Technical & Product Guidance
How to Prepare Walls Before Painting
If you take one thing away from this article, let it be this: preparation is not a shortcut you can skip. A beautifully applied top coat will fail within months if the substrate underneath is not sound, clean, and properly primed. In our experience working across Westminster period homes and modern apartments alike, the single biggest reason paint peels, cracks, or looks patchy is inadequate prep work.
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How to Prepare Walls Before Painting
Assessing the Existing Wall Condition
Before you touch a roller or brush, spend time actually looking at the walls. Run your hand across the surface and feel for soft spots, hairline cracks, and areas where old filler has shrunk back. Tap lightly with your knuckles — a hollow sound on plasterwork often indicates it has blown away from the lath behind, which is extremely common in Victorian and Edwardian properties around Pimlico. Note every defect with a pencil mark so you can come back systematically. Pay special attention to corners, around window reveals, and along skirting boards. These are stress points where buildings move seasonally, and cracks tend to reappear if not treated properly. In older Westminster properties you may find layers of lining paper hiding a multitude of sins beneath. Resist the temptation to paint over them — if the paper is lifting at edges or bubbling, you are building on a failing foundation. Strip it back, assess the plaster, and decide whether a skim coat or localised patching is needed. Also check for damp staining, especially on external walls and chimney breasts. A yellowish-brown tide mark often indicates a historic or active leak. Painting over active damp is pointless — the moisture will push the paint off the wall within weeks. Address the source of moisture first, let the wall dry out fully, and then apply an appropriate stain-blocking primer before any finish coats.
Filling Cracks, Holes and Surface Defects
For hairline cracks in plaster, a flexible filler is your best friend. Products like Toupret or Gyproc Easi-Fill work well, but the key is technique: rake out the crack slightly with the corner of a scraper to give the filler something to key into, dampen the crack with a mist spray, and then press the filler firmly in rather than just skimming over the top. Overfill slightly because every filler shrinks as it dries. On deeper cracks or where plaster has genuinely failed, you may need two applications, allowing each to dry fully before adding the next. For screw holes and picture-hook damage, a quick-drying lightweight filler is fine. Wipe it flush with a damp finger or flexible filling knife and you'll save yourself sanding time. Larger patches — say where a shelf has been removed and rawl-plug holes remain — benefit from a two-stage approach: pack the holes with a deep-repair filler first, then skim over with a finishing filler once that has set. One mistake we see repeatedly is people using the wrong filler for the job. Powdered fillers that you mix with water set by chemical reaction and are much harder once cured, which makes them ideal for high-traffic areas or where you need to screw back into the repair. Ready-mixed tub fillers are softer and easier to sand but can shrink more and are not suitable for large voids. Match the product to the problem and you will get a far better result.
Sanding and Keying the Surface
Once all your filler repairs are dry, sanding is the step that transforms a patchy wall into a smooth, paint-ready surface. Use 120-grit paper for general keying and knocking back filler, moving to 150 or 180 grit for a final pass if you are going for a flat matt or very smooth finish. A sanding block or pole sander keeps things flat — free-handing with a loose sheet of paper creates dips and uneven spots that will show through the finish coat, especially in raking light. Don't forget to key the entire wall, not just the filled areas. Previously painted surfaces that are in good condition still need a light sand to provide a mechanical key for the new paint. Glossy or satinwood surfaces in particular must be flatted back or the new coat simply won't adhere properly. If you are dealing with old oil-based paint that has been built up over many layers, sanding also helps you identify any areas that are flaking or delaminating before they become a problem under your new finish. After sanding, dust removal is critical. Vacuum the wall with a brush attachment or wipe it down with a damp microfibre cloth. Any dust left on the surface will be trapped under your primer or paint and create a rough, gritty texture. In period properties with ornate cornicing or ceiling roses, dust gets trapped in the moulding details — a soft brush and vacuum are essential here.
Priming: When and What to Use
Priming is the step most DIY painters skip and most professional painters insist on. A primer serves multiple purposes: it seals porous surfaces so your top coat goes on evenly, it provides adhesion for the paint system, and specialist primers can block stains, seal knots in timber, or inhibit alkali in new plaster. Skipping it is a false economy because you will end up needing extra top coats to compensate, and adhesion will be compromised long term. For standard plaster walls in good condition, a mist coat of diluted emulsion (roughly 70% paint to 30% water) works well as a primer on new or bare plaster. This lets the plaster absorb the thinned paint and creates a sealed surface for full-strength coats to follow. On previously painted walls where you have done localised repairs, spot-prime the filled areas with a dedicated primer or a thinned coat of your chosen emulsion to even out suction before applying full coats. For stain blocking — where you have had water damage, nicotine staining, or mystery brown spots bleeding through — you need a shellac-based or specialist stain-block primer like Zinsser BIN or Cover Stain. Water-based stain blockers exist but in our experience they are not reliable on heavy staining. Apply the stain blocker, let it dry, and then proceed with your normal undercoat and top coats. On new plaster that is fully dry, an acrylic primer-sealer is a good modern alternative to the mist-coat approach, especially if you are painting with a high-end emulsion that the manufacturer advises against diluting.
Common Preparation Mistakes to Avoid
The most common mistake we encounter on jobs in Westminster is people painting over wallpaper that is past its best. If the paper is firmly adhered across its entire surface with no lifting edges, you can sometimes get away with painting over it, but you must seal it first with a coat of primer to prevent the water in the emulsion from reactivating the old paste and causing bubbling. More often than not, though, you are better off stripping it and starting fresh. Steamer hire is inexpensive and a few hours of effort will save you from a disappointing result. Another frequent mistake is not allowing enough drying time between stages. Filler needs to be bone dry before you sand it, or it clogs the sandpaper and pulls out of the repair. Primer needs to fully cure before you apply top coats — check the manufacturer's recoat time and add extra time in cold or humid conditions, which is relevant in London properties during autumn and winter. Rushing leads to paint defects that are expensive and time-consuming to fix. Finally, neglecting to protect the room properly before you start is a preparation step in its own right. Dust sheets on the floor, masking tape on woodwork you are not painting, and removal of switch plates and socket covers make a huge difference to the quality and speed of the job. Cutting in around a light switch freehand when you could have removed the face plate and painted cleanly behind it is wasted effort and a compromised finish.
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Frequently Asked Questions
Common questions about this topic.
Yes, a light sand with 120-grit paper provides a mechanical key for the new paint to grip. This is especially important on previously glossed or satinwood surfaces. You do not need to sand back to bare plaster — just scuff the existing surface enough to remove the sheen and any nibs.
You can, but the cracks will usually show through the new paint once it dries, especially in raking light from windows. It takes very little effort to fill hairline cracks with a flexible filler and sand them smooth. This five-minute job per crack makes a significant difference to the finished appearance.
New plaster needs to dry fully, which typically takes four weeks in normal conditions but can take longer in winter or in poorly ventilated rooms. You can test by taping a piece of cling film to the wall overnight — if condensation forms underneath, the plaster is still releasing moisture. Once dry, apply a mist coat of diluted emulsion as your first coat.
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