What is Masking Fluid? Hi world! Today I am going to show you a couple of ways I use masking fluid in my watercolor works. Masking fluid is a removable liquid latex mask with ammonia that resists paint and can be used on unsized papers. It is usually a slightly pigmented off-white that yellows as it dries and is a very sticky, brush destroyer. I use Winsor & Newton’s ‘Water Colour Art masking Fluid.’ Why Masking Fluid is Amazing Masking fluid is an incredibly useful material