Identify and treat moisture before repainting

You have noticed dark marks in a corner, paint bubbling, or a persistent water stain on the wall. Before repainting, it is essential to understand where the moisture is coming from. Treating only the symptoms guarantees that the problem will return quickly, often with more extensive damage and additional costs.

This article helps you distinguish between the three main families of moisture problems (condensation, saltpetre, infiltration), make the right diagnosis, and choose the appropriate treatment before any painting work.

The three types of moisture that damage your surfaces

Condensation: when humid air meets a cold wall

Condensation appears when humid air meets a surface whose temperature drops below the dew point.1 This phenomenon is very common in poorly ventilated or insufficiently heated homes. Water stains and mould spots are the main signs of condensation in homes, most often visible in cold, poorly ventilated places: ceiling corners, wall angles, wardrobe interior walls.

This harmful process starts as soon as the humidity level in the air exceeds the critical threshold of 60 %.2 The upper corners of rooms, north-facing walls, and thermal bridges are particularly exposed. In Paris, Lyon or Lille, old homes with single glazing often see this problem worsen in winter, when intermittent heating allows surfaces to cool down.

Saltpetre: moisture rises that deposit salts

Saltpetre looks like a white or greyish powder, sometimes slightly glossy, which reappears in the same spot after cleaning.3 In a home, it often indicates that water is circulating through the wall, dissolving mineral salts, then leaving them to crystallise on the surface.

This phenomenon mainly affects the lower walls in cellars, ground floors of old houses in Bordeaux, Toulouse or Marseille, and poorly drained basements. Unlike condensation, saltpetre does not disappear by ventilating: you must treat the source of moisture (capillary rises, failed drainage) to eliminate it permanently.

Infiltration: water that penetrates from outside

Infiltrations come from cracks in the facade, degraded joints, blocked gutters, or failed waterproofing. They manifest as localised stains that worsen after rain, often near windows, under window sills, or in angles of exposed walls.

A localised stain after rain points more towards infiltration than condensation. In Strasbourg, Rennes or Nice, facades exposed to prevailing winds and driving rain should be monitored regularly. Repainting without repairing the crack or joint will only mask the problem temporarily.

Diagnose the real cause before any work

Simple tests you can do at home

A simple test involves placing a sheet of aluminium against the suspect wall for 24 hours: if the moisture is found between the wall and the sheet, the problem comes from inside the wall, whereas if the moisture appears on the external face, it is indeed condensation. This test allows you to quickly determine whether moisture is coming from outside (infiltration, capillary rise) or from indoor air (condensation).

Also observe when the marks appear:

• Marks worse in the morning, in corners or near windows: likely condensation.
• White deposit that always reappears in the same place, often at the bottom of wall: saltpetre.
• Stain that spreads after heavy rain or near a gutter: infiltration.

When to call a professional to measure humidity

If doubt persists, a surface or contact hygrometer measures the wall's humidity level. A humidity tester (surface hygrometer, 20 to 40 €) allows you to check that the support's humidity level has dropped below 5 %. Below this threshold, the surface can receive paint without risk of bubbling or rapid mould growth.

If measurements show a level above 5 %, or if several walls are affected, have a professional come in for a full diagnosis. Our platform validates that all painters are properly registered with a SIRET, ensuring you work with properly qualified professionals.

Treat each type of moisture with the right method

Condensation: ventilate, heat and insulate

A clogged mechanical ventilation unit, a poorly heated room, or furniture pressed against the wall can worsen the problem. Start by checking that ventilation grilles are not blocked, that extraction outlets are working, and that air circulates well in each room.

Slightly increase heating in rooms where condensation appears, especially bedrooms that are little used or bathrooms. Move furniture away from cold walls to allow air to circulate. If the problem persists, consider installing mechanical ventilation or replacing single glazing with double glazing.

Saltpetre: clean then block the salts

Eliminate saltpetre by vigorous brushing and scraping. Apply our ANTI-SALTPETRE treatment to permanently eradicate saltpetre. The anti-saltpetre treatment penetrates the surface, blocks the rise of salts, and hardens crumbly plaster.

Remove remnants of wallpaper and old paint; eliminate saltpetre deposits, mould, and other whitish salts by brushing. We recommend then rinsing with acidified water (10%), rinsing again, and allowing to dry for a few days. This preparation step is essential for the treatment to adhere properly.

