a dry paper, place it on a board of plexiglass or on a table surface that doesn´t absorbe water. Tape it to the surface on all four sides with transparent packaging tape. Paint, let it dry, remove the tape. Works, but creates a bit of rubbish...