Sgraffito on paper works differently to sgraffito on canvas because the layer of paint is (generally) so thin you’re scratching the paper as well as the paint. Where you scratch or indent the surface of the paper, the wet, top paint will collect in it, rather than revealing the white of the paper. If the paint's beginning to dry, less will flow in.
Using a knife, sharp blade or sandpaper to scratch the surface of a watercolour can be very effective for creating texture, but remember you’ll have ‘damaged’ the surface of the paper and it’ll be very absorbent (porous) if you paint on it again.
If you add a little gum arabic to your watercolours, the paint will have more body and sgraffito marks will be more prominent, or defined.
