Field guide
Concrete Surface Preparation for Coatings: Grinding, Profile, and Moisture
Surface preparation decides whether a coating lasts fifteen years or peels in eight months, and it accounts for more coating failures than every other cause combined. Mechanical grinding, a measured concrete surface profile and a documented moisture test are the three things that separate a floor that holds from a floor that comes back.
Surface preparation decides whether a coating lasts fifteen years or peels in eight months, and it accounts for more coating failures than every other cause combined. Mechanical grinding, a measured concrete surface profile and a documented moisture test are the three things that separate a floor that holds from a floor that comes back.
The uncomfortable part is that prep is where most of the labor sits and none of the visible result does. A customer sees the flake and the gloss. They do not see the CSP 2 profile or the relative humidity probe reading, which is exactly why those steps get skipped on price-driven jobs and exactly why those jobs fail.
This page covers the whole prep sequence: grinding versus acid etching and why grinding wins for high performance coatings, how to pick a CSP profile, how moisture testing actually works with in-situ relative humidity probes and calcium chloride, what readings are too high, when a moisture vapor barrier primer is required, how long new concrete has to cure, and how to handle coating over existing coatings, sealed concrete and painted floors.
Why mechanical grinding beats acid etching
Acid etching had a place when the alternative was nothing, and it still has a narrow place under thin water based sealers. It has no place under a professional two component coating.
Acid dissolves cement paste. It does not remove oil, grease, silicone, curing compound, existing sealer or previous coatings, and those contaminants are the actual reason coatings fail. On a contaminated slab an acid etch often leaves the contaminated areas untouched while etching everything around them, so you get an inconsistent surface you cannot see. Acid also leaves salt residue that has to be neutralized and rinsed.
Grinding removes material. A diamond grinder takes off weak surface laitance, opens the pore structure, removes contaminants and previous coatings, and produces a repeatable, measurable profile. It flattens high spots and reveals defects while they are still cheap to fix. If the coating is a high performance two component system, grind it.
- Acid does not remove oil, silicone, curing compound or existing coatings. Grinding does.
- Acid etch profile is inconsistent and unmeasurable. Grinding produces a repeatable CSP.
- Acid residue left behind becomes a bond breaker under your primer.
- Grinding with a dust extractor keeps the job dry and controls respirable silica dust.
Concrete surface profile and picking the right CSP
Concrete surface profile is the standardized way the industry describes how rough a prepared slab is. The scale runs from a barely textured CSP 1 up to a heavily scarified CSP 10, and each number corresponds to a physical reference chip you can hold against the floor and compare.
The general relationship is that the thicker the coating, the more profile it needs. Thin film coatings and sealers sit at the low end. Standard roller applied systems sit in the middle low range. Heavy build systems, slurry coats and mortars need a deeper profile to key into. Matching profile to coating matters in both directions: too little and the coating has nothing to grip, too much under a thin film and you telegraph the grinding pattern, burn through on the peaks and use far more coating than you planned for.
- Diamond grinding typically produces the low to mid range profiles used for roller applied systems.
- Shot blasting produces a deeper, more uniform profile and is the usual choice on large industrial floors.
- Scarifying and milling produce the deepest profiles, used ahead of mortars and heavy build systems.
- Follow the profile the coating manufacturer specifies rather than a general rule.
- Keep an ICRI profile chip set on the truck. Judging profile by eye is how crews end up arguing with a warranty department.
Tooling, dust and silica
Diamond tooling selection changes the cost of the job. In broad terms, metal bond segments are for aggressive removal and profiling, PCD tooling is for stripping thick coatings, glue and mastic without loading up the diamonds, and resin bond tooling is for refining rather than profiling.
Segment hardness runs opposite to slab hardness, which is the part that trips people up. Soft bond goes on hard concrete and hard bond goes on soft concrete. Run it backwards and you either glaze the diamonds or burn a set of segments in an afternoon.
Dust control is not optional. Grinding concrete generates respirable crystalline silica, and the accepted control is a dust extractor with appropriate filtration on a shrouded grinder, plus whatever exposure controls your jurisdiction requires. A dry, contained grind is also faster and far easier to sell in an occupied building than a wet grind and a slurry cleanup.
- Soft bond on hard concrete, hard bond on soft concrete.
- PCD for coating and mastic removal, metal bond for profiling, resin for refining.
- Vacuum thoroughly after grinding. Residual dust in the profile is a bond breaker.
Moisture testing: relative humidity probes and calcium chloride
Moisture vapor moving up through a slab is the second great coating killer after poor prep, and it is the one that shows up months later as blisters or disbonding. Concrete is porous, vapor moves through it, and a coating less permeable than the slab traps that vapor at the bond line.
There are two established test methods and they do not measure the same thing. In-situ relative humidity testing, described in ASTM F2170, sets sleeved probes into drilled holes at a specified depth and lets them equilibrate before reading, which gives the internal humidity of the concrete itself. Calcium chloride testing, described in ASTM F1869, sits a sealed dish of anhydrous calcium chloride on the surface for a set period and weighs the moisture gained, which gives a surface vapor emission rate.
