Painting Cast Iron Radiators (per section)

A column radiator is made of individual sections joined side by side. Each section has an uneven surface on both sides, with ridges, bumps and narrow gaps that a brush struggles to reach. That is why the job is not measured by area like a wall. It is counted in units: one section, one unit. A typical radiator under a window has a few dozen sections. The large radiators found in older Victorian and Edwardian houses often have more.

Painting a section usually includes cleaning, degreasing, light sanding and the top coat. Rust removal, an anti-corrosion primer or taking the radiator off the wall are often treated as separate items. Agree in advance exactly what the job covers, so there are no surprises.

First the heating is switched off and the radiator is left to cool completely. On a warm surface the paint dries too fast, leaves patches and does not grip properly. Then the floor, the wall behind the radiator and anything nearby are covered with sheeting or a dust sheet, because paint drips easily from between the sections.

Next comes cleaning. Dust is brushed out of the gaps with a narrow radiator brush, and the surface is washed with soapy water and degreased. Loose flakes of old paint and rust come off with a wire brush, sandpaper or a sanding attachment on a drill. Before painting, the surface needs to be clean, dry and free of flaking spots.

Rusty areas are treated with a rust converter or an anti-corrosion primer. On older radiators that have been painted many times, a primer also helps the top coat stick. The painting itself is done in thin coats, using an angled radiator brush for the gaps and a small roller for the flat faces. The second coat goes on only after the first has dried, following the times on the tin.

Ordinary emulsion for walls and standard wood varnish do not belong on a radiator. Heat makes them yellow, crack and peel. Use paints that the manufacturer clearly labels as suitable for radiators, and always check the heat resistance on the label.

• Water-based (acrylic) radiator enamel: dries quickly and has little smell, so it suits a home you are living in. It is fine for normal central heating.
• Solvent-based radiator enamel: harder and more resistant to scuffing, but slower to dry and with a strong smell. It needs good ventilation, so it is more often chosen for cellars, workshops or utility rooms.
• Paints with an anti-corrosion component: useful on older radiators with small patches of rust. Some can go straight onto cleaned bare metal.

Colour is a matter of taste too. White remains the classic choice, but a radiator can just as well be anthracite, matt black or the same colour as the wall so it blends into the room. With metallic finishes, bear in mind that some manufacturers say they can slightly reduce the heat the radiator gives off.

A radiator goes quickly when the old paint is sound and only faded, it is easy to reach and it stays on the wall. It just needs cleaning, a light sand and paint.

More time is needed when there is:

• rust, or cracked, thick paint built up over many years
• large or unusual sections with deep gaps
• a radiator very close to the wall, under a deep windowsill or behind furniture
• a need for primer and extra coats
• a change from a dark colour to a light one, where a single coat does not cover

For a really thorough finish, the radiator can be taken off the wall. That means working on the heating system, though. In a block of flats, draining the system and removing a radiator must be agreed in advance with the building management or owners' association, and the work should be done by a heating engineer. A painter normally paints the radiator in place.

• Painting a warm radiator. The paint dries unevenly and may start peeling within a few days.
• Coats that are too thick. The paint runs, forms drips and patches, and builds up in the gaps. Two thin coats are always better than one thick one.
• Skipping cleaning and sanding. New paint will not hold on dust and grease. The preparation decides how long the finish lasts.
• The wrong paint. Wall or wood paint turns yellow and cracks on hot metal.
• Turning the heating on too soon. Heat on paint that has not cured causes marks, blisters or a change of colour.
• Painted valves. The thermostatic valve head, bleed valve and fittings should not be painted, or they may stick and become hard to undo.

You can paint a radiator in place yourself, as long as you have the patience for the gaps between the sections. Paint on radiators in older homes may contain lead. Do not dry-sand old paint without a suitable respirator, and wipe up the dust afterwards with a damp cloth.

1. Turn the thermostatic valve off and let the radiator cool completely.
2. Cover the floor and the wall behind the radiator with sheeting, and tape over the valves and valve head.
3. Brush the dust out of the gaps with a narrow brush and wash the surface with soapy water.
4. Remove loose paint and rust with a wire brush or sandpaper, then lightly sand the remaining paint.
5. Wipe off the dust, degrease the surface and let it dry.
6. Treat rusty spots with rust converter or anti-corrosion primer.
7. Paint the inner parts and gaps first with an angled brush, then the front faces of the sections.
8. Let it dry for the time given on the tin and apply a second thin coat.
9. Remove the tape, air the room and switch the heating back on only after the time recommended by the paint manufacturer.

• The paint is even and covers the gaps between the sections
• There are no drips, runs or paint build-up anywhere
• No rust or old colour shows through
• Valves, valve head and fittings are free of paint and still turn
• The floor, wall and surroundings are clean, with no splashes
• After the first heating cycle the paint does not yellow, blister or feel sticky