Peeling
Why do floor coatings peel?
A coating peels when the bond to the concrete was never made, or when water moving up through the slab breaks that bond from underneath.
The short version
- A piece that lifts clean, with smooth concrete on the back, never bonded to the slab.
- Concrete on grade carries water up through itself, and that vapor collects at the bond line.
- A coating over an old paint, sealer or mastic is only as stuck down as the layer under it.
- Oil, salt residue and an old curing compound all sit between the resin and the concrete.
- The underside of a lifted piece tells you which one it was.
The bond that was never made
A coating holds onto a profile, not onto a smooth surface. Grinding opens the top of the slab so there is a texture for the resin to key into, and a vacuum pulls the dust off the floor while that happens. A hard-troweled garage slab is closer to glass than most people expect, and a coating laid straight onto one has very little to hold.
Contamination does the same thing from the other direction. Oil soaked into the concrete, salt residue, an old curing compound, a sealer somebody rolled on years back: any of them sits between the resin and the slab. Grinding takes most of it away, and oil that has gone deep gets dealt with before anything else goes down.
Then there is a new coating put over an old one. That is the right call on some floors, and it also means the new system is only as attached as the layer underneath it. When the old layer releases, everything above it comes along, and from the top it looks like the new coating was the problem.
Water coming up through the slab
Concrete sitting on the ground moves water up through itself, and you cannot see it happening. A slab can read dry on top while it is carrying moisture through the middle. Put a coating over that and the vapor has nowhere left to go, so it gathers at the bond line, first as small blisters and then as sheets that lift.
It gets more likely with a slab poured without a vapor barrier under it, ground that holds water against the foundation, a downspout emptying at the wall, and spring, when the frost comes out and everything under the floor is wet at once. A moisture reading taken before the work starts is how that gets caught.
Reading a piece that has lifted
Pick a piece up and turn it over. Smooth gray concrete on the back, with nothing stuck to it, says the coating never keyed into the slab and the prep is where it went. Grains of concrete stuck to the back say the opposite: the bond held and the top of the slab came away with it, which is a concrete problem rather than a coating one.
A different coating on the back means the older layer released and not the new one. An oily film, or a chalky powder, means the slab was not clean when it got coated. Those are four different findings leading to four different repairs, which is why the piece is worth keeping in a bag until somebody has looked at it.