Filling cracks
Do cracks have to be fixed before a floor is coated?
Cracks get opened up and filled before the coating goes down, and how a crack behaves afterward depends on whether the ground under it is still moving.
The short version
- A shrinkage crack is narrow, wanders, and sits level on both sides. It opened once and stopped.
- A moving crack has a height difference across it, changes width by season, or runs off a corner of the foundation.
- Cracks are cut open into a clean channel, filled level and ground flush before the coating goes on.
- A coating covers a filled crack. It does not hold two pieces of moving concrete together.
- A crack the ground is still working on can show back through the finish as a fine line.
The two kinds of crack
Most cracks in a garage or basement floor are shrinkage cracks. Concrete gives off water as it cures and gets slightly smaller doing it, and a slab that cannot shrink evenly cracks somewhere. Those are narrow, they wander across the floor, and both sides sit at the same height. They opened once and then they stopped.
The other kind is a crack the ground is still working on. You can usually see it: one side sits higher than the other, or the gap changes width between a wet spring and a dry fall, or it runs straight off a corner of the foundation. Something under the slab is moving, and the crack is what that movement looks like at the surface.
What gets done to them
A crack gets chased out first, which means running a blade down it to open it into a clean channel with sound concrete on both sides. Filling a crack in the shape it cracked in gives the filler almost nothing to hold. Then it gets filled level with the floor and ground flush, so the coating runs across it without a ridge.
Bigger damage gets the same treatment at a bigger scale. A spalled edge gets cut back to solid material and rebuilt, and a low area gets brought up before the flake goes down, because a coating follows the floor it is put on and does not level anything.
What a coating can and cannot bridge
A finished floor covers a filled crack and you generally do not pick it out afterward, especially under flake. What the coating cannot do is hold two pieces of concrete together. If the slab moves again, the film over that line moves with it, and the crack shows through as a fine line in the finish.
So a moving crack is a conversation before the work rather than after it. The person quoting it can look at what the crack has already done, tell you which of the two it looks like, and say plainly that a line the ground is still opening can come back through the finish. Ask about it while somebody is on the floor with you, rather than the following spring.