Paint

Polishing
Before painting, many fresco artists prefer to give a slight polish to the intonaco. Wait until the layer is firm yet malleable like soft leather. Use a striking light (light from an extreme side angle) to see imperfections on the surface. Don’t overdo the polishing! It can make your surface too smooth to accept paint, leaving you with bald spots or even lime puddles. Your goal should be to quickly address the larger imperfections, then leave the plaster alone to continue firming. If your trowel leaves faint gray marks (carbon trails) on the plaster surface, your intonaco is already too firm to polish. If small bubbles develop, you’re intonaco is too wet to polish. The solution is to give the plaster more time to dry before attempting to trowel out the bubbles.

Cartoon Transferring
There are two common methods to transfer drawings from traced cartoons to the fresh painting coat. The first is called “pouncing”, which requires poking small holes into the drawn lines of the cartoon, with about 1/8” space between holes. Once the holes are made, the cartoon is carefully placed over the painting coat and held firmly. A muslin bag is filled with vine charcoal or pigment (use a mask) and gently pounced against the surface. Charcoal from the bag will pass through the holes in the cartoon leaving a dot-to-dot drawing on the damp plaster. 

Pouncing creates quite a bit of charcoal dust.
it's important to protect the wet plaster from over spray. 

After pouncing you have a charcoal dot to dot. 
From here you need only paint. 

The second transfer method is called “incising”. Instead of poking holes, simply place the cartoons against the damp plaster and use a pointed tool such as a pencil to press along the lines of the cartoon. This will leave indentations in the plaster. Similar to polishing, a striking light can be used to better see the indented lines when outlining them with paint. If you're speedy, you may even be able to polish out the indentations once they've been reinforced with a painted line. 

Here I'm incising over the contours of the image. 
It's resting on the soft plaster

After incising an indented contour is visible

Checking for Readiness
After the cartoon has been transferred, check to see if the plaster is ready for painting by placing a swatch of tracing paper on the horizontal plaster surface (for tiles). If the paper becomes saturated and sticks to the surface, the plaster is too wet to paint. If it can be blown away with a gentle breath, painting can likely go forward. 

A second check uses a soft clean brush dipped in distilled water. Lightly drag the wet brush across the surface of the plaster. If the water trail disappears quickly into the plaster you’re ready to paint. At the same time, look at the tip of the brush or paint on your forearm to see if any lime is present. If a white milky residue is appears on the brush or your arm, it’s too early to paint. Remember that painting too soon will cause the brush to erode the smooth surface of the intonaco. It will create an unpleasant gritty texture and the paint will look muddy and chalky in appearance. 

Conditioning the Paint
Many fresco painters choose to condition their paints to promote bonding to the plaster surface. After pouring a small amount of ground pigment in a small cup (a tablespoon or more) you can stir in a dollop of lime putty about the size of a single unshelled peanut. The resulting mixture will have the thickness of heavy cream. This master mix can be divided into cups and mixed with varying amounts of water to create washes that range in color density. In cases where you’ve added too much water, simply let the cup sit for ten minutes until gravity separates the pigment and water. Pour off the clear water to get the color density you’re looking for.

Alternatively, some fresco painters choose to grind pigments directly into lime water and/or use lime water instead of distilled water for rinsing brushes and dampening (by spraying) the plaster’s surface. To make lime water, put ¼ cup of lime putty in a glass jar and fill to the top with distilled water. Shake vigorously until cloudy and let the mixture sit overnight. The next day, carefully pour the clear lime water into a new container and label for future use.

Underpainting
Once the plaster is ready to accept paint, outline the pounce/incision lines with an underpainting color. Many frescoists use a gray-green paint called “Verdaccio”. Its simple form is made by mixing yellow ochre and mars black together. Diego Rivera used black as his underpainting, which is technically referred to as a “Grisaille”. 

After the outline is in, slowly build up values in the work by applying modest transparent washes. Remember the underpainting provides value structure but is not fully rendered. It should be lighter than desired in the final work, as the transparent color washes will finish darkening process.

