Schmincke Liquid Frisket Medium is a removable, colourless gum emulsion that is used for masking off specific areas. Apply directly to areas that you don't want the paint to cover.
Once dry, you can then paint over the medium and the surface underneath will remain paint free. Remove as soon as possible. Once the painting is finished and dry, simply rub off the masking fluid with your finger or an eraser. Easy to remove. Does not contain ammonia.