When you paint on paper, it goes wrinkly. There is a proper way to avoid this when painting watercolours: Before painting, you soak your paper in water, which stretches it, and stick it to your board with gummed tape. In the morning, your paper will have shrunk tight and flat, and you can now paint your painting.
The problem is the time the stretched paper takes to dry. You can't finish one painting, go and trace the next and start painting it immediately.
So I have developed a system which seems to work (The management takes no responsibility for damage to paintings or any other unforseen circumstances :-) )
- Trace your picture to your paper. For a large painting with lots of wet washes, you'll need something like 425gsm paper. A smaller or drier one can be done on 300gsm, which is rather easier to trace onto.
- Stick your paper to the board with masking tape. This will keep it flattish. Enough to paint on.
- Paint your picture.
- Damp the back of the painting with a sponge. Make sure no water gets on the front. And don't over-wet it, esp with thinner paper.
- Place it on one board, and put another board on top. You could pile up several, with paintings in between. Put big heavy books on top and press for a while until flat.
- Tape paintings to board with gummed paper tape and leave to dry.
- Cut off board - one perfectly stretched painting!
You may be able to get away without step 5. In previous projects I didn't use it, but it wasn't working this time. Thanks to my dad for suggesting it, as I was in a slight panic.