Epoxy floor preparation is the work done to a concrete slab before any coating goes on. It means cleaning the slab, filling the cracks, grinding the surface rough with a diamond grinder and checking that it is dry. It is the part of an epoxy garage floor installation you cannot see afterward, and it decides whether the floor lasts. Below: what each stage does, how the industry measures it, and how to tell whether yours was done right.
What epoxy floor preparation has to do
A coating holds to the top of the slab, not to what is deep inside it. So the top of the slab has to be four things.
- CleanNo oil, grease, old sealer or paint between the coating and the concrete.
- SoundCracks and divots are filled, so the floor does not follow the slab’s flaws.
- RoughAn open, rough surface gives the coating something to grip.
- DryMoisture rising through the slab can lift the coating from underneath.
Without the grind, a coating starts to peel or bubble. Two to three years is the usual window. Store kits skip the grind, and many of our calls come from owners who started with one. Owners also call me after a floor from another company has failed.
The six stages, in the order they happen
Walk a slab through all six stages
Tap a stage, or press Play. The drawing shows what changes in the concrete. The same words are written out below it.
Stage 1: the slab is inspected. Oil, an old coating and