Outgassing, the most common cause
Concrete is porous, and those pores contain air. When a slab is warming, in the morning, or as the sun comes onto it, that air expands and moves upward, which is fine on a bare floor and a problem when there is wet resin sitting on top. The air pushes through and leaves a pinhole. The prevention is timing rather than product: coat on a falling or stable slab temperature, in the afternoon rather than in the morning, so the air in the slab is contracting rather than expanding.
Moisture blisters, the serious one
Blisters larger than a pinhole, particularly ones that appear days or weeks after the install and contain liquid, are vapour drive from below rather than trapped air. This is a slab moisture problem and it does not resolve itself. It means the floor went down without a moisture test, or with a test result that should have called for a vapour barrier primer. The affected area has to come off, and the primer has to go in.
Technique bubbles
Rolling too aggressively whips air into the resin, and a roller nap that is too short or the wrong material can hold and release air as it goes. These bubbles are small, uniform and appear immediately, and on a self-levelling system most of them release on their own within the working window. A spiked roller run through the wet film is the standard tool for releasing what remains, which is why you see crews walking a floor in spiked shoes.
What can be done afterwards
Cosmetic pinholes in an otherwise sound floor are sanded and filled with a further topcoat pass, which closes them completely. Moisture blistering cannot be topped over: the coating in the affected area is removed, the slab is tested, mitigation goes down and that area is rebuilt. Any contractor offering to fill moisture blisters with more resin is proposing to trap the same vapour under a thicker lid.
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