The practical difference is depth. Calcium chloride only characterizes the top portion of the slab, so it can read acceptably on a slab that is wet deeper down. Relative humidity probes read the condition inside the slab, which is why most coating manufacturers now prefer or require them. Either way, run the method properly. A probe read an hour after drilling is a guess, not a test result.
- ASTM F2170 in-situ relative humidity: probes set into drilled holes, allowed to equilibrate, read as a percentage.
- ASTM F1869 calcium chloride: a sealed dish on the surface, weighed before and after, reported in pounds per 1,000 square feet per 24 hours.
- Condition the building to service temperature and humidity before testing, and test in enough locations to represent the floor.
- ASTM F710 covers surface pH testing, which matters because a high pH surface attacks some adhesives and coatings.
What moisture reading is too high
There is no single universal number, and any page that gives you one without a caveat is guessing on your behalf. The threshold is set by the coating manufacturer for the specific product, and it appears on the technical data sheet.
That said, the trade has a well known set of reference points. Many coating and flooring manufacturers set their limit for standard systems somewhere in the region of 75 to 80 percent internal relative humidity, and somewhere around 3 pounds per 1,000 square feet per 24 hours on a calcium chloride test. Readings above the product's stated limit mean you do not apply that product directly, full stop.
What you do instead is apply a moisture vapor barrier primer rated for the reading you measured. These are typically 100 percent solids epoxy primers formulated specifically to tolerate high moisture conditions, and the good ones publish the relative humidity level they are rated up to. The barrier primer goes down on properly profiled concrete, and your coating system goes on top of it.
Document the readings. Photograph the probe display, note the location and date, and keep it with the job file. When a floor blisters two years later, a moisture test record is the difference between a warranty conversation and an argument.
How long new concrete has to cure before coating
The conventional answer is 28 days, and it is a reasonable default, but 28 days is the standard age at which compressive strength is measured. It is a strength milestone, not a certificate that the slab is dry.
A 28 day old slab can still hold plenty of moisture, particularly a thick slab, a slab poured with a high water to cement ratio, a slab in a humid environment, or a slab on grade with no vapor retarder underneath. Natural drying in the flooring trade is measured in weeks per inch of thickness under good conditions, and conditions are rarely good.
So on new construction: wait the minimum age the coating manufacturer specifies, test moisture, then decide. If the readings are high and the schedule will not wait, that is what moisture vapor barrier primers exist for. One more new construction issue is curing compound. Many new slabs are sprayed with a compound designed to sit on the surface and slow water loss, and that compound is a bond breaker. Grinding is the only reliable way to know it is gone.
Coating over existing coatings, sealed and painted concrete
Recoating an existing floor is a judgment call about what is underneath. The first question is what the existing coating is and how well it is stuck. The second is whether the new system is compatible with it.
Start with a bond test. A lattice of scored cuts with a sharp blade, firm tape applied over it and pulled, tells you quickly whether the existing film is sound. A pull-off adhesion test using the method in ASTM D7234 gives a number instead of an impression. If the existing coating comes up, it all comes off. If it is sound and compatible, you still have to abrade it mechanically, because a glossy cured coating is close to a non-stick surface for the next layer.
Sealed concrete is the trickiest case because sealers are often invisible. Drop water on the slab in several places. If it beads and sits, the slab is sealed and the sealer has to be ground off. Painted concrete is nearly always a removal job, since most floor paint is a thin single component film and your system would only be as well attached as the paint under it.
- Water drop test in several locations tells you whether the slab is sealed.
- Tape and scored cut test, or a pull-off test to ASTM D7234, tells you whether an existing coating is sound.
- Sound and compatible coatings still need mechanical abrasion before recoating.
- Always run a full system test patch, let it cure, and try to fail it before you coat the whole floor.
Repairs, joints and cracks
Repairs happen after grinding and before priming. Grinding first exposes what actually needs fixing and keeps you from grinding away a repair you just made.
Static cracks and spalls are opened up, cleaned out, filled with a repair mortar or a rigid epoxy or polyurea repair product, then ground flush. Control joints are a separate decision. Filling a control joint and coating over it gives a continuous surface, but the joint exists because the slab is going to move, so a coated joint may telegraph or crack. Honoring the joint and terminating the coating at it is more conservative and much less likely to come back.
Moving cracks, structural cracks and slabs with active differential movement are a slab problem, not a coating problem. Coating over active movement puts a hard film across a joint that is going to open, and the film loses. Flag it to the customer in writing before you coat.
Step by step
The surface prep sequence, step by step
- STEP 01
Assess the slab before you quote
Walk the floor and identify existing coatings, sealers, oil and grease staining, curing compound, cracks, spalls, control joints and any signs of moisture such as efflorescence or dark patches. Do a water drop test in several spots to check for sealers. Every one of these items changes the labor number, so find them before the price is fixed.
