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Preparing Concrete Before Coating

Nearly every concrete coating that fails, whether epoxy, polyurethane, sealer or paint, fails at the bottom of the film, not the top. The coating itself was fine. What it was stuck to was not. That is why professionals spend more time on grinding, testing and repair than on the visible coating step, and why a floor that looks clean can still be a poor candidate for a coating until a few simple measurements have been made.

Concrete is not a solid surface

It helps to picture concrete accurately. It is a hardened paste of cement, sand and stone, full of microscopic capillaries, and on top it carries a thin layer of weak, fine cement paste created when the slab was troweled, called laitance. Below it, the slab is often sitting directly on soil, which holds moisture that can migrate upward as vapor. A coating has to bond to the sound layer under the laitance, and it has to resist whatever moisture arrives from below. Preparation handles the first, and testing finds out about the second.

Step one: clean for real

Surface contamination blocks bonding. Oil drips under the car are the obvious problem, but the concrete also absorbs grease, brake fluid and tire residue deep into the capillaries, where surface cleaning does not reach. Degreasers and a scrub brush remove the surface layer. Stubborn stains may need a poultice or a stripping product. Any oil that remains can bleed back up through a coating and cause blisters or fish eyes, so heavily stained areas often get an extra grind and a test patch before the full coating is committed.

Old coatings need identification. A film that scratches off with a fingernail is a different problem than a hard epoxy that has to be ground off entirely. A quick test of a few drops of water helps: if water beads, there is a sealer or coating, and if it darkens and soaks in, the surface is open.

Step two: profile the surface

Coatings grip a rough surface and slide off a smooth one. Roughening is called creating a profile, and the industry measures it against a scale of standard textures, with most thick coatings wanting something like medium sandpaper. There are two main ways to get there.

Diamond grinding. A heavy floor machine fitted with diamond segments removes the laitance and a thin layer of concrete and leaves a consistent, open profile. It also levels small lips and removes old coatings and light stains. It makes a lot of fine dust, so a vacuum attached to the machine is standard practice, and silica dust is a real health concern that calls for a proper respirator.
Acid etching. A diluted acid, usually muriatic or a safer citric or phosphoric based product, is spread and scrubbed, then rinsed. It cleans and lightly roughens the surface. It is cheap and requires no machine, but the result is inconsistent, it cannot remove dense laitance or old coatings, and rinse water that is not fully removed leaves residue that weakens bonding. Many coating manufacturers prefer grinding, and some floors never meet the standard through etching alone.

A simple check after etching or grinding is to sprinkle water on the floor. It should soak in nearly at once. If it beads, the surface is still too tight or contaminated.

Step three: test for moisture

This is the step most often skipped, and it matters more than the others in a slab on grade. Water vapor travels upward through concrete continuously. A coating that is vapor tight traps it, and the pressure builds under the film until it pushes up blisters or pops the coating loose in patches.

Several tests exist, in order of how simple they are.

Plastic sheet test. A square of clear plastic is taped tightly on all edges to the bare slab and left for a day or more. Condensation under the plastic, or a dark, damp patch on the concrete after removal, signals moisture moving through. It is rough but cheap and tells much.
Calcium chloride test. A sealed dish of calcium chloride crystals is weighed, placed under a dome on the slab for a set period, then weighed again. The weight gain measures the vapor emission rate. It is a standard method with published limits for various coatings.
In situ relative humidity probes. Small holes are drilled into the slab to a depth based on its thickness, and a sensor measures humidity inside the concrete. This is the most reliable method and is widely used for flooring installations.

If moisture is high, the options include a moisture mitigating primer or membrane designed for the purpose, a breathable coating system, or fixing the source, such as poor drainage along an outside wall, leaking downspouts or irrigation spraying onto the slab edge. Applying a regular epoxy over a wet slab is a predictable failure.

Step four: repair cracks, spalls and joints

Cracks come in a few kinds, and the right repair differs.

Hairline shrinkage cracks. Often tight enough to be filled by a coating, but a repair with a low viscosity epoxy or crack filler keeps them from reflecting through the surface.
Wider cracks, a quarter inch and up. These are routed or chased with a grinder into a small V shaped groove, cleaned of dust, and filled with a flexible or rigid repair material suited to the movement.
Active cracks. Cracks that open and close with temperature or settlement will reappear in a rigid coating. A flexible joint sealant is the honest answer there, and the coating is cut with the crack.
Spalls and pitting. Chipped or flaking areas are ground back to sound concrete and patched with a repair mortar, then ground flush.

Control joints, the straight cut lines in a slab, are designed to let the concrete move. They are normally left open or filled with a flexible sealant, not bridged by a rigid coating, which would simply crack along them.

Step five: final cleaning and conditions

After grinding and repairs, dust is vacuumed thoroughly, not just swept, and the floor is wiped or checked for residue. Temperature and humidity are noted. Concrete should typically be above the dew point by a margin the product states, and the slab should be dry on top. In warm dry weather, a coating on a slab that is heating up through the day may bubble as trapped air escapes, so planning the pour for a cooling slab, usually later in the day, reduces pinholes.

Signs that a floor is not ready

It darkens and stays damp under a taped plastic sheet.
Water beads on it after etching.
A chalky, powdery film rubs off on a hand.
Fresh concrete under a month old has not cured enough for most coatings.
Oil keeps appearing at the same patch after cleaning.

Good preparation is time consuming and not visible in the finished product, which is exactly why it is the part cut by low bids. A floor ground to a consistent profile, tested dry and carefully repaired gives any coating chemistry the best chance. A floor that skipped those steps may look beautiful for a few months before it starts to lift.