How to Prep Concrete for Epoxy
Prep is not one job, it is four gates. The slab has to be sound, profiled, clean and dry, and every one of them is a pass or fail before the first pail is opened. Miss one and the floor still looks perfect on handover.
Prep is four separate gates, and a floor has to pass all of them. The slab has to be sound, so nothing under the coating is going to let go; profiled, to the concrete surface profile the coating’s data sheet names; clean, meaning no laitance, sealer, curing compound or oil between the coating and the concrete; and dry enough that the slab is not pushing moisture up into the film. Grinding or shot blasting are what produce a profile repeatably. Every one of the four is a pass or fail before the first pail is opened, and none of them is visible on the day you hand the floor over.
Prep is four gates, not one job
Most prep advice collapses into a single instruction: grind the floor. That is the biggest part of the work and it is also the part that gets treated as the whole of it, which is why floors that were genuinely ground still fail. Grinding answers one of the four questions above. It does nothing about a slab that is delaminating underneath, it does not fix oil that has soaked in below the depth you are cutting, and it does not change what the slab is giving off from below.
Reading them as four gates is useful because it tells you what to do when one of them fails. A profile problem is more grinding. A contamination problem is a chemical step before the grinding. A moisture problem is not a prep problem at all, and no amount of grinding will move it. Sorting the failure into the right gate is most of the diagnosis.
The order matters too, and it is the order in the diagram above. Repairs come before profiling because a repair is a new surface that also has to be profiled. Degreasing comes before profiling because grinding through contamination drives it deeper. Profiling comes before the final clean because grinding makes dust. The moisture test can be run at any point, and it should be run early, because it is the only one of the four that can stop the job outright.
Gate 1: the slab has to be sound
A coating is only ever as strong as what it is stuck to. If the top layer of the concrete can be pulled off, the coating will eventually come off with it and the failure will look like a coating failure even though the coating did its job perfectly.
What to look for
- Hollow sounding areas when you drag a chain or tap with a hammer, which usually mean a delaminated top layer
- Spalling, popouts and crumbling edges, particularly around joints, drains and door thresholds
- Cracks, and whether they are static or still moving, because those get different treatments
- Slab flatness, since a coating follows the floor rather than correcting it
What to do about it
- Cut out and patch unsound areas rather than coating over them, then profile the patch like the rest of the floor
- Fill static cracks and joints with a rigid filler, and treat moving joints as joints rather than filling them solid
- Level low areas before coating if the finish has to look flat, because a thin film cannot do it
- Let repairs cure to the manufacturer’s stated time before you grind them, or you will pull them out
The repair side has its own arithmetic. The crack repair calculator turns crack lengths and dimensions into filler volume and cartridges, and estimating crack filler volume works through where those numbers come from. Where the problem is flatness rather than damage, the self leveling underlayment calculator sizes the pour.
Gate 2: the profile the coating asks for
Concrete surface profile, CSP, is the ICRI scale from 1 to 10 that describes how open a prepared surface is. It runs from a barely textured acid etched surface at the low end to heavy scarified concrete at the top, and most epoxy and polyaspartic floor systems call for CSP 2 to 3.
The important habit is that you do not choose the number. The coating manufacturer does, on the data sheet, and it is one of the few figures on that sheet that is a requirement rather than a typical value. If the sheet names a profile, that is the target, and reading the sheet properly is where the rest of the job’s conditions come from too.
Two things the profile is doing at once, and it helps to keep them separate. It is removing the weak layer, which on a power troweled slab is laitance: fine cement and water that rose during finishing and set far weaker than the concrete beneath it. And it is opening a texture for the coating to key into. A surface can be textured and still have laitance on it, which is exactly the state acid etching tends to leave behind, and it is why a floor can pass a visual check and still lift later with gray dust on the back of the film.
Choosing how to open the slab
Four methods do most of the work, and they are not interchangeable. The choice comes from what is on the slab, how much of it there is, and what the finished floor has to look like.
