Technical & Product Guidance
Damp and Stain Treatment Before Painting
Every decorator has seen it: a homeowner has painted over a damp patch or stubborn stain, only for it to bleed straight through the fresh paint within weeks. Treating the symptom without addressing the cause is the most common mistake in decorating, and it wastes both time and money. This guide explains how to properly diagnose and treat damp, water stains, nicotine discolouration, and other surface problems before any paint goes on.
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Damp and Stain Treatment Before Painting
Diagnosing the Source of Damp
Before you can treat damp effectively, you need to understand where the moisture is coming from. There are three main categories: penetrating damp (water coming through the wall from outside), rising damp (groundwater being drawn up through the wall by capillary action), and condensation (moisture from indoor air depositing on cold surfaces). Each has different characteristics and different solutions, and misdiagnosis is extremely common — particularly by companies that make their living from installing damp-proof courses. Penetrating damp typically appears on external walls, often linked to specific defects: a leaking gutter, cracked render, failed pointing, or a flat roof junction. The damp patch may move or worsen during heavy rain and improve during dry spells. In Westminster properties, common culprits include failing stucco, deteriorated window seals, and blocked or leaking parapet gutters. Fixing the external defect is essential — no amount of internal treatment will solve a problem that is being continuously fed from outside. Rising damp is far less common than the damp-proofing industry would have you believe. Genuine rising damp creates a tide mark typically no more than one metre above floor level, with salt deposits (efflorescence) on the wall surface. However, the same symptoms can be caused by condensation at floor level, bridged damp-proof courses, or high external ground levels, all of which have simpler and cheaper solutions. Before accepting a diagnosis of rising damp and commissioning an expensive chemical damp-proof course, get an independent assessment from a chartered surveyor who does not sell damp treatments.
Treating Water Stains and Tide Marks
Once the source of moisture has been identified and fixed, and the wall has had time to dry out — which can take weeks or months depending on the severity — you can treat the visible staining. Water stains leave behind dissolved minerals and salts that create the characteristic yellowish-brown marks. These stains are not just superficial; they are carried into the plaster by the water and will bleed through standard emulsion paint no matter how many coats you apply. The professional solution is a stain-blocking primer. Shellac-based products like Zinsser BIN are the gold standard for stain blocking — they seal virtually any stain, including water marks, nicotine, soot, crayon, and even fire damage. Apply one coat by brush or roller, allow it to dry (typically 45 minutes), and then check whether the stain is fully sealed. Heavy staining may require a second coat. Once the stain blocker has cured, you can apply your normal emulsion over the top with confidence that nothing will bleed through. Water-based stain-blocking primers exist and are marketed as low-odour alternatives. In our experience, they work adequately on light staining but are not reliable on heavy water damage or persistent marks. If you are dealing with anything more than a faint mark, use a shellac or oil-based stain blocker. The odour during application is strong — ventilate the room thoroughly — but it dissipates within a few hours. The alternative is watching your new emulsion turn yellow over the following weeks, which is considerably more annoying than a few hours of strong primer smell.
Dealing With Nicotine and Smoke Damage
Nicotine staining from years of indoor smoking creates a pervasive yellowish-brown film on every surface in the room — walls, ceilings, woodwork, and even the inside of cupboards. This is one of the most stubborn staining problems in decorating and it requires thorough treatment. Standard emulsion will not contain nicotine staining; the tar compounds in cigarette smoke are soluble in water-based paint, and they will migrate through multiple coats of emulsion, turning your fresh white walls a sickly yellow within weeks. The treatment process starts with washing all surfaces with a sugar soap solution or a dedicated nicotine cleaning product to remove as much of the surface residue as possible. This is unpleasant work — the brown water that runs off the walls shows just how much contamination is present. After washing and allowing the walls to dry, apply a full coat of shellac-based stain blocker to all affected surfaces. Do not spot-prime — nicotine staining is everywhere, even where it is not obviously visible, and areas you miss will show through. For severe nicotine contamination — typically properties where someone has smoked heavily indoors for many years — a single coat of stain blocker may not be sufficient. Apply a second coat and test the result before proceeding with emulsion. In extreme cases, the ceiling may need a specialised treatment or even re-skimming if the staining has penetrated deep into the plaster. Woodwork that has absorbed nicotine may need sanding back and priming with an aluminium primer before repainting. The effort is significant, but cutting corners with nicotine treatment is a guaranteed route to disappointment.
