Q: I want to use oil glazes over a fast-drying underpainting,
and I was told that casein is a good medium to use. Is it wise to paint
on canvas this way?
A: Casein may be used as an underpainting for oil glazes.
However, it's far too brittle to be painted on a flexible support, so
you shouldn't use this technique on a stretched canvas. Once the casein
underpainting has thoroughly dried, it will be far too absorbent to
properly receive oil glazes: Your brush will snag and grab the painting
surface, making smooth delivery of the oil glazes nearly impossible.
Therefore, brushing or spraying a very thin size to seal the
underpainting is required. Bleached white shellac that has been reduced
with five parts of denatured alcohol works well for this purpose.