Ox Gall (Winsor & Newton) Ox Gall is the traditional wetting agent for watercolour. A drop or two in water with Gouache or Watercolour improves flow and will provide good adhesion. Ox Gall will clean paper prior to painting by removing greasy finger marks. It is also used on very hard sized papers to reduce surface tension. If a paper is resisting a watercolour wash, leave it to dry before covering the paper with diluted Ox Gall. Once this is dry the painting can be continued as normal. Do not push the brush into Ox Gall as this may introduce bacteria.