The heavier the watercolor paper, the less it tends to warp. Basically very wet washes cause the watercolor fibers to try and snap back to when the factory rolled the paper fibers into sheets because the water seeps through the sizing into the fibers below. That is why some artists prepare the watercolor paper by stretching. This involves soaking the sheets of paper, but not too wet, then taping or stapling the paper on a watercolor board, or stretching on stretcher bars similiar to a canvas. Taping on a watercolor board or plexiglass is the most common because sometimes tacks or staples tear the paper as the paper shrinks or dries. The stretched method on stretcher bars is similiar to needlepoint. Sometimes an artist will use a spray on the back of the paper and the surface of the watercolor board to try and glue the paper on the board with water. The artist works quickly with the wash before the back of the paper dries, then allow the painting to dry completely flat between each application. I am lazy so I just buy watercolor blocks although some artists dislike these tablets because the artists think the tablets are not sized as well as individual sheets. I should also mention a fellow artist has been known to iron her finished watercolor work on the back with a steam iron to try and make it flatter...works for her but I have never dared try it.