Mould Treatment and Prevention
Mould on internal walls is a moisture problem, not a paint problem, and treating it with mould-resistant paint without addressing the underlying cause is a temporary fix at best. Mould needs moisture and organic material to grow. In residential properties, the moisture typically comes from condensation — warm, humid air from cooking, bathing, and drying clothes meets a cold wall surface and the water vapour condenses, creating the wet conditions that mould thrives in. The most effective long-term solutions are improved ventilation and, where possible, better thermal insulation of cold walls. For immediate treatment before painting, kill the existing mould with a proprietary mould wash or a dilute bleach solution (one part household bleach to four parts water). Spray or sponge the solution onto the affected area, leave it for at least 30 minutes, then wipe off with a clean damp cloth. Allow the wall to dry fully. Do not just paint over visible mould — the spores are alive and will grow through new paint surprisingly quickly. The mould must be killed before any coating is applied. Once the mould is treated and the wall is dry, apply a mould-inhibiting primer or use a paint that contains anti-mould additives for the finish coats. Products like Zinsser Mould Killing Primer or Dulux Trade Sterishield are designed for this purpose. These products slow mould regrowth by creating a hostile surface environment, but they are a supplement to — not a substitute for — addressing the moisture source. If you paint with anti-mould products but the room continues to suffer from condensation, the mould will eventually return. Install or upgrade extractor fans, ensure adequate ventilation, and consider whether the wall needs insulating to raise its surface temperature above the dew point.
Salt Deposits and Efflorescence
Efflorescence — the white crystalline deposits that appear on masonry and plaster surfaces — is caused by water dissolving soluble salts within the wall and depositing them on the surface as it evaporates. It is common on new plaster, new brickwork, and walls that have experienced damp penetration. The salts themselves are not harmful, but they indicate moisture movement through the wall and they will disrupt any paint applied over them. The treatment for efflorescence is mechanical, not chemical. Brush the salt deposits off with a stiff dry brush and do not wash the surface with water, as this simply re-dissolves the salts and drives them back into the wall to reappear later. Keep brushing them off as they appear — on new plaster, this process can take several months as residual construction moisture works its way out. Once the salts stop appearing, the wall has reached a stable moisture content and is ready for decoration. If you need to decorate before the efflorescence has fully resolved — perhaps on a new-build handover or after emergency repair work — apply a salt-resistant primer or a diluted PVA solution to create a barrier between the salts and the paint. This is a temporary measure and the primer should be overcoated with emulsion to provide a decorative finish. Be aware that if significant moisture continues to move through the wall, the salts may eventually push through the primer as well. The only truly reliable solution is to wait until the moisture movement has stopped and the efflorescence has ceased naturally.
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Frequently Asked Questions
Common questions about this topic.
Never paint over active damp. The moisture will prevent the paint from adhering and will push the new coating off the wall. Fix the source of the damp first, allow the wall to dry out fully — which may take several weeks — then treat any staining with a stain-blocking primer before painting.
Shellac-based primers like Zinsser BIN are the most effective stain blockers for water damage. They seal virtually any stain in one or two coats and dry within an hour. Water-based stain blockers are available but are less reliable on heavy staining. For serious water damage, always use a shellac or oil-based product.
Mould returns because the underlying moisture problem has not been resolved. Mould-resistant paint slows regrowth but cannot prevent it indefinitely if condensation or damp continues. Address the root cause — improve ventilation, install extractor fans, reduce indoor humidity sources, and consider insulating cold walls to prevent condensation forming.
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