- STEP 02
Test moisture and pH
Condition the space to service conditions, then run in-situ relative humidity probes to ASTM F2170 or calcium chloride tests to ASTM F1869 in enough locations to represent the floor area. Allow the full equilibration or exposure period specified by the method. Record the readings with dates and locations and keep them in the job file.
- STEP 03
Clean and degrease
Remove loose debris and treat oil and grease contamination before grinding so you are not driving contaminants deeper into the pore structure. Heavy petroleum staining may need a degreaser and several cycles. Contaminated concrete that will not clean up sometimes has to be removed mechanically to sound material.
- STEP 04
Grind to the specified profile
Grind the full floor with appropriate diamond tooling to the concrete surface profile the coating manufacturer specifies, running a dust extractor throughout. Overlap passes and change direction between passes to avoid leaving a directional pattern. Edge and corner work with a hand grinder should match the profile of the main floor, not just look clean.
- STEP 05
Repair cracks, spalls and joints
With the floor ground and the defects visible, open and fill cracks and spalls with an appropriate repair product and grind the repairs flush. Decide joint by joint whether control joints get filled and coated over or honored and left open. Filled joints give a continuous floor, honored joints are less likely to crack.
- STEP 06
Vacuum thoroughly
Vacuum the entire floor including edges, corners and joints, then vacuum it again. Dust left in the surface profile sits between the concrete and the primer and becomes a bond breaker. On a large floor this is a real time allocation, not a five minute step.
- STEP 07
Prime or apply a moisture vapor barrier
Prime porous, thirsty or heavily profiled concrete to improve wetting and reduce outgassing. If moisture readings exceeded the coating manufacturer's limit, apply a moisture vapor barrier epoxy primer rated for the level you measured. Follow the primer's recoat window closely, because a primer left past its window has to be abraded.
- STEP 08
Run a test patch and confirm the system
On any floor with an existing coating, an unknown history or a mixed-manufacturer system, apply the full stack to a small test area, let it cure and attempt to remove it. Confirm adhesion, color and finish before committing material to the whole floor. An hour spent here is the cheapest insurance on the job.
Frequently asked
Is grinding or acid etching better for concrete floor prep?
Grinding is better than acid etching for any high performance coating because a diamond grinder removes surface laitance, contaminants, curing compounds and old coatings while producing a repeatable, measurable profile. Acid etching only dissolves cement paste, leaves oil and sealers in place, produces an inconsistent profile and leaves salt residue that must be neutralized and rinsed.
What CSP profile do I need for an epoxy floor?
The concrete surface profile you need is set by the coating manufacturer and appears on the product data sheet, with thin film coatings requiring less profile and heavy build systems, slurry coats and mortars requiring more. Standard roller applied coating systems generally fall in the lower to middle part of the ICRI scale, which diamond grinding produces reliably.
How do you test concrete for moisture before coating?
Concrete moisture is tested either with in-situ relative humidity probes following ASTM F2170, which are set into drilled holes at a specified depth and read after equilibration, or with calcium chloride tests following ASTM F1869, which measure surface vapor emission. Relative humidity probes read the slab's internal condition and are generally preferred by coating manufacturers.
What moisture reading is too high to coat concrete?
A moisture reading is too high when it exceeds the limit printed on the specific coating manufacturer's data sheet, since thresholds vary by product and there is no single universal number that applies across the category. Many standard systems set that limit in the region of 75 to 80 percent internal relative humidity or around 3 pounds per 1,000 square feet per 24 hours.
When do I need a moisture vapor barrier primer?
You need a moisture vapor barrier primer whenever the measured slab moisture exceeds the limit stated for the coating system you plan to install, which is a common situation on new slabs, on grade slabs with no vapor retarder and any floor showing efflorescence. These primers are typically 100 percent solids epoxy formulations rated to a specified relative humidity level.
How long does new concrete need to cure before coating?
New concrete is generally given at least 28 days before coating, but 28 days is a compressive strength milestone rather than proof that the slab is dry, so the 28 day mark tells you the concrete is strong enough and not that it is dry enough to coat. Wait the specified age, test moisture, then decide. Curing compounds must be ground off completely.
Can you put epoxy over an existing coating?
You can coat over an existing floor coating only if that coating is sound, compatible with the new system and mechanically abraded first, because a glossy cured film is close to a non-stick surface for whatever you put on top of it. Test adhesion with scored cuts and tape or a pull-off test to ASTM D7234, then run a full system test patch.
How do I know if my concrete is sealed?
Test sealed concrete by dropping water on the slab in several separate locations and watching what happens, because a sealer is often invisible and is frequently applied unevenly across a floor. If the water beads and sits, the concrete is sealed and the sealer must be ground off. If it darkens and soaks in within about a minute, the pore structure is open.
Do I have to grind concrete before applying a coating?
Mechanical grinding is required before any professional two component coating because it removes weak surface laitance, contaminants and old films while creating the profile the coating needs to bond, and no chemical etch achieves the same result. Skipping mechanical preparation is the single most common cause of peeling, hot tire pickup and delamination within the first two years.