Gouache (pronounced "gwash," to rhyme with squash) is a form of water-based paint. True gouache is made by mixing pigments with gum Arabic. Unlike transparent watercolors, gouache is opaque. It dries quickly with a matte surface. It is used often used by designers. It is useful for scratch printing as it is water soluble: it can be allowed to dry and then reactivated with water.
In this technique, a drawing is made on a piece of stiff card. This is then covered with thick, smooth layers of glossy acrylic medium -- like acrylic paint, but transparent.The acrylic medium must be free of brushstrokes as these will interfere with the printing process; a sponge brush may be used instead of a bristle brush to get a smoother finish. Once the medium has dried completely, the drawing underneath is scratched into the surface using a sharp knife. Plate tint (shaded areas) can be created by painting some areas of the image with matte medium. The finished piece is called a scratch plate.
Instead of card and acrylic medium, a sheet of transparent rigid plastic (Plexiglas, or Perspex) can be used as a scratch plate. The original image is placed underneath the plastic and copied onto the surface using a sharp knife. The image will be reversed when printed; if this is undesirable, it can be drawn on tracing paper and laid upside down underneath the plastic. Plate tint can be created by roughening the surface a little with sandpaper.
Once the scratch plate is finished, it is covered with gouache. The paint must be worked well into the scratches and wiped off the rest of the plate. The plate is then laid face up. Damp paper is laid on top and rubbed firmly all over with a roller or the back of a large spoon. The moisture from the paper reactivates the gouache, which is then transferred onto the paper by the pressure from the block or spoon. Progress can be checked by carefully lifting up a corner of the paper. If the image has not transferred completely, the paper must be replaced and rubbed or rolled some more.