An underpainting is a first layer that establishes the structure of a picture before any of the final color goes on. It is one of the oldest working methods in European painting and it solves a problem most beginners feel without being able to name.
The problem it solves
A white canvas is a liar. Every color you put on it looks darker than it is, because white is the lightest value available and everything is being judged against it. Painters mix too light as a result, and then wonder why the finished picture looks weak and washed out.
Cover the white with a mid-tone and the problem disappears. Now your lights read as light and your darks read as dark, because they are being judged against something in the middle rather than against the brightest thing in the room.
The second problem it solves is decision order. Painting well means getting the values right, which is a separate job from getting the colors right, and trying to do both at once is what makes a picture go muddy. An underpainting lets you settle where the darks and lights go while the only decision available is how dark to make something.
The three traditional forms
Imprimatura is the simplest: a thin, transparent wash of a single color over the whole canvas to kill the white. Burnt sienna, raw umber and a warm gray are the usual choices. Ten minutes of work and the most useful of the three.
Grisaille is a full monochrome painting in grays, worked up to a finished tonal image before any color is applied. Slower, and the most instructive thing an amateur can do, because it forces you to see the picture as light and dark rather than as objects.
Verdaccio is a green-earth underpainting, used historically beneath flesh tones. Green sits opposite red on the color wheel, so a warm skin tone laid thinly over a cool green ground is subtly neutralized where the ground shows through, which produces a convincing complexion rather than a pink one.
That last principle generalizes, and it is worth stealing. A warm red-brown under green foliage, or a warm ochre under a blue sky, makes the top layer vibrate wherever it does not quite cover. Our note on the basic color wheel covers why complementary pairs do this.
How acrylic changes the method
All of the above was worked out for oil, where an underpainting has to dry for days before anything can go over it. That waiting is the reason a lot of painters skip the step entirely.
With acrylic it is a non-issue. A thin imprimatura is dry in ten or fifteen minutes, and a full grisaille can be finished and painted over in the same afternoon. This is the single strongest argument for underpainting in acrylic: it costs you almost nothing.
Three things to adjust, though.
Keep it thin. A heavy underpainting leaves texture you will be fighting for the rest of the painting. Thin the paint with an acrylic medium rather than with water; past roughly a quarter water you are diluting the binder as well as the pigment and the layer dries chalky and can lift when you work over it. A numbered kit does this step for you in the printing, which is part of what the pre-primed canvas is buying you.
Acrylic is opaque, so plan for it. In oil, thin upper layers naturally let the underpainting show through. Acrylic covers more completely, so if you want the ground to influence the top layer you have to work deliberately: glaze with a transparent color thinned with medium, or leave the ground exposed at the edges of your strokes.
Fat over lean does not apply. That rule exists because oil layers dry at different rates and a lean layer over a fat one cracks. Acrylic layers dry the same way regardless of order, so you can put anything over anything. Any guide telling you to observe fat over lean in acrylic has copied it across from an oil manual.
How to do one
- Tone the canvas. Thin a warm earth color with medium to the consistency of milk and scrub it across the whole surface with a large brush or a rag. Aim for a mid-tone you can still see your drawing through. Ten minutes to dry.
- Block in the darks. Using one dark color, put in the largest shadow masses. Do not draw outlines; place shapes. Squint at your reference and paint what is left when the detail disappears.
- Put in the lightest lights. A little white or a pale mix in the two or three brightest areas only.
- Stand back. If the picture reads correctly in three values from across the room, it will read correctly in full color. If it does not, no amount of color will rescue it, and now is the cheap moment to fix it.
- Paint over it. Full color, working from the largest areas down, leaving the underpainting to do the work in the shadows.
Underpainting and gesso are not the same thing
Gesso is a primer. It seals the surface, gives it tooth and stops the fabric absorbing the binder out of your paint. It goes on before anything else and it is not optional on raw canvas. An underpainting is a painting layer, applied over primed canvas, and it is entirely optional. Our note on what gesso actually is covers the difference.
A note about numbered canvases
None of this applies to a paint by number kit, and it is worth saying so directly, because the two subjects sit next to each other online. A numbered canvas already carries a printed guide, and anything you put over the whole surface would bury it.
The underlying physics still shows up, though, in a way that catches people out. Every region has a black printed number in it, and pale colors have the least covering power. That is why yellows, creams and pale blues need a second coat on a numbered canvas: they are doing exactly the job an underpainting exists to make easier.
For working with layers generally, see layering acrylic paint, and for blending while the paint is still open, wet on wet acrylic. If you are assembling colors to try this, our guide to acrylic paint sets covers what to buy.
