I've tried all methods for stretching watercolour paper. There is nothing worse than creating a lovely work of water colour art on a paper that is warping and buckling. When a watercolour paper is properly stretched it no longer warps or buckles. It stays perfectly flat - a wonder to behold! So I'm going to share my most favourite way of flattening watercolour paper.
What you will need to flatten paper in my favourite way:
1. Grafix Incredible Art Board (it's a special light weight foamboard - usually a little on the pricey side but I love it)
2. Staple gun - if you don't have a staple gun you can substitute with thumb tacks.
3. A tray that can be filled with water and big enough to fit the paper size you want to stretch.
4. Staple pulling tool.
Soak paper for 5 -10 minutes, it should become quite flexible and don't leave it in too long as that can affect the paper in a way that it won't accept paint properly. Use a timer if you have to.
Take paper out of the tray and place on the board. Blott off excess water from the surface and then apply staples.
Staples should be applied about 1/2" in from the edge of the paper.
They should be approx. 1 1/2" apart from each other.
Lay it flat to dry.
Use a staple removing tool to remove the staples once dry. Paper should be flat and ready to accept washes of paint without buckling.