How to Prepare Walls for Painting — The Right Way

Every decorator will tell you that preparation is the most important part of a quality paint job. Most customers who ask what makes one decorator better than another are surprised when the answer isn't a particular brand of paint or a specific technique with a brush — it's the hours spent on preparation before a brush or roller touches the wall.

In this article I'll explain exactly what professional wall preparation involves, why it matters as much as it does, and what happens when it's skipped or rushed. If you're planning to hire a decorator, this will help you ask the right questions. If you're thinking about doing it yourself, this is the part that most DIY guides underestimate.

Why Preparation Matters More Than Paint

Paint is only as good as the surface it adheres to. Apply premium paint to an unprimed, dusty, or damaged surface and it will peel, crack, or look uneven within months. Apply entry-level paint to a properly prepared surface with the right primer and you'll get a durable, even finish that lasts years.

The most common DIY decorating failure I see — when customers hire me to redo a room they've previously painted themselves — comes from skipping or rushing preparation. The paint looks fine for a few weeks and then starts showing every crack, bump, and damp patch that was underneath it. By that point, correcting the job properly means removing the new paint as well and starting again, which costs more than doing it right first time.

Step 1: Clear and Protect the Space

Before any work begins, everything movable should be out of the room or covered. We use dust sheets on furniture and flooring and mask skirting boards, architraves, and switches before touching any wall surface. Any paint that lands on unprotected furniture or flooring is a problem — masking tape on skirting boards is a ten-minute job that prevents an hour of careful cleaning later.

If we're stripping wallpaper (which is often the case in older properties in areas like Wandsworth or Wandsworth where Victorian properties may have layer upon layer of old paper), additional dust sheets go down because wet stripped paper makes a mess that is time-consuming to clean from carpets or timber floors.

Step 2: Strip Existing Wallpaper or Loose Paint

If the room currently has wallpaper, it almost always needs to come off before painting. The exception is very old wallpaper that is absolutely solid and shows no sign of lifting — in some Victorian properties with traditional lime plaster, old wallpaper can actually be better left on if it's genuinely bonded to the wall. This is a judgement call that requires examining the wall and the paper's condition, and getting it wrong in either direction causes problems.

Stripping paper properly involves soaking it to release the adhesive and then removing it in sections without gouging the plaster underneath. Rushing this step and tearing paper off dry leaves adhesive residue on the wall that will show through paint. It also damages plaster edges and corners that then need more filling work.

Loose or flaking existing paint also needs to come off. Painting over loose paint transfers the adhesion problem — the new paint bonds to the old paint which then comes away from the wall and takes everything with it. Scraping and sanding back to a firm layer takes time but is not optional.

Step 3: Wash the Surface

Grease, nicotine, mould, and household grime all prevent paint adhesion. Kitchens and bathrooms are the worst offenders — cooking vapour and steam leave invisible deposits on walls that make paint fish-eye and peel. We wash these surfaces with sugar soap before any other preparation begins.

Any mould — visible as dark spots, usually in corners or behind furniture — needs to be treated with a specific mould-killing solution before the wall is painted. Painting over mould conceals it temporarily but doesn't kill the spores. They'll come back through the paint within months. If the mould is persistent or extensive, the underlying damp problem needs to be investigated.

Step 4: Fill Cracks, Holes, and Imperfections

This is the most skilled and time-consuming part of interior preparation. Every crack, nail hole, screw hole, and chip needs to be filled, allowed to dry, sanded flat, and then inspected again — because filler shrinks slightly as it dries and often needs a second pass.

Fine cracks in plaster (hairline cracks that follow the plaster joints) are common in all older buildings and particularly common in houses that are settling. They need to be opened slightly with a scraper before filling, otherwise the filler sits on top of the crack rather than into it and falls out within weeks. This is a detail that separates professional preparation from amateur preparation.

