Updated: Sep 6, 2021
You could apply wet paint on top of wet paint but for beginners this “direct”, wet-on-wet approach often results in mud. It's one big advantage is that painting alla prima allows you to create an entire painting in one session, useful if time is short or you’re working outdoors. Much safer is the classical “indirect” method, where you start with an underpainting and build up layers allowing each one to dry before applying further paint. I like to combine the two methods, working from indirect to direct.
Here’s a quick guide to how to create an underpainting:
Start by using a pre-primed board (see my blog on how to prepare your board).
Next, create a "soup" by thinning down a transparent paint to the consistency of single cream using your preferred medium. I mix equal quantities of Galkyd (alkyd resin) and Gamsol (odourless mineral spirit).
1. Prepare a thin dark “soup” as above (try burnt sienna with a touch of ultramarine blue).
2. Using a graphite stick, oiled charcoal or small brush dipped into fluid paint, lightly sketch in the main subject matter (or “stuff”) as a mass (i.e., an outline with no interior edges).
Include the shadows but not the background. So “stuff” could be trees, buildings, people, but not the sky or empty landscape.
3. Taking a large brush, roughly block in the mass or “stuff" with your transparent paint. Use a rag or paper towel to wipe out forms and create areas of light and dark (see example below).
4. Work from dark to light. Strengthen your darks. Then add opaque paints and colour to model forms, building up to areas of brightest lights.
5. As you build up layers, use thicker paint. Add final details with very thick paint (“impasto”).
6. Avoid adding white until the final stages as it can contaminate and weaken the darks. And it’s slow to dry. Also avoid adding medium over an area that has already been painted or it will take off the underlying paint.
7. Once the work is completely dry you may add glazes of thinned transparent paint to intensify colours or tone them down.
The principles are the same as for method one: establish tonal values from the start (i.e., darks and lights); work big to small, from transparent to opaque and thin to thick.
The main difference is that you start by covering the whole board with a single layer of transparent paint not just the “stuff”. This layer is called an imprimatura.
You can have a split imprimatura, based on two colours, one for the sky or sea, the second for the land.
Or create a “colour beginning” applying glazes of pure colour over three or four large areas so the underpainting looks like a loose watercolour.
After applying paint with a large brush, wipe away any excess so you are left with a dry, evenly-toned board.
Then paint loosely over the imprimatura, allowing it to show through creating depth and texture (e.g., add a broken layer of blue over an orange underpainting for a lively effect - any two complementary colours will work).
Choose the right colour to tone your board:
Warm red-toned supports are a good choice for cool wooded landscapes and mountain scenes
Rich earth colours suit traditional portraits
Yellow ochre plus white works for bright skies, or to really dazzle, try overpainting a pink with a thin wash of blue-green
Blue-violet creates an harmonic base for a seascape
Yellow-green is perfect for sunlit woods; blue-green for a cool pine forest
Grey provides a neutral base for rich colours, really making them sing. Perfect for a still-life with flowers
A warm yellow underpainting stops a wintry scene from being excessively cold
Violet is a safe choice for gardens and foliage
Mid-tones work best overall (i.e., not too dark and not too light). Also consider drying times as colours dry at different rates, with earth colours like burnt sienna being the fastest and cadmium hues taking the longest.
Check your underpainting is completely dry before going any further.