If you do not stretch your watercolour paper you will usually find that your paper has cockled (the creasing or wrinkling of a surface) or warped during painting to a varying degree and when you try to frame it your painting will not sit flat on its mat. I have a technique I use to fix this which will re-flatten watercolour paintings. I must say at the outset however that you do this at your own risk because if you let any water flow onto the painted surface of your watercolour painting you will ruin it!
Removing watercolour paper wrinkles or cockles
To carry out this technique you will need the following material:
Flat table or bench top
A sheet of mat board cut into two so each half will fully cover the watercolour paintings you would like to flatten.
A sheet of waterproof plywood about 6mm or 1/4″ thick
About 7 strong clamps and or lots of heavy books
A trigger water spray bottle
Here is how to do it:
Lay one of your sheets of mat board on your table or bench top.
Place the watercolour paintings you want to flatten painted side down beside one another with a gap of about 2.5cm or 1″ between each.
With your spray bottle give the back of each piece of watercolour paper a good spray of water, make sure no water runs under the paper where it can damage the actual painted surface.
I then run my hand over the back of each painting to make sure the paper is evenly wet. If need be I can add an additional spray of water.
I then place the second piece of mat board over the paintings and on top of that I place my plywood.
I then clamp all the way around my board and in the middle of the board I also place a stack of heavy books.
I leave all this to dry overnight.
The next day your paintings should be very flat and ready for framing.
Below you can see a short video I have produced on how to flatten cockled or warped watercolour paintings which you can have a look at. It should make all the above steps a little bit clearer. Hopefully this will help improve the overall presentation of your work and give your watercolours a more professional look.