Add colour and seal your plaster base all at once with Water Undercoats. Use Water Undercoats to add realism and the illusion of depth. Use multiple colours to create authentic transitions between shallows and depths. How to Use Water Undercoat To Seal and Colour Your Plaster Base Video Specially formulated to represent authentic colours on the Forel-Ule Scale, Water Undercoat lays a base for creating dark depths to shallow shorelines, and underwater scenes for any aquatic ecosystem. Create customized colours by mixing Water Undercoat together.