Painting techniques: watercolour, gouache, acrylic and oil; supports and mixed media
This domain tests whether you can plan a painting in layers and explain why a medium suits a subject. Expect questions on drying behaviour, layering order and the preparation of supports.
Watercolour and gouache
- Watercolour is transparent: white is reserved paper, not paint. Wet-on-wet gives soft edges, wet-on-dry crisp ones, dry brush gives broken texture. Lifting, masking fluid and salt extend control. Values dry lighter than they look wet, so paint slightly stronger than intended, and uneven drying produces blooms.
- Gouache is opaque watercolour with added white or chalk: matte, re-wettable and capable of light-over-dark. Thinned gouache behaves like watercolour and lifts easily, so keep opaque passages fairly dry.
Acrylic and oil
- Acrylic is a polymer emulsion: it dries fast, becomes waterproof and cannot be re-wetted. Mediums control gloss, transparency, texture and open time. It accepts almost any primed surface.
- Oil dries slowly by oxidation, allowing blending and reworking. Follow fat over lean: each layer should contain more oil than the one beneath, or the film cracks. Glazes go over dried opaque passages; impasto uses stiff paint and a knife.
Supports, preparation and mixed media
- Stretch canvas or use rigid panel, prime with gesso, and sand between coats for a smooth surface. Acrylic may be used under oil once fully cured; oil under acrylic is unsafe because the acrylic cannot key to the oily film.
- Mixed media succeeds when you respect drying order and adhesion: water-based layers first, oil-based last, and test collage adhesives for staining.
Worked example
A portrait with a strong shadow side. Choose oil: an underpainting thinned with turpentine and earth colours establishes drawing and value; opaque mid-tones go on once it is touch-dry; the darks are glazed thinly with more oil; final highlights are impasto. Each stage is fatter than the last, so the surface will not crack, and the slow drying lets you rework edges across sessions.
Traps that cost marks
- Inverting fat over lean and blaming the canvas when the paint cracks.
- Painting acrylic over an oily or waxy surface and watching it bead.
- Using white watercolour for highlights and losing the paper's luminosity.
- Over-thinning acrylic with water until the binder fails and the film dusts off.