A large 250ml bottle of Pebeo Drawing Gum. Use this blue peelable latex masking fluid to mask off areas where you don't want colour. It can be used with watercolour paints, inks and gouache.
Apply with a brush, pen or colour shaper. Wait 10-15minutes for it to dry and then apply your paint and watch it resist it.
Once the paint has dried, you can remove this Pebeo Drawing Gum by gently rubbing it off with your finger or a clean eraser.
Also available in a smaller 45ml size and marker pens.