Rollo Q asks: "I have a couple of questions about the Casein. Roughly how long do you find the paint is wet (moveable rather than liftable) on the surface. Obviously I realise this must depend on climate and surface and paint thickness etc, but I've seen people call it fast drying and its hard to tell what that means (I paint in flashe vinyl which is notably slower than acrylics but faster by far than oils. Open time is 30 - 60 mins for flashe I'd guess). Also do you use an acrylic varnish? Or a resin?"

James Gurney answers: You're right, time it takes for a freshly painted area to dry to the touch depends on the temperature, humidity, wind, paint thickness and wetness, absorbency of the ground, and even the type of pigment. Cadmiums are slower drying than umbers, for example, and titanium white dries relatively fast. This pigment variable is especially noticeable on the paint blobs squeezed out on the palette. 

So how long does it take for a passage to dry? On a nice day an average paint stroke starts to set up in anywhere from a few minutes to ten minutes or so. While it's still wet you can blend all you want, but once it's dry to the touch, a stroke can go down over it without much fear of previous layers lifting or reactivating, unless the new stroke is really wet or there's a lot of scrubbing. 

Paint that is newly dry presents a relatively closed surface, meaning that, unlike gouache, it resists reactivation. The strength of the paint's glue-like binder is not as strong as acrylic, which has a powerful bond which sticks hard to your palette and wreaks havoc on your brushes. Because of that weaker binder, casein techniques involving thick, textural impasto should only be painted on a rigid surface such as a panel or illustration board. 

With casein, paint that feels dry from evaporation isn't fully cured yet. The milk protein molecules keep bonding, which can take anywhere from overnight to a couple weeks. Curing time can be accelerated by putting the Painting in a sunny, dry, and warm (but not too hot) location. 

After it's dried and cured you can decide whether or not you want to varnish it. Like gouache, casein is meant to dry matte, and that can look good in a light, high-key painting. I almost never varnish sketchbook paintings. Paintings with a matte surface certainly photograph well, and the digital version of a painting can be adjusted in Photoshop to give it the full range of values. The manufacturers suggest buffing the surface of a dry painting using an old T-shirt to add some semi-gloss luster to the surface.

The main reason I varnish some of my casein paintings is to deepen the darks in an overall dark painting or to protect a painting that I want to frame without glass. Dark-keyed paintings look better varnished, and sometimes varnish is needed to unify the surface sheen of the painting. You can varnish casein just as you would varnish an oil painting. The advantage of casein compared with oil for plein-air painters is that you can varnish it the same day, rather than waiting weeks or months for the paint to dry. 

I've tried various brush-on varnishes, but I've been using a non-yellowing spray gloss varnish, which works well, but acrylic gloss varnish should work well, too. 

For the varnish to work with one or two coats, the substrate beneath the paint needs to be relatively non absorbent, such as a hardwood panel or gesso-primed canvas mounted to board. I've been painting on acrylic primed canvas panels, which respond well to varnish. If the painting consists of thin casein washes on soft watercolor paper, the surface will absorb the varnish, and it will take a lot of coats before you start to get a glossy surface. After it's varnished, a casein painting can be framed without glass just like an oil.