I was using this technique long before I learnt the name of it. Sgraffito is a drawing technique where a sharp tool is used scratch into a surface to produce drawings with fine lines.
Sometimes sgraffito is used as a painting technique as well and this is how I have used it. For example you could be painting a portrait and use sgraffito to add some precise detail to the hair. You could apply for example a light layer of paint on a dark ground/background then scratch into the wet paint.
Of course you can’t just scratch into any surface, watercolours for example can’t really be scratched into unless they are used thickly. Sgraffito lends itself best to oil pastel and oil paint. You can scratch into acrylics but the quick drying times can make this challenging.
Sometimes painters deliberately use thick paint by mixing in a thickening medium (impasto) so that there is more to scratch into. This creates a textural quality and a kind of relief effect.
I like sgraffito because it is a simple technique that isn’t difficult for beginners to learn. I have taught it in art lessons to young children by using oil pastels and then scratching out with a pen nib. For a more interesting result you can use different implements to scratch into the surface; I’ve used palette knives, scalpels and a plastic picnic knife. These implements all produced varied lines and interesting results.
Support means the surface so for example watercolour paper is a support, so is mount-board. The surface also makes a difference to the type of marks that can be achieved and it is worth experimenting with supports.
I like to use board because it is a bit more robust and can cope with heavy scratching into. I prime the board with gesso because this makes the surface that is being scratched into more robust and leaves a clean white line. If you are not sure what gesso is see my post What is Gesso? I then add acrylic paint to the gesso if I want colour lines, the acrylic paint dries and does not shift when it is completely dry. This layer of acrylic works well when you add an oil based pastel or paint and scratch into the surface, it leaves a clean coloured line.