The problem it solves
Concrete on grade is in constant contact with soil moisture, and where there is no vapour barrier under the slab, that moisture moves up through the concrete as vapour and reaches the surface continuously. A coating applied over it is a lid: the vapour arrives at the bond line, cannot escape, and builds pressure that eventually lifts the coating in blisters or sheets. The coating did not fail. It was asked to hold back a physical process.
How the primer works
An MVB primer is formulated to be extremely dense and low in permeability, and it is applied in one or two thick coats directly onto a properly ground slab. Rather than trying to stop the vapour, it reduces transmission to a level the floor system above it tolerates, which is measured and rated by the manufacturer to a specific relative humidity. Above that rating, a topical primer is not the answer and a different approach is needed.
When it is called for
The trigger is a test result, not a hunch. An in-situ relative humidity probe drilled into the slab and left to equilibrate gives a number, and most coating manufacturers set their limit around 80 to 85 percent. Above that, the primer goes in the quote. This is why we test before quoting anything below grade or on any older slab-on-grade, and why a quote given over the phone cannot tell you what your floor will cost.
What happens when it is skipped
The floor looks perfect for a season and then blisters, usually in patches that map to where the slab is wettest. By then the only fix is grinding the whole floor off and starting again with the primer that should have been there, which costs more than the primer would have. Skipping the moisture test is the cheapest thing an underpriced quote can do, and it is the most expensive thing that can happen to a floor.
Want this answered for your slab?
Free site visit with a moisture reading, then a written fixed price. Across the BC Interior.