How to Prepare Exterior Walls Before Painting: Wire Brush, Fungicidal Wash & Stabiliser

Why Preparation Determines Everything

The single most important factor in the longevity of any exterior paint job is the quality of preparation. A premium paint applied over a poorly prepared surface will fail within two to three years. A mid-range paint applied over a properly prepared surface will last seven to ten. This is not a marginal difference — it is the difference between value and waste.

London's exterior walls face a demanding environment. Pollution deposits, biological growth (algae, lichen, moss), wind-driven rain, freeze-thaw cycles, and UV radiation all attack painted surfaces. The preparation process addresses each of these threats, creating a clean, stable, well-bonded substrate that allows the paint system to perform as intended.

This guide covers the preparation steps for the most common exterior wall types found across London: render, masonry (brick and stone), pebbledash, and concrete.

Step One: Assessment

Before any physical work begins, assess the wall carefully.

Paint condition. Is the existing paint intact but chalky? Flaking in patches? Peeling in sheets? Or has it been painted over so many times that the surface is thick and uneven? Each condition requires a different approach.

Substrate condition. Tap the wall in several places. Hollow sounds indicate render that has debonded from the substrate — this is known as blown render and will need cutting out and patching before painting. Check for cracks, particularly around windows, at corners, and where extensions meet the original building.

Biological growth. Green algae, black mould, and orange lichen are common on London's exterior walls, particularly on north-facing and sheltered elevations. These must be killed and removed before painting — painting over biological growth traps it beneath the film, where it continues to develop and eventually pushes the paint off.

Damp. Check for signs of moisture ingress: tide marks, salt deposits (white crystalline patches known as efflorescence), or persistently damp areas. Painting over damp walls traps moisture and accelerates deterioration.

Step Two: Wire Brushing and Scraping

The first physical step is mechanical removal of loose material.

Wire brushing. A stiff wire brush removes loose and flaking paint, powdery surface material, and light biological growth. Work systematically across the wall, paying particular attention to areas around window frames, beneath cills, and along horizontal ledges where moisture accumulates.

For larger areas, a cup wire brush attachment on an angle grinder speeds the process considerably. Wear eye protection and a dust mask — old exterior paint may contain lead, particularly on properties painted before the mid-1990s.

Scraping. Where paint is peeling in sheets or lifting at edges, a broad scraper removes failed material back to a sound edge. Do not try to scrape paint that is still firmly adhered — the goal is to remove what is loose, not to strip the wall bare.

Power washing. A pressure washer can clean large wall areas efficiently, but use caution. Too high a pressure damages render and mortar joints. A medium-pressure setting (around 100-120 bar) with a wide fan nozzle is appropriate for most masonry walls. Allow the wall to dry thoroughly after washing — typically two to five days depending on the weather and wall orientation.

Step Three: Fungicidal Wash

After mechanical preparation, treat the entire wall with a fungicidal wash. This step is essential even if biological growth is not immediately visible — spores are present on virtually every London exterior wall and will germinate under a fresh paint film if not treated.

Application. Dilute the fungicidal wash according to the manufacturer's instructions (products such as Barrettine Mould and Mildew Cleaner, Dulux Trade Weathershield Fungicidal Wash, or Ronseal Anti-Mould Concentrate are all effective). Apply by brush, roller, or low-pressure sprayer, ensuring complete coverage.

Contact time. Leave the wash on the surface for the period specified — usually a minimum of 24 hours and ideally 48. Do not rinse off. The active ingredients continue working during this period, killing embedded spores that brushing alone cannot remove.

Removal of residue. After the contact period, brush off any remaining dead growth with a stiff bristle brush (not wire — you do not want to scratch the substrate at this stage). Some products specify rinsing with clean water after the contact period; follow the product instructions.

Step Four: Stabilising Solution

If the existing surface is friable (crumbly), chalky, or powdery — as is common on older render, weathered concrete, and previously painted surfaces where the paint has broken down — apply a stabilising solution before priming.

What it does. Stabilising solution is a clear, penetrating liquid that soaks into the loose surface material and binds it together as it dries. This creates a firm, cohesive layer that paint can grip, rather than sitting on top of powder that will eventually let go.

Application. Apply liberally by brush, working the solution into the surface. On very absorbent surfaces, two coats may be needed — the first will soak in almost immediately, and the second builds the binding layer. Allow to dry fully, which usually takes 24 hours.

When not to use it. Stabiliser is not a substitute for removing genuinely loose material. If render is debonded and hollow, stabiliser will not reattach it. Cut out the failed section and patch with new render.

Step Five: Crack and Defect Repair

With the surface clean, treated, and stabilised, address any cracks and defects.

Fine cracks (hairline to 2mm). Fill with a flexible exterior filler. Rigid fillers crack again as the wall moves with temperature changes. Products such as Toupret Fibacryl or Everbuild Flexible Decorators Filler cope with seasonal movement and can be painted over once cured.

Larger cracks and holes. Rake out loose material, dampen the area, and fill with an exterior-grade repair mortar. For render repairs, match the texture of the surrounding surface as closely as possible — a smooth patch on a textured wall will be visible through the paint.

Around window and door frames. Gaps between frames and masonry should be sealed with a paintable exterior sealant. Cracked or missing pointing around frames is a common entry point for water and should be repointed before painting.

Step Six: Priming

The final preparation step before topcoating is priming any bare or repaired areas.

On bare render and masonry. An alkali-resistant primer is essential. Fresh render and concrete are highly alkaline, and standard primers can be attacked by the alkali content, leading to saponification (a soapy breakdown of the paint film). Dulux Trade Weathershield Stabilising Primer, Johnstone's Stormshield Alkali-Resistant Primer, or similar products neutralise this risk.

On bare timber (fascias, window frames). An alkyd wood primer, followed by an undercoat if a gloss topcoat is planned.

On repaired areas. Prime filled and patched areas to equalise their porosity with the surrounding surface. Without priming, repaired areas absorb paint differently, producing visible flashing — areas of different sheen or colour intensity.

Timing and Weather

Exterior preparation is weather-dependent. Ideally, carry out the work during a settled dry spell. The wall should be dry at the start of preparation and remain dry through the stabiliser and primer stages. In London, the most reliable windows for exterior work are typically May through September, though dry spells in spring and autumn can also be productive.

Avoid preparing walls during or immediately after rain. Moisture trapped in the wall by subsequent coatings will cause adhesion failure. If rain interrupts the process, allow the wall to dry for at least 48 hours before resuming.

Professional Exterior Preparation

Thorough preparation is labour-intensive and unglamorous, but it is what separates a professional exterior paint job from an amateur one. Our team prepares and paints exterior walls across Belgravia, Chelsea, Kensington, Pimlico, Fulham, and the wider London area, following a methodical preparation process on every project. The result is a paint finish that looks clean, performs well, and lasts.