Warning: Before applying an anti-saltpetre treatment, always identify the source of the moisture: treating only the effects without treating the cause will make the problem recurring. If saltpetre keeps returning, you probably need to drain the soil around the foundations or install a moisture barrier by injection.

Infiltration: repair the building envelope

Infiltrations require repair to the facade, joints, or roof before any interior painting. Have gutters, rainwater downpipes, window frame sealing, and the condition of external renders checked.

Before any surface treatment, it is essential to stop water intake. Infiltrations require: masonry work, crack repair, joint and render work. Once the water source is cut off, let the wall dry completely (this may take several weeks depending on thickness and initial moisture level) before repainting.

Choose the right paint once the problem is solved

Paints suitable for damp rooms

For a bathroom or kitchen, choose a class 1 or 2 paint for humidity resistance (according to the NF EN 13300 standard). These paints contain antifungal agents that create a protective barrier against microscopic fungi.

Our platform always requests photos and videos of surfaces before finalising a quote, allowing the painter to recommend the right product and anticipate preparation needs.

Breathable surfaces and microporous finishes

Make sure the coating you choose is microporous (in other words: "breathable") so the paint retains its properties. An airtight paint traps moisture in the wall and worsens damage in the medium term.

Prefer matte or satin finishes in damp rooms: they better mask small irregularities and offer better permeability to water vapour. Glossy finishes are reserved for areas exposed to splashes (around the bath, sink), where washability matters more than breathability.

Application conditions for lasting results

Application should ideally be done at temperatures between 15 and 25°C, with ambient humidity below 70 %. These optimal conditions enable drying and allow active agents to set properly in the paint film.

Never paint a wall that is still damp, even with so-called "anti-moisture" paint. Never paint directly over mould. Before any painting work, you must identify the source of the moisture (condensation, infiltration, capillary rises, lack of ventilation...), clean contaminated surfaces, and treat the surface with an appropriate product.

Properly prepare surfaces before painting

Clean mould marks and deposits

First, you must start by removing any mould present. Use bleach solution. Let it work for 15 to 30 minutes, then rinse. Wear gloves and ventilate the room well during this operation.

For saltpetre, brush vigorously when dry rather than soaking the wall. Vacuum the dust instead of dispersing it into the room. Remove all plaster or paint that is no longer adhering.

Wait for the surface to be truly dry

This patience is essential for the treatment to be lasting. The drying time depends on wall thickness, initial humidity level, and room ventilation. A thick wall after infiltration may take several weeks.

Use a contact hygrometer to check that the level has indeed dropped below 5 % before starting painting. If you are pressed by a sale or rental timeline, tell the painter: they can recommend an accelerated drying treatment or a realistic timeline that avoids unpleasant surprises.

Apply a primer suitable for the treated surface

After anti-saltpetre or anti-moisture treatment, apply a specific primer that adheres to the treatment product and prepares the surface for the finishing paint. This intermediate layer improves adhesion and evens out the surface's absorption.

On our platform, all job details (preparation, products used, number of coats) are visible online to the client, the professional, and the platform. This ensures everyone sees the same scope and prevents misunderstandings during work.

Organise your work without redoing the same room twice

Logical sequence: diagnosis, treatment, drying, painting

1. Diagnosis: identify the cause of moisture (condensation, saltpetre, infiltration).
2. Treatment of the cause: ventilation, drainage, facade repair, anti-moisture injection.
3. Cleaning and surface treatment: eliminate mould and saltpetre, then apply an appropriate product.
4. Complete drying: wait for the surface's humidity level to drop below 5 %.
5. Painting: primer then two coats of finishing paint, in the right weather conditions.

If you reverse the steps or skip drying, the paint will bubble quickly and you will repaint in a few months. This sequence is valid in Paris, Marseille, Strasbourg, or any other city in France, regardless of local climate.

Timeline and overall cost of a complete treatment

Payment on our platform is done in two parts: 50 % at the start (to reserve the date and buy materials) and 50 % at the end, after verification of work. This formula protects you in case of disagreement about quality or scope completed.

Avoid quotes based only on surface area

When a quote is calculated per m², keep in mind it remains indicative until the painter has seen the real condition of the surfaces. Two 20 m² walls can require very different preparation times if one has saltpetre, cracks, or residual moisture.

Our platform systematically requests photos and videos before finalising a quote, allowing the price to be adjusted based on the condition found and avoiding unpleasant surprises during work. The professional can assess treatment needs, primer, and preparation time, whether from images or during an on-site visit. Both approaches work the same way and provide a reliable quote.