Painting a facade with active damp or mould residue without properly preparing the surface is one of the most common mistakes in exterior painting work. The result is usually disappointing: stains that reappear within weeks, paint that peels off, or efflorescence that seeps through the new coat. Before opening the first tin of paint, it is essential to understand what type of damp affects your facade and how to treat it correctly.
Why damp and mould appear on facades
Facades are exposed to varied conditions: from constant humidity in coastal areas to extreme temperature swings between day and night in many regions. These conditions favour three main types of damp.
Infiltration damp appears when water penetrates through cracks, deteriorated window joints, or blocked gutters. Stains usually appear in specific spots and worsen on rainy days, sometimes accompanied by flaking paint and salt deposits.
Condensation damp is more common than you might think on poorly insulated facades. It occurs when a cold wall comes into contact with warmer air, creating water droplets that encourage mould growth, usually black or greenish in colour, especially on north-facing walls or areas with poor ventilation.
Capillary damp mainly affects the lower part of facades in older buildings. Water from the ground rises through the material's pores and can reach considerable heights, leaving a distinctive horizontal pattern with white efflorescence (mineral salts) and progressive deterioration of the coating.
Diagnosis before you start: what type of damp do you have
Before buying any products, take time to inspect your facade. Locate where stains appear, when they worsen (after rain? in winter?) and whether visible mould is present. Take photos and document the affected areas. This will help whether you decide to do the work yourself or need to get quotes from a professional.
If stains are in the lower part of the facade, running horizontally with white salt crystals, it is likely capillary damp. This type of moisture cannot be solved with paint alone: it requires waterproofing the foundations or installing chemical barriers. Painting over it only masks the problem temporarily.
If stains appear after rainy days, in specific areas near windows, eaves, or downpipes, you are dealing with infiltration. Here too, painting without first addressing the cause will not help: you must first seal cracks, repair joints, and ensure water cannot enter again.
If stains are superficial, with black or green mould, on north-facing walls or in cold corners, it is likely condensation. In this case, thorough cleaning and the use of anti-condensation and anti-mould paints may be sufficient, provided the wall is dry before application.
Surface preparation: the step you cannot skip
Once you have identified the type of damp and addressed the source (remember: if the problem persists, paint will not fix it), it is time to prepare the surface. This step makes the difference between work that lasts for years and work that fails within months.
Start by removing all visible mould residue. Use a specific fungicidal cleaner, applying it with a sprayer, sponge, or brush, and leave it to work for 15 to 20 minutes before rinsing. If there are areas with heavy ingrained mould, you will need a brush. Ensure the surface is completely clean and dry before continuing.
If old paint is loose, flaking, or lifting, remove it completely. Sand the areas that remain to help the new paint adhere well. If there is white efflorescence (mineral salt stains), remove it with a metal brush and water, and allow to dry thoroughly. Painting over salts is pointless: the stain will seep through the new coat within weeks.
In cases of stubborn damp stains that have already penetrated the coating, many professionals recommend applying a stain-blocking primer before the finishing paint. This type of product isolates the stain and prevents it from reappearing. Do not apply standard plastic paint directly over an old damp stain: the problem will return.
Choosing the right paint and applying it correctly
Once the surface is clean, dry, and prepared, it is time to choose the paint type. For facades with a history of damp, look for products specifically formulated for exterior use with breathable properties, mould resistance, and, if the problem has been condensation, with anti-condensation or thermal-insulating properties.
Quality acrylic exterior paints are a good foundation, but if your facade is in a very humid area or north-facing, it is worth investing in paint with fungicidal additives. These products not only cover, but actively prevent mould from returning.
As for application, do not skimp on coats. Apply the first coat well, respecting the drying time recommended by the manufacturer (typically 24 hours), and a second coat to reinforce protection. Work in suitable weather conditions: avoid days with high humidity, temperatures below 5 degrees, or applying in full sun on hot surfaces. Proper drying is key to the paint developing all its properties.
Remember that our platform always requests photos and videos of surfaces before finalising any quote. This allows professionals to assess the true condition of the facade, identify possible damp problems, and adjust both the type of treatment and the final price, avoiding surprises during the work.
