Repairing cracks before painting: why filler doesn't solve everything

Many homes in Portugal have cracked walls, especially in older buildings or apartments that have gone through several cycles of temperature and humidity changes. The temptation is simple: buy some filler, cover the crack, sand it down, and paint over it. But the result isn't always long-lasting. After some time, the mark reappears or, worse still, the crack opens again.

The reason is straightforward: the current condition of the walls (whether they have cracks, humidity, mould, or old paint) directly influences the effectiveness of any repair. Not all cracks behave the same way, and not all repair products work for every type of damage.

Not all cracks are the same and each requires a different approach

Cracks don't appear for the same reason. Some are superficial and result from paint contraction or poorly applied plaster. Others are deeper and follow the wall structure, often linked to building movement or humidity problems.

It is important to identify cracks (stable or moving), holes, nails, cavities, and other elements1. A stable crack is one that no longer moves, is dry, and can be safely repaired. A moving crack continues to open and close depending on temperature, humidity, or structural settlement. In the latter case, applying common filler won't solve it, because the crack will break the repair material again.

In apartments in Porto, Coimbra, or humid areas like the Aveiro coast, it's common for walls to have cracks linked to changes in relative humidity throughout the year. In these cases, repair requires not only the right product but also certainty that the underlying cause is under control.

Crack preparation is as important as the product chosen

Before applying any filler, the area must be prepared. Slightly widen the crack with a spatula (to receive filler). Apply repair filler, which may be filler paste, fine filler, or filler specific to cracks. Allow to dry completely (do not skip this step!). Sand to level2.

This step of widening the crack may seem counterintuitive, but it's essential. A narrow, tight crack won't allow the filler to penetrate well, and the result is a superficial repair that comes loose easily. By slightly opening the crack with the tip of a spatula or a craft knife, space is created for the product to grip in depth.

After application, complete drying is mandatory. Sanding before the filler is completely dry compromises adhesion and leaves the surface uneven. Only after it's dry and sanded is the wall ready to receive paint.

The type of filler makes a difference depending on the size and location of the crack

Not all repair fillers have the same composition or flexibility. For fine, superficial cracks, a finishing filler usually suffices. For deeper cracks or areas subject to movement, a filler with greater elasticity or even a fibre-reinforced filler is needed.

In exterior areas with significant damage, reinforced fillers should be used, such as universal exterior pastes, mortars, or resistant plasters. These products have high resistance to extreme temperatures and humidity. Fillers with fibreglass or resin: allow repair of structural damage or areas with heavy wear. The same principle applies to interiors where there is risk of future movement.

In practice, for most interior walls in apartments in Lisbon, Setúbal, or Faro, a standard repair filler is sufficient, provided the crack is stable and dry. But if the wall shows signs of humidity, filler alone won't solve the problem. The source of humidity must be treated before proceeding with repair and painting.

Painting over a poorly repaired crack only delays the problem

Many people believe that paint will hide the crack or somehow reinforce the repair. In fact, the opposite happens. Painting over it doesn't solve anything, ever. If the base isn't well prepared, the paint will dry, shrink slightly, and reveal exactly where the crack was.

Moreover, if the filler hasn't adhered well or if the crack is still moving, the paint will crack in the same spot. The final result is a visible line on the painted wall, often more noticeable than the original crack.

When a professional sees the space through photos and videos or during a visit, they can identify whether cracks are active, whether there are signs of humidity, or whether the wall is safe to repair. This initial assessment determines the type of intervention needed and the type of product to use. On our platform, we always ask for photos and videos of surfaces before closing any quote, precisely to avoid surprises during the work.

When it's worth repairing cracks before painting and when it's better to go deeper

If the wall has few fine, stable cracks with no signs of humidity, repair with filler and paint will work. But if the cracks are numerous, deep, or follow structural joints, it may be a sign that the wall needs more than a layer of filler.

In old buildings in central Porto or in Braga, it's common for lime mortar or old plaster walls to show settlement cracks. In these cases, the solution involves evaluating the general condition of the wall before proceeding with spot repairs. Repairing crack by crack may work in the short term, but if the structure continues to move, the problem will repeat.

When the quote is based on a real assessment of the wall (not just square metres), the professional can forecast the amount of filler, the type of preparation, and the number of paint coats needed for a lasting finish. This type of planning avoids extra costs during the work and ensures the final result matches expectations.

In the end, repairing cracks before painting isn't just a technical matter. It's a question of understanding how the wall behaves, choosing the right product, and applying each layer with adequate drying time. When these steps are respected, the paint looks smooth, uniform, and without visible marks.