Nice comment from facebook:
Meredith McPherson The rough and smooth sides of raw single xuan can be subtle in their differences. I like to write calligraphy on the smooth side since it allows the brush to move faster and more freely. Wet mounting is basically pasting two pieces of paper together. From a physical perspective, paste will adhere better to a rough texture than to a smooth surface. For that reason alone, I apply wet mounting paste to the back rough side of the painting and adhere the wet mounting paper’s rough side next to the painting’s rough side. That is … rough side to rough side for best adherence. Any other ideas on this?