Can You Varnish Casein?

Scottie Jensen in my YouTube channel asks:
"I love watching your videos and have purchased a few of your “In the Wild” series. I am greatly inspired by you and have built a sketchbook easel, started painting in casein and have went out into the wild to paint all thanks to you. I have one question about varnishing casein, I read on your blog you use spray on varnish - is this to avoid smudging once dry?

Also, how long should I wait before varnishing a casein painting? Will a couple days after it is dry to the touch be sufficient or is there a longer curing process to avoid cracking? Thanks so much for your knowledge and infectious passion!"

Incident on Kelly Street, casein on board, winner of the NSPCA Award

James Gurney: Thanks for the support and feedback. The main reason I varnish some of my caseins is to deepen the darks in an overall dark painting or to protect a painting that I want to frame without glass. It shouldn't smudge without the varnish, and I almost never varnish sketchbook paintings, because I can always deepen the darks in photoshop.

Keep in mind that casein paintings are not usually varnished. The matte surface is an attractive quality, especially in high key paintings. The manufacturers suggest buffing the surface of a dry painting using an old T-shirt to add some semi-gloss luster to the surface.

You should wait a couple of days before varnishing and during that time, put the painting in a warm place to make sure the paint is fully dry. The glue-like protein binder gets stronger over time. The best defense against cracking is to make sure you paint on panel or illustration board. If you paint on watercolor paper, don't use thick impastos, because casein does not have a strong emulsion.

As with any unconventional technique, experiment first on a scrap and make sure it does what you want.
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The painting is documented on my feature tutorial Fantasy in the Wild
Free Facebook group: Sketch Easel Builders
Previously on GJ: Painting a Magical Light Effect

On Amazon: Artist Casein Colors, Set of 12