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Linda says:
"As a watercolorist ready to try some new media, I am still confused about the difference in the effects of gouache and casein. I like the idea of underpainting with casein, but do I have to use gouache on top or can I continue with watercolor?"

Linda, I've had pretty good experience with combining pretty much any water media, such as watercolor, gouache, tinted gesso, acrylic gouache, acrylic, or casein. 

You can mix them or layer in any way you want as long as the resulting paint layer maintains adhesion. So you can do an underpainting in watercolor and finish with thick casein. Or you can put down casein first and then put thin washes of gouache over it. 

Some manufacturers caution against mixing diverse mediums together (or even diverse brands together) while they're wet on your palette, but I haven't had any issues so far with doing that. So you can try using acrylic paint to tint white gesso, and you can use gouache colors with white casein on the same palette. However, there are a few cautions.

Caution #1 is that if you have a layer that ends up being glossy or impermeable, a thin layer applied on top of that when it's dry may not have very strong adhesion. In other words it might tend to bead up while you're applying it, and it might tend to rub off after it's dry. (Incidentally, both of those effects might be desirable in certain techniques. If it beads up it will dry in a really interesting pattern. And if you're worried about it rubbing off, you can seal down the whole surface with finish varnish.)

Caution #2 is that if you're painting on flexible paper, your paint application should be relatively thin to keep the paint from flaking or breaking off. The glue-like emulsion of casein is not very strong compared to, say, acrylic. When it dries, casein is kind of like chalk. So if you want to use casein for thick, textural impastos, you should use a stiff panel or board as your base layer.  

Caution #3 is that you should experiment on a scrap first to see what happens with various combinations. You don't want to be in the middle of an important painting when you're experimenting with mixed media.

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"Casein Painting in the Wild"
HD Digital download or stream on Gumroad 
DVD at Kunaki (ships worldwide) or Amazon

Jack Richeson Casein The Shiva Series Underpainting with 37ml Tubes, Set of 6
M. Graham 1/2-Ounce Tube 5 Color Gouache Paint Primary Set