Cleaning walls before you paint: what to use and what to skip

What you'll need

• A dry microfiber cloth or a vacuum with a brush attachment, for dust.
• Two buckets, one for cleaner and one for rinse water.
• Mild dish detergent or an all-purpose cleaner, and a degreaser for kitchens.
• Soft sponges and clean towels. Skip scouring pads.
• Rubber gloves and eye protection.
• A step stool and a fan to speed up drying.
• A drop cloth, so wet runs don't land on the floor.

Why a clean wall matters

Paint grips a clean, dull surface. Grease from cooking, soap film in bathrooms, smoke residue and the oil from a thousand hands leave a layer that paint sits on instead of bonding to. The result is paint that beads, crawls or peels in the places people touch and cook most.

Dirt also feeds mildew. Missouri Extension notes that greasy films, like those that form on kitchen walls, contain many nutrients for mildew-causing molds. Cleaning isn't only about how the paint goes on. It's also about whether the wall stays clean underneath.

The basic method

1. Clear the room and cover the floor. Remove outlet covers and anything hung on the walls.
2. Dust from the top down. Wipe the ceiling line, corners, door casings and baseboards, then the wall faces. Dust left on a wall turns to mud when you wet it.
3. Test a small, hidden spot with your cleaner first, so you know it won't lift or dull the existing paint.
4. Mix mild detergent into warm water in one bucket and fill the other with clean rinse water.
5. Wash one section at a time with a sponge that's damp, not dripping. Work in gentle circles, and don't scrub hard.
6. Rinse the section with a clean sponge and clear water, and dry it with a towel.
7. Open a window and run a fan. Let the walls dry completely before you do anything else.
8. Look at the walls again in raking light. Spot prime any stains that remain before painting.

What to use on what

Skip anything abrasive. Scouring pads and harsh powders dull paint and can scuff drywall. EPA's advice for older homes is to wipe with a damp sponge, to avoid steel wool, scouring pads and abrasive cleaners, and not to rub spots excessively. That's good advice for any wall.

Kitchens and bathrooms

Kitchen walls collect airborne grease, especially near the range and over the sink. Use a degreaser or a stronger detergent solution there, work in small sections, and rinse well, because leftover cleaner leaves its own film. Rinse again if the sponge still comes away sticky.

Bathrooms collect soap film and often mildew. EPA says to scrub mold off hard surfaces with detergent and water and dry completely, and not to paint or caulk moldy surfaces, because paint applied over mold is likely to peel. If the mold keeps returning, the moisture source is the problem. EPA suggests keeping indoor humidity below 60 percent, ideally between 30 and 50, and running the bath fan or opening a window when showering. Our guide to painting a bathroom covers that side.

If you use bleach on mildew, EPA warns never to mix it with cleaners that contain ammonia, because toxic fumes can result. Keep the room ventilated, wear gloves, and rinse afterward. EPA also notes that if the moldy area is more than about 10 square feet, it's a bigger job than a sponge.

Gentle cleaning, smoke and old paint

Flat and matte paints are the easiest to damage. Scrub one hard and it burnishes into a shiny patch, or the color wipes off on the sponge. If a spot won't come clean with a light touch, stop washing it and prime it instead. A stain-blocking primer will cover a mark that scrubbing would only spread.

Glossier paint in hallways and kitchens takes washing better, but it's still worth testing a hidden spot first. If the sponge picks up color, the paint is chalky or poorly adhered, and that tells you the wall needs primer, not just paint.

Smoke and nicotine leave a sticky yellow film that smears when you wash it. Wash in small sections, rinse often, and change your rinse water as soon as it turns brown. Even after a good cleaning, smoke stains and odors can bleed through, so plan on a stain-blocking primer over the whole wall, not just the worst spots.

If your house was built before 1978, EPA says its paint is more likely to contain lead. Wet cleaning with a damp sponge is the safer method for dusty old paint. Avoid dry scraping and sanding, and find out before you disturb it. For paint that's flaking, our post on prepping walls for painting covers what to do next.

Drying, dust and ventilation

The surface can feel dry while the wall behind it is still damp, especially around a tub or a window that sweats. Give washed walls a full day if you can, and longer in the bathroom. A wall that's dry looks even in color, with no dark patches, and feels the same temperature as the wall beside it.

If you sanded repairs, wipe the dust off with a damp microfiber cloth once more before you prime. Dust on a washed wall stays loose, and primer over it ends up gritty. Use a vacuum with a brush attachment for baseboards and corners, where dust collects.

Cleaning products give off fumes too. EPA lists degreasing and cleaning products among household products that contain organic solvents, and advises increasing ventilation when you use them. Open windows, run a fan out of the room, wear gloves, and never combine cleaning products. Our guide to low-VOC paint covers the paint side of indoor air.

Where DIY gets hard

• Too much water. A soaked sponge can soften drywall and lift paint or tape seams.
• Streaks and drips. Wash in small sections and wipe down drips right away, or they leave clean stripes on dirty walls.
• Not waiting. Painting over a damp wall is how paint fails, and a closed-up bathroom can take much longer to dry than you'd think.
• Hidden moisture. Mold that returns after cleaning means a leak, bad ventilation or a cold wall.
• Tall walls. Washing a stairwell or vaulted wall means ladders and awkward reaches.

What we do before the paint comes out

On our interior painting jobs, we cover floors and furniture first, repair small holes, and clean, sand and prime before any paint comes out. Most rooms take one to two days. The written quote is the price, no extra charge if the job takes longer than expected. If your walls have smoke damage or heavy grease, mention it when you call and ask for a free estimate.