Hairline cracks in plaster before painting: are they a problem?
The Abimeister team · Updated · 5 min read
You are about to paint a wall and notice a web of fine cracks in the plaster that you never paid attention to before. Most of the time this is completely ordinary and has nothing to do with the wall failing. But a crack is also the plaster telling you something, and it helps to read what before you cover it with paint and lose sight of it for years. This guide walks through what a normal shrinkage crack looks like, which cracks along joints are expected, which patterns mean you should call someone, and how to deal with the harmless kind before you paint.
What an ordinary shrinkage crack looks like
Plaster and the skim coat over it shrink slightly as they dry, and in older Tallinn flats and wooden houses this can happen years after the original work if the wall has ever been damp and dried out again. The result is usually a fine, hair-thin crack, often in a rough map or star pattern, with no difference in level either side. You cannot fit a fingernail into it, and it does not run in a single straight line across the whole wall.
These cracks tend to cluster where the plaster is thinnest or where it was applied in a hurry, near corners, above door frames, and around old repairs. They do not grow if you watch them over a few weeks, and pressing either side of the crack does not make one edge dip below the other. This is the kind of crack you fill and paint over without worrying further.
Cracks along joints and board edges
A straight crack that follows a perfectly straight line, especially one that reappears after being filled before, usually marks a joint underneath: between two plasterboards, at the corner where a partition meets a solid wall, or along an old repair line. These move slightly with temperature and humidity even when the building itself is entirely sound.
In panel buildings from the Soviet period this is very common along the joints between concrete panels, which telegraph through the plaster in a dead straight line. It is worth filling with a flexible filler rather than a rigid one, because a rigid filler will simply crack again along the same line within a year or two. This is cosmetic maintenance, not a sign of a structural problem.
When a crack points to something more serious
A few patterns are worth taking seriously rather than just filling over. A crack wider than a couple of millimetres, one where you can feel a clear step between the two sides, or one that runs diagonally from a corner of a door or window frame all suggest movement in the structure behind the plaster, not just the plaster itself.
Cracks that keep reopening within weeks of being filled, that run through more than one wall or across the ceiling into the wall, or that appear together with a door or window that has started sticking are also signs of ongoing movement rather than old, settled shrinkage. In an apartment block this is a matter to raise with the korteriühistu, since it may involve the building rather than just your flat.
We do not touch load-bearing walls, and if a crack looks structural the right step is a structural engineer or the local building authority, not filler and paint. What we can help with is everything short of that: assessing whether a crack looks like ordinary shrinkage, and putting the wall right once you know it is.
- Crack wider than about 2 mm or with a visible step between the edges
- Diagonal crack running from the corner of a door or window frame
- Crack that reopens within weeks of being filled
- Crack running across the ceiling and down into the wall
- Door or window that has started sticking around the same time
- Any of the above in a building you do not own alone — worth raising with the korteriühistu
How to fill an ordinary plaster crack before painting
For fine shrinkage cracks, widen the crack slightly with a scraper or the corner of a filling knife so the filler has something to key into, then brush out the dust. A flexible acrylic filler works better than a rigid gypsum-based one on cracks that follow joints or corners, because it tolerates the small seasonal movement that caused the crack in the first place.
Apply the filler with a narrow flexible knife, pressing it into the crack rather than just skimming over the top, and leave it slightly higher than the surface since it shrinks a little as it dries. Once dry, sand it flush with fine sandpaper and check with your hand rather than your eye, since a shadow under raking light will show up any hollow you missed.
A patch of new filler is more absorbent than the surrounding plaster and will show through paint as a slightly duller patch unless you seal it first, so prime the filled area, or the whole wall if there are many patches, before the topcoat. If you would rather hand the whole wall over, our painting and decorating work starts from 12 € per job and covers exactly this kind of preparation and finishing together.
If the crack is in plasterboard, not plaster
Plasterboard walls crack in a more specific place: along the taped joints between boards, and at the corners of window and door openings where the board was cut. A straight crack that appears right along a joint line, sometimes with the tape lifting slightly, is a sign the joint tape has failed rather than anything wrong with the board itself.
This needs re-taping rather than simple filler, since filler alone over a failed joint will crack again within months. The area is cut back to sound plasterboard, new joint tape is set into fresh joint compound, and it is finished in two or three thin coats before sanding and priming. Our plasterboard work starts from 28 € per job, which covers this kind of joint repair as well as larger partition work.
Fine, hairline cracks with no step between the edges are almost always shrinkage and easy to fill before painting, but a wide, stepped, or reopening crack is worth a proper look before you cover it up.