What it actually is
Concrete contains soluble calcium and other salts. Water moving through the slab dissolves them, carries them to the surface, evaporates and leaves the salts as a white crystalline bloom. The deposit itself does no harm to the concrete and brushes or grinds off easily. What matters is what it proves: water is moving through this slab, in this area, often enough to leave a visible record.
Why it matters before coating
Two reasons. First, the salt layer is a bond breaker, so any coating applied over efflorescence bonds to the deposit rather than to the concrete and releases with it. Second, and more importantly, it tells us to test. Efflorescence in a garage near the door might be melt-water and road salt from above. Efflorescence in the middle of a basement slab is vapour drive from below, and that changes the specification.
How it gets dealt with
Diamond grinding removes it along with the top layer of paste, which is part of the standard preparation for any coating we install and is one of several reasons acid etching is not an acceptable substitute. Etching does not reliably remove salts and, done badly, leaves its own. After grinding we take an in-situ relative humidity reading in the area that was blooming, because that is where the slab is telling us to look.
When it comes back through a coating
It does not, in the sense that efflorescence cannot appear on top of an intact coating: there is no path. What can happen is that vapour drive continues under the film and eventually blisters it, and salts appear at the failure. That is the same moisture problem in a different presentation, and it is prevented by testing and, where the numbers require it, a vapour barrier primer under the system.
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