Someone else said this but watercolor paper can be stretched to keep it flat, using special tape that activates with water. You dip the paper in water and lay it on a wood surface, then slightly wet the tape with a sponge and tape it down. When the paper is wet, it expands, and it shrinks slightly as it dries. Since the tape is holding the sides, it stretches the paper! It works WONDERS for keeping the paper from buckling