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The success of a painting, whether glossy, satin or matte, depends to a large extent on the preparation of the support. This operation, unavoidable when it comes to old walls, requires only common equipment, and a little patience...

Filling a crack on a wall

Practical advice

Always work with perfectly clean and sharpened painter's knives: asperities at the end of the thread would leave furrows on the coatings.
● After applying the paint, check the flatness of the substrate with a grazing light. With a solid color, the defects will appear.

First, cover the floor with tarpaulin or newspaper, secured with masking tape. In pencil, circle the most discreet imperfections so that no one escapes you, especially on white backgrounds. Spot them with a strong source of grazing light.
● Identify the nature of the defects: micro-cracks or cracks result from temperature variations. In most cases, it is merely to fill them up, without preparation, with the smoothing plaster. The other cracks can be caused by a wrong dosage of the plaster or mortar layer, too fast drying, a lack of application, etc. As for the disorders described as "structural", generated by the movements of ground, they require the intervention of professionals.

The preparation

Expand "V" cracks less than 1 cm wide with a triangular scraper. Then, dust off with a brush or a semi-hard brush to facilitate the adhesion of repairs. Similarly, all friable parts must be removed. Enlarge the small holes to encourage the coating to stick. In any case, finish with a vacuum. For large bleeds and gaps, flush them to the bottom using one of the tips of the scraper. Using a fine brush, moisten the bottom of the crack to avoid the absorption of water contained in the coating. Note: on old and powdery plasters, the application of a hardener is recommended.

Refilling

Filling and smoothing coatings are ready to use or to spoil. To treat thin cracks, an acrylic cartridge putty can replace traditional plaster. It will be particularly appreciated to treat defects flush with the joinery. Its flexibility allows it to follow the dimensional variations of the wood.
● For the filler, apply the paste with the painter's knife, perpendicular to the cracks, pressing strongly. Then quickly return to the passes, in the direction of the length, in order to remove the surplus. When only one layer is sufficient, smooth carefully, knowing that the coatings are drying during drying, much more in thick layers. After drying, sand evenly with a fine sheet-abrasive mounted on a shim. If the cracks are large or may change, tap a band of calico in a first layer of filler. On the other hand, treat the deepest holes with one or two loads, the first one possibly armed with tow. In any case, finish after hardening with a careful pass of smoothing compound.

Open the cracks

Open the cracks

After protecting the floor, open the V-shaped crack with the triangular scraper. Carefully brush off the dust and moisten the substrate so that it does not absorb water from the plaster.

Acrylic putty for small cracks

Acrylic putty for small cracks

To be reserved for small cracks or joints of joinery, the acrylic putties are smoothed in a single movement of blade or with the finger wet with soapy water. Beware of silicone sealants, most can not be painted.

Spoil the coating

Spoil the coating

Rub the coating (by pouring the powder into the water) in a small trough, until you get a creamy consistency. Beware of lumps, work long and pressing with the blade.

Apply and smooth

Apply and smooth

Load the crack with small strokes, then smooth in one stroke in the direction of the length. Then avoid reworking the applied coating, you would only deteriorate it.

Sand and wipe

Sand and wipe

Dry plaster, sand in circles (use a sanding block to work flat). Then vacuum the plaster dust or wipe with a damp cloth.

Clear the big cracks

Clear the big cracks

For large cracks, clear to support. Use a sharp scraper and, if necessary, a chisel. Also scrape the old paint on which the plaster would adhere badly.

Stuff with plaster

Stuff with plaster

Then fill with filler by pressing hard on the blade. Do not try to smooth the repair on the first try, several passes are usually necessary.

Integrate a strip of calico

Integrate a strip of calico

The integration of a strip of calico is essential for large or progressive cracks. As soon as the first coat is applied, sink the strip into the fresh coating.

Bridging imperfections

Bridging imperfections

The smoothing coating, with its very fine grain, makes it possible to fill the last imperfections up to 2 or 3 mm thick. Use a blade wider than the crack, and carefully pull


Video Instruction: How to fix cracks in ceilings and walls