Larger cracks — anything that is more than hairline width, or cracks that recur after filling — can indicate movement in the structure and should be monitored rather than simply filled and painted over. If I see something that concerns me on a survey visit, I'll tell the customer before quoting.

Step 5: Sand

Once filling is complete and fully dry, the filled areas need to be sanded smooth so they're flush with the surrounding surface. The wall also benefits from a light overall sand to key the existing paint surface so the new coat has something to grip. This is done with fine-grit paper by hand around edges and corners, and with a pole sander on large flat surfaces.

After sanding, all dust must be removed. We use a vacuum followed by a tack cloth or damp sponge wipe. Paint applied to dusty surfaces has dust particles in it — visible as a rough texture in raking light — and dust between the old and new coat compromises adhesion.

Step 6: Prime or Mist Coat

This is the step that most DIY jobs and even some professional jobs skip, and it's the one that causes the most problems.

Fresh or bare plaster is highly porous. If you apply standard emulsion directly to new or newly exposed plaster, the plaster absorbs the water from the paint unevenly, causing it to dry patchy and lose its adhesion properties. The correct approach is to apply a mist coat — standard emulsion diluted approximately 20 to 30 percent with water — which is absorbed into the plaster and creates an even, sealed base layer. This takes a full day to dry properly before any finish coats go on.

For bare wood or previously painted wood surfaces, a primer is applied before finish coats. The primer bonds to the surface and provides the adhesion layer for the finish paint. Skipping primer on bare wood leads to uneven coverage and poor adhesion, particularly on hardwood surfaces where natural oils can prevent paint bonding altogether.

Our interior painting service includes all preparation as standard — we don't separate it out as an optional extra because without it the job isn't done properly.

Preparation for Exterior Surfaces

Exterior preparation follows similar principles but with additional requirements because surfaces are exposed to weather. Masonry and render surfaces need to be cleaned of dirt, moss, and algae — power washing is often used on exterior surfaces that wouldn't be appropriate indoors. Any loose or blown render needs to come off and be replaced before painting, because painting over hollow or damaged render means the paint will lift when the render eventually fails.

Timber on exterior surfaces — windows, doors, fascias, bargeboards — requires the most preparation because it's the most vulnerable to movement and weather. Old paint on timber builds up over years and becomes so thick that it cracks and falls away rather than flexing with the wood beneath it. On properties in Richmond and similar areas where the housing stock includes a lot of Victorian and Edwardian timber windows, stripping paint back to bare wood and starting again is sometimes the only approach that produces a finish that will last.

Exterior preparation includes treating bare timber with a wood hardener and then primer before any finish coat goes on. Any defects in the timber — rot, split sections, gaps around frames — are dealt with before painting. Painting over rot conceals it and allows it to spread; treating it first stops it progressing and gives the repair the best chance of holding.

We cover exterior painting across South West London and Surrey — the specific products and preparation approach we use are matched to the exposure and condition of each property.

How Long Does Preparation Take?

The honest answer is: longer than most people expect, and longer than it looks from the outside.

A typical average-size room with reasonable surface condition — some cracks to fill, existing paint in reasonable condition, no wallpaper — takes a full day of preparation before painting starts. A room in a period property with more extensive cracking, wallpaper to strip, and areas of loose plaster might take two days of preparation before any finish coat goes on.

This is why a quote that promises to do a room "in a day" should be treated with caution. A properly prepared room takes a day. A room that's painted in a day has not been properly prepared.

What to Check When Hiring a Decorator

When you're getting quotes for decorating work, preparation is the area where the biggest differences between decorators appear. Here are the right questions to ask:

• What preparation is included in your quote?
• Will you be filling all cracks and holes before painting?
• Will you apply a mist coat or primer to new or bare surfaces?
• How many coats of finish paint are included?
• Will sanding between coats be done?

A decorator who answers all of these clearly and specifically is showing you that they know what they're doing. One who gets vague or dismissive on any of them is telling you something about the standard of work you should expect.