An important note! Thin washes fuse more successfully than thick marks! Too thick a brush mark may clog the surface and prevent the lime crystals from encapsulating the pigment particles. Moreover, thick marks set poorly and take excessive time to bond. They may prevent you from working areas for long periods of time. Dense or opaque colors are better achieved through a slow buildup of layers.

Building Color
Once the underpainting is complete, it’s best to give the surface time to crystalize before adding washes of color. Diego Rivera planned his lunch to occur after the grisaille was finished, giving his painting time to cure while he ate. You can test the underpainting by dragging a clean wet brush across an inconspicuous area of the underpainting. If the marks move or lift, the underpainting needs more time to cure. If the marks stay put, you can begin applying thin washes of local color over the area. In the same way that you let the underpainting cure, you must let each layer of color cure before subsequent washes are applied. As layers of color build, they become more densely pigmented and even increase in their opacity. The photo sequence below shows how colors develop using the method I have described.

Completed verdaccio for WKU Fresco by Nichols

Paper towels can be used to to control the amount of fluid in your brush. 

Fully pigment fresco with many layers of transparent color.

As hours go by, you’ll notice the plaster’s character changes from delicate and wet to firm and thirsty. In the early stage, anything but a dry brush results in drips. It’s advised that you squeeze excessive fluid from your brush using fingers (covered in rubber gloves), or dab the bristles onto a paper towel. In this early stage paint will seem to float unfused on the surface. It may take 20 minutes for some areas to begin locking into the surface. If you’re gentle, changes can be made with a clean damp brush. However, don’t dwell in an area or the added wetness and activity may erode the delicate plaster surface. In the early stages the plaster is barely dry enough for painting.

The middle stages of painting require more water in the brush because the plaster becomes increasingly thirsty. At some point it will only take a minute or two for brush marks to fuse on the surface. In the later stages of painting, the intonaco will be very firm and brush marks will fuse instantaneously. Changes cannot be made in this stage as marks are locked in as soon as you put them to the plaster.
However, it is very pleasing as the plaster welcomes wash after wash of paint with little waiting between layers. Near the end of this stage the plaster is fiercely thirsty. It will seem that brush is not wet enough. 

Being familiar with the character of the late stage will help you to identify when the plaster no longer accepts paint. One minute every mark locks into place instantaneously, and the next minute subsequent marks lift and move. With experience, you’ll notice this in time to reinforce some areas before the plaster locks you out. Most marks past this stage will float unfused on the surface. Sometimes, the plaster may refuse liquid all together. Other times, particularly if you’re working with more opaque colors you may see your mark lightening in the span of a few seconds. In this case, the day’s painting is finished and you should cease working to prevent damaging the delicate new lime crust.

Opaque Colors using Lime White 
There are several choices of white for fresco. One option includes mixing lime putty into the colors. In addition to lightening colors and increasing opacity, lime white promotes binding. Lime white has a thickness to it, which may build marks in a slight impasto. It’s still recommended to make opaque colors through the building of translucent washes, rather than with a thick mark. If the grittiness of the lime putty is an aesthetic issue for you, it can be sieved using a paint strainer from your local hardware store (See image below). 
 
Lime white has many advantages, but it dries substantially lighter and more opaquely then is seen in its initial application. Fresco painters must anticipate how the color will look when dry. Use an already dry plaster tile as a testing surface, as marks on it will dry in seconds revealing the color’s final character. A second option for white includes using titanium white. Unlike lime white, Titanium white does not build in thickness and its value doesn’t shift so dramatically. But again, remember dense opaque colors are more safely achieved through repeated layering of translucent washes.

Finally, you want your plaster to cure, rather than dry. Don’t be afraid to mist water on your surface or neighboring wall to keep the surface moist and active. For fresco tiles, you’ll notice a lightening on the edges. Spray the corners and sides to keep it well hydrated. While you’re in the learning stage, you may choose to wash your fresco tiles to gauge how much pigment fused. To do this, wait at least 48 hours before attempting to wash. Then, allow a gentle stream of water to pour over the surface. At the same time, you can lightly, without scrubbing, glide your fingers over the painting surface to help remove any unfused pigment. When colors “powder”off it is likely that the plaster was too dry to accept paint. Although, it could also be a number of other factors such as pigment type, or the sequencing of more particular colors.