When it goes on
A curing compound goes down the same day as the pour, right after the finish is put on and the surface has firmed up enough to spray without marking it. Timing matters on both ends, early enough that it does not disturb the finish, before moisture has already started leaving the surface faster than it should.
It goes on as a fine spray, evenly across the whole slab, and forms a thin film as it dries. On a broom finished driveway or a troweled floor, that spray step is often the last thing the crew does before packing up, right after the joints are cut.
What the film actually does
Concrete cures through a chemical reaction between the cement and the water already mixed into it, not by drying out. The faster that water leaves the surface, especially in sun and wind, the weaker and more likely to dust or crack that top layer ends up. The film a curing compound leaves behind slows that moisture loss way down, so the reaction underneath keeps running instead of stalling out while water disappears from the top.
It is a stand-in for the more hands-on ways of doing the same thing, a wet cure that keeps a slab damp with a hose, or covering it with plastic or blankets, and it works around the clock without anyone checking on it.
What a slab looks like without one
Skipping curing protection entirely, in sun or wind or dry air, is how a top layer ends up weak enough to dust under a broom or craze into a web of hairline cracks before it ever gets used. None of that shows up on the day of the pour. It shows up in the weeks after, once the surface has already lost the water it needed.
A curing compound is one way to prevent that. Which method makes sense for a given slab depends on the finish, the weather, and how the surface will get used, and that is a call we make on-site rather than the same way for every pour.
Hot, windy days push us toward the spray more often, since that is exactly the combination that pulls surface moisture away the fastest. A shaded slab poured on a mild day can sometimes get by on a simple wet cure instead, and either way the goal is the same, keep that water in the concrete long enough for the reaction to finish its work.