I have used Matisse Gel Medium over the years for mixed media work on canvas, with textile and paper. Medium is easy to work with allowing you to move your work around, once dry (glossy and clear) you are able to paint directly on to it. I have also had no issues with this medium turning yellow, also no cracking or separation over time.

Matisse Gel Medium can also be added directly to your paint as a colour extender, or to thicken paint to enhance the texture of brush and knife strokes.