The main thing to remember when doing sgraffito with oils or acrylics is that the colour you want to show through must be totally dry before you apply the layer of paint you’re going to scratch away. Otherwise you’ll scratch off both layers.
When initial colour has dried, apply the colour you’re going to scratch through. The top layer of paint should not be runny, otherwise it’ll just run back into the areas you’ve scratched. Either use the paint quite thick, so it holds its form, or let it dry a little before you scratch into it.
Sgraffito is particularly effective with impasto painting, providing another level of texture as well as the contrasting colour. If you like having text on a painting, you should try using sgraffito – you may well find it easier than trying to paint on words.