Do I need to sugar soap walls before painting?

Sugar soap is an alkaline cleaner — traditionally sodium carbonate and sodium phosphate — that cuts grease and lightly etches an old paint film, which is why decorators have used it for a century. It contains no sugar. Whether you need it depends entirely on what is on the wall, and the honest answer is that some walls need it badly and some do not need it at all.

What you'll need

• Sugar soap, concentrate or ready-mixed spray
• Two buckets, sponges and clean cloths
• Rubber gloves and eye protection
• A stepladder and dust sheets

Step by step

1. Decide whether the wall is actually dirty.Wipe a clean damp cloth over a section and look at it. Kitchen walls, above radiators, around switches and handles, hallways, smokers' rooms and anywhere with candles or a wood burner will show grey or yellow residue. A bedroom wall painted five years ago and never touched often shows nothing.
2. Wash where grease or contamination is present.Grease, nicotine, soot, hairspray, cooking residue and the oily film that builds above a radiator all prevent adhesion, and paint over any of them will fail. These are the walls where washing is not optional.
3. Mix to the stated dilution and work from the bottom up.Follow the dilution on the container — stronger is not better and leaves more residue. Working upwards stops dirty runs streaking down dry wall and drying into marks that are hard to remove.
4. Rinse with clean water, in a second bucket.This is the step people skip and it matters more than the wash. Sugar soap residue is alkaline and hygroscopic; left on the wall it interferes with adhesion and can show through the new paint. Rinse with clean water and change the water when it stops being clean.
5. Let it dry fully before painting.A wall that feels dry can still hold moisture in the plaster after a thorough wash. Give it a day in a ventilated room, longer in cold weather. Painting a damp wall causes patchy drying and poor adhesion.
6. Use it deliberately on old gloss, not just to clean.On sound old gloss and satinwood, a sugar soap wash both degreases and slightly dulls the surface, which helps a new coat grip. It is not a substitute for abrading, but it is a genuine part of the preparation on trim.
7. Use the right product for the actual contamination.Mould needs a fungicidal wash, not sugar soap. Heavy nicotine needs a strong degreaser and often a stain block afterwards. Sugar soap is a general degreaser and light etchant, and it is not the answer to everything.

Tips

• Ready-mixed sugar soap sprays are convenient for switch surrounds, door frames and small greasy areas and still need wiping off with clean water.
• Wear gloves. It is alkaline and it will dry and crack your hands over an afternoon's work.
• If the wall is only dusty rather than greasy, a vacuum with a brush head and a wipe with a damp cloth is quicker, cheaper and leaves no residue at all.

Common mistakes

• Not rinsing afterwards — Alkaline residue left on the wall attracts moisture and interferes with the new film. It can show as patchy sheen or poor adhesion in exactly the areas you cleaned hardest.
• Using sugar soap on mould — It removes the visible growth but does not kill it, so the mould returns through the new paint. Use a fungicidal wash with its stated dwell time.
• Washing bare plaster or unsealed filler — It soaks in, takes a long time to dry, and can leave the surface with uneven suction. Wash painted surfaces, not bare porous ones.

Questions people ask