Before painting, some watercolour painters soak or stretch their paper. To prevent the paper from curling when wet media is applied to the surface, this is typically done on lighter-weight watercolour papers.
Stretching is a simple technique for producing a smooth painting surface for watercolour and gouache that involves soaking the paper, taping it down wet, then shrinking it only until the tape will allow it to move when additional layers of water are applied as it dries.