Diamond grinding
- The default for coating prep: predictable profile, works up to the wall with the right edge tooling
- Runs dry with a vacuum on most coating jobs, which keeps the slab dry and the dust contained
- Tooling has to be matched to the concrete, since a bond that is too hard glazes over on soft slabs
- Slower than blasting on very large open floors, and that gap grows with the area
Shot blasting
- Fast and consistent over large open areas, and it leaves a uniformly textured surface
- Self contained and dust free in normal operation, with the abrasive recycled as it runs
- Cannot reach edges, corners or obstructions, so it is always paired with grinding for the perimeter
- Struggles with soft or thick coatings, which can smear rather than shatter
Scarifying
- Aggressive removal for thick coatings, heavy build ups and high spots
- Leaves a far coarser profile than a coating usually wants, so it is a first step and not the last one
- Expect to grind afterwards to bring the surface back to the profile the data sheet asks for
- Can fracture the surface if it is run too deep, which turns a prep step into a repair
Acid etching
- The one to avoid for a floor you intend to warranty, and the reason is mechanical rather than a matter of taste
- It attacks the surface chemically and can leave the laitance it was supposed to remove
- Residue has to be neutralized and rinsed, and the rinse puts water back into the slab
- Where there is genuinely no alternative, treat the result as unproven and test the bond before coating
Whatever the method, plan the machine time properly rather than absorbing it into the coating rate. Prep is the first thing squeezed when a quote is tight, and the one whose absence writes the callback. Production rates by pass covers what moves the rate on site, and the grinding cost calculator turns those hours into a priced line item.
Put hours against the prep before you price it
Floor area, a production rate for one pass, and the number of passes. Returns the grinding hours and the days, so prep arrives in the quote as time rather than as a guess.
Change the rate to whatever your own machine did on a trial cut, because that beats any published figure.
Open the live calculator →Gate 3: clean, and the test that proves it
Cleaning and prepping are different jobs that get the same word. Sweeping, blowing and vacuuming a slab removes what is sitting on it. Gate 3 is about what is bonded to it or soaked into it, and there are four common culprits.
- Curing compounds. Nearly every new slab has one, and it is a film designed to stay put. It comes off with the profiling pass, but it makes that first pass do more work than the schedule assumed
- Sealers. Often invisible, often applied years ago, and often only in some areas. They are the usual reason a slab beads water in one corner and drinks it everywhere else
- Oil and grease. Below the surface rather than on it, so degreasing has to come before grinding. Grinding an oily slab moves the contamination deeper and smears it along the machine path
- Grinding dust. The one you create yourself. A slab that was perfectly prepped and then left dusty gives the coating a powder layer to bond to
The check is the water drop test, and it costs seconds. Drop water on the prepared slab and watch it: it should darken the concrete and soak in within seconds. If it beads or sits, something is still sealing the surface and the coating will bond to that instead of to the concrete. Do it in several places, because sealers are applied unevenly and contamination is local by nature. One good spot in the middle of the floor does not clear the room.
Where the job warrants it, a pull off test to the value the coating manufacturer states is the only direct measurement of what the bond will actually do. Everything else on this page is a proxy for that number.
Gate 4: dry, and why this one can stop the job
The first three gates are things you can fix with more work. This one is not. A slab that is passing moisture upward will push a coating off from underneath however good the profile is, and the fix is either a barrier or waiting, neither of which is a grinding decision.
Test rather than count days. The concrete moisture calculator reads a relative humidity probe or a calcium chloride result against the bands that decide whether the floor gets coated, gets a barrier first, or gets left alone until the source is found. Where a barrier is the answer, the moisture vapor barrier calculator sizes it, and the companion guide covers when one is genuinely required.
There is a second moisture question that has nothing to do with the slab, and it lands on the same day. Air that is close to its dew point will condense on a cool slab, and a film applied onto that traps water under it. The dew point calculator settles it in seconds from the air temperature, the humidity and the slab temperature, and it is the difference between a clear finish and a milky one.
The order the work goes in
A prep day that runs in this order rarely has to repeat a step. Every one of these is here because doing it later undoes something done earlier.
- 1. Test the slab for moisture. Early, because it is the only result that can cancel the coating rather than change it
- 2. Walk the floor and mark it. Hollow areas, cracks, spalls, oil stains, changes in surface texture that suggest a sealer, and anything that beads water
- 3. Degrease the contaminated areas. Before any grinding, and let the slab dry afterwards rather than grinding it wet
- 4. Repair what is unsound. Patches, crack filling, joint treatment, and leveling if the finish demands it. Let it cure
- 5. Profile the whole floor, repairs included. To the CSP the data sheet names, with the edges and corners taken to the same profile as the open field
- 6. Vacuum, then check. Water drop test in several places, a look at the edges, and a last pass with the vacuum
- 7. Check the conditions. Slab and air temperature, humidity and dew point, against what the data sheet asks for, immediately before mixing
Two judgment calls are worth flagging inside that sequence. The first is edges: an edge ground to a different profile than the field is a visible line in the finished floor and a weak spot in the bond, and it is the most commonly skipped step on the list. The second is the gap between prep and coating. A prepared slab is an open surface that collects dust, traffic and moisture, so a floor prepped on Friday and coated on Monday is a floor that needs checking again on Monday.
