Preparing canvas surfaces for oil painting outdoors

Why the surface decides the painting

Outdoors, your canvas is fighting the light. Sunlight bounces off a white, glossy ground and straight back into your eyes, which makes it almost impossible to judge a tone accurately. Squint at a bright primed board at midday and every mid-tone looks darker than it is; paint a passage you think is subtle and you get home to find it chalky and thin. Toning the surface before you leave the studio is the single cheapest fix for this. A mid-tone ground absorbs glare, gives you a ready-made middle value, and lets you work both lighter and darker from the first stroke.

The other benefit is tempo. Plein air painting rewards speed, and a toned surface means you never have to cover an intimidating expanse of white. You are painting into a picture from the outset rather than filling in a void.

Choosing a support for field work

Weight and rigidity matter more than prestige. A flapping canvas in a coastal breeze is misery, and stretched cotton on a light frame will drum against your arm all afternoon. Practical options include:

• Canvas boards and panels: cheap, rigid, stackable, and they take a ground well. Cardboard-core boards can warp in damp air, so look for ones with a sealed back.
• Birch ply or MDF panels: cut to size, sand the edges, and seal both faces with a coat of acrylic medium or shellac so moisture cannot get in and bow them.
• Linen or cotton on rigid panels: glue the fabric down with acrylic medium, wrap the edges to the back, and prime once dry.
• Aluminium composite panels: light, completely stable, and beautifully smooth for knife work, though the surface needs a keying rub with fine abrasive first.

Whatever you choose, prepare a batch in one session. Twenty small panels prepped in an afternoon will carry you through a whole season of sketching trips.

Sizing and priming without a headache

Oil paint will rot raw fabric over time, so the cloth needs isolating before anything else touches it. A traditional rabbit-skin glue size is excellent but must be kept hot and used fresh, and it smells. For most outdoor painters, an acrylic dispersion primer does the same job with far less fuss. Two thin coats, brushed in opposite directions, sanded lightly between them with fine paper, give a sound tooth without clogging the weave.

If you want a more absorbent, traditional feel, apply an oil-based ground over a well-sealed surface, never directly to the fabric. Keep coats thin. Thick grounds crack, and a ground that is fatter than the paint layers above it will fail.

Work in a ventilated room, away from food, and let each coat dry fully before the next. Acrylic grounds dry fast enough that you can complete a batch in an evening. Oil grounds need days, so plan ahead and keep a stock of prepared panels turning over.

Toning: colour, consistency and drying

A tone is simply a thin, even wash of colour laid over the dry ground. The usual choices are raw umber, burnt sienna, or transparent red oxide, alone or mixed with a little terre verte or yellow ochre to warm or cool the result. For landscape work in Britain, a warm mid-brown or a soft ochre-grey tends to sit comfortably under greens and sky blues alike.

Keep the layer genuinely thin. Scrub it on with a rag or a wide brush, then wipe back with a dry cloth so the weave shows through and no ridges remain. Thin, lean layers dry reliably; a thick, oily tone will stay tacky for days and will lift when you paint over it.

Acrylic toning is the fast route. Thin acrylic paint with water to a stain and it will be touch-dry in twenty minutes, ready to pack in the car. An oil tone is more sympathetic to oil paint above it, but needs at least a day indoors before travelling and considerably longer in a cold car boot. Oil drying slows dramatically below about 10°C, so a tone applied outdoors on a November morning may still be wet the following week.

If you need speed from an oil tone, add a small amount of alkyd medium and keep the layer thin. Do not be tempted to add driers or strong solvents beyond what the medium already contains.

Drying, storing and travelling with wet panels

Drying is a matter of air, temperature and time. A dry, airy room at normal household temperature is ideal. Stacked panels, sealed boxes and cold studios are not. Leave toned boards standing on edge with space around them, and give them at least a day longer than you think they need.

For transport:

• Use a carrier with grooved slots so wet surfaces never touch.
• Separate panels with spacers, or face wet panels inward against a dry partner.
• Wrap finished wet paintings in a breathable cover, not cling film, which traps solvent and slows drying.
• Keep the car out of direct sun; heat makes fresh paint slide.

Prepare more panels than you think you will use, tone them all in one go, and store them flat and dry. The habit pays for itself the first time the light is extraordinary and you have a board ready to go.