Moisture-barrier primer
Two hundred microns, and the only layer holding the other two on
A concrete slab is not dry and never will be. Ground water moves up through it constantly, and on a slab poured without a membrane under it (which is most slabs in this city built before about 2010), it arrives at the surface as vapour, at a pressure high enough to lift a coating off from underneath.
The primer is what stands in the way. It is the thinnest thing on this page, roughly a tenth of the base coat above it, and it is the layer that decides whether any of the rest is still attached in five years.
The reading comes first
We take a relative humidity reading in the slab before we choose the primer, not after. Below 75% RH a standard solids primer is enough. Between 75 and 95 it takes a moisture-tolerant epoxy at a heavier film. Above 95 the honest answer is that the slab needs a different conversation, and we have walked away from jobs at that number rather than pour over it.
No test, no pour. It is the one line on the load-out we do not negotiate, and it costs a day at the start to avoid rebuilding the floor at the end.
Why it has to soak in
Resin does not bond to concrete. It bonds to the profile that grinding leaves in concrete: a landscape of open pores and cut aggregate. The primer is thin on purpose so it runs into that landscape and sets inside it, which is what turns a mechanical key into a chemical one.
That also means it has an expiry. Left more than about sixteen hours, a cured primer film is smooth enough that the base coat is bonding to primer rather than into it, and it has to be abraded again before anything goes over it. On a two-day job this is the reason the second day starts early.
Where it is genuinely not needed
On a young, dry, suspended slab with a functioning membrane under it and a reading well under 75%, a moisture-barrier primer is belt and braces, and we will tell you so. What we will not do is skip the reading that establishes it.
How to tellTwo years on, look where the slab meets an outside wall or a downpipe. A floor primed against moisture is flat there. An unprimed one has blisters the size of a coin, or a dull patch that has lost its bond and sounds hollow when you tap it with a key.
What to ask
Ask for the moisture reading in writing, with the date and where on the slab it was taken. A contractor who has not measured it is not choosing a primer, they are guessing at one, and the guess is free to them and expensive to you.