What a prep failure looks like when it surfaces
The reason to take all four gates seriously is that none of them announces itself on the day. The coating goes down, it looks right, it gets handed over, and the failure arrives later, which is also why prep faults are so often blamed on the product.
Peeling and delamination
The classic profile or contamination failure. Turn a piece over: gray dust on the back means it took the laitance with it, and a clean back means it never bonded at all. Why epoxy floors peel splits the four causes apart.
Fisheyes and craters
Contamination the prep did not remove, showing up in the wet film within minutes as the coating pulls away from it. Fisheyes and craters covers what to do when it is already in the coat.
Bubbles and pinholes
Air coming up out of an open slab as it warms, which is a conditions and timing problem as much as a prep one. Bubbling and pinholes separate the two.
Hot tire pickup
Usually a cure and handover problem rather than a prep one, but a marginal bond makes it far more likely. Hot tire pickup covers the difference between walking on a floor and driving on it.
If a floor is already failing and you are working backward from the symptom rather than forward from the slab, start at the diagnosis tool, which ranks the candidates from what you saw, when it appeared and the conditions on the day.
Where to go next
For the schedule and the price of the prep itself, how long concrete grinding takes is the planning half and grinding cost per square foot is the pricing half. For the repairs that come before it, estimating crack filler volume and sizing a self leveling pour cover the two most common ones.
For the conditions the coating itself imposes, every one of them is printed on the product data sheet, and how to read a technical data sheet goes through it field by field: the profile, the moisture limit, the temperature range, the dew point rule and the recoat window that decides what the rest of the day looks like.
Frequently Asked Questions
How much does the concrete actually need to be ground?
Enough to reach the profile the coating asks for, which is a number printed on the product data sheet rather than a judgement made on site. Concrete surface profile is the ICRI scale from 1 to 10, and most epoxy and polyaspartic systems call for CSP 2 to 3.
That is a target, not a time. How long it takes to get there depends on what is on the slab, how hard the concrete is and how flat it starts, which is why the honest way to plan the day is a rate per pass and a pass count rather than an hours-per-floor figure. A trial cut on the actual slab beats any published rate, including ours.
Can I acid etch instead of grinding?
For a floor you intend to warranty, no. Etching attacks the surface chemically, and the layer it has to remove is laitance: the fine cement and water that rose during finishing and set weaker than the concrete under it. Etching can leave that layer in place while still looking like it worked.
It also creates two problems that grinding does not. It leaves residue that has to be neutralized and rinsed, and the rinse puts water into the slab you then have to wait out. Grinding and shot blasting produce a profile mechanically and repeatably, and they take the weak layer with them.
How do I know the slab is clean enough to coat?
Drop water on the prepared surface. It should darken and soak in within seconds. If it beads or sits on top, something is still sealing the concrete: a curing compound, a sealer, a polished layer or contamination, and the coating will bond to that instead of to the slab.
The test is worth repeating in several places rather than once in the middle of the floor. Sealers get applied unevenly, and traffic patterns, oil and spills are local by nature. One good spot does not clear the room.
Do I have to degrease before grinding, or will grinding take the oil off?
Degrease first. Oil that has soaked into concrete is below the surface, so grinding a contaminated slab removes the visibly stained layer and drives the rest deeper while spreading it across the machine path.
The order that works is degrease, let the slab dry, then profile, then check again. Contamination that survives prep is the classic cause of fisheyes and craters in the wet film, and by the time you see them the coat is already down.
Does new concrete need different prep?
The same profile, plus two extra questions. New slabs usually carry a curing compound, and that is a coating in its own right that has to come off, so it does not change the method, only how much work the first pass does.
The two extra questions are moisture and time. A young slab is still giving up water, and a coating applied over it can be pushed off from underneath no matter how good the profile is. Test the slab rather than counting days, because a calendar rule cannot see the slab thickness, the mix or the building.
The old coating is still stuck down. Do I have to take it off?
Not always, but you have to prove it rather than assume it. An existing coating that is genuinely well bonded can be abraded and recoated, and abrading is not optional because a cured film gives the next coat a mechanical bond at best.
Prove the bond before you commit: check the edges and any thin areas, cut a test patch, and where the job warrants it pull test to the value the coating manufacturer states. If the old film is failing anywhere, treat the whole floor as removal work. A recoat over a failing coating fails with it, and it takes your material and your labor with it.