Why isn't simple filler enough for a crack?

Filler sits on the surface with no depth of adhesion. V-cut first so the material enters deep like a wedge.

Which material should I use for interior cracks?

Ready-mixed acrylic filler - it stays slightly flexible. Avoid gypsum/powder: they set rock-hard and re-crack.

How a Crack is Properly Repaired: Opening, Cleaning and Filling

We have all done it: we see a crack in the wall, we take some ready-made filler, we cover it superficially, we apply a coat of paint and we think we are done. A few months later (or with the first change of weather), the crack returns exactly to the same spot, "mocking" our effort.

The truth is raw, but it must be told: you cannot repair a crack if you don't enlarge it first. Surface skim-coating offers no mechanical strength. For the wall to "bind" and the crack not to reopen, you must follow the strict procedure of professionals. Let's look at the 3 basic steps.

Step 1: The Opening (The "V" Shape)

It sounds paradoxical to have to damage your wall in order to fix it, but it is the most critical step. Cracks are usually very narrow at the surface. If you try to put material inside a hairline crack, the filler will only stay on the outside.

What you need to do: Take the sharp corner of a metal spatula (or a special triangular scraper/chisel) and press it into the crack.
The Goal: Drag the tool along the crack, scraping the render, so as to give it a "V" shape. The opening at the surface must become wider than the "bottom" of the crack.

This "V" shape creates a huge bonding surface (a larger "belly") so that the repair material can enter deep inside, act like a wedge and hook firmly on the left and right.

Metal spatula opens crack into V: wide at surface, narrow at bottom - bonding surface

Step 2: Meticulous Cleaning and Priming

After you open the crack, its interior will be full of render debris and fine dust. As we have said, dust is the biggest enemy of adhesion.

Brush dusts inside V-crack, acrylic primer applied with small brush

1. Dusting

Take a dry, stiff brush (or a small broom) and vigorously clean the inside of the "V". You can also use your vacuum cleaner to suck up the stubborn dust.

2. Priming (The Professionals' Secret)

Do not put the filler directly onto the dry, bare render. The old render will immediately absorb the filler's moisture, drying it out before it has time to stick. With a small brush, coat the inside of the crack with an acrylic water-based primer (or at least wet it lightly with a damp sponge). Let it dry according to the manufacturer's instructions.

Step 3: Proper Filling (Building in Layers)

The material you choose makes all the difference. Do not use simple skim-coat filler (putty) or plaster for moving (active) cracks, because they dry, become like "stone" and break.

The Ideal Material: For interior cracks choose an acrylic filler (in a tube or tub) that maintains a slight elasticity. For exterior ones, choose an elastomeric acrylic sealant or polyurethane (paintable).
The Application:
1. Take material with a flexible metal spatula.
2. Press the material vertically into the crack, so it reaches the bottom of the "V" and no trapped air remains.
3. Do not try to fill it all at once if the crack is deep. The filler shrinks (settles) as it dries.
4. Apply a first coat, let it dry (it will usually make a small depression) and then apply the second, final coat, so it comes "flush" with the rest of the wall.

Flexible spatula presses acrylic filler into V, first and second coat with dip between

The Final Finish

After the repair material has fully dried, you will notice that its surface is slightly rough or protrudes slightly.

Sand lightly with a fine sandpaper (e.g. number 150 or 180) to "erase" the boundaries of the repair with the old wall.
Dust off, apply a little primer locally over the patch, and your wall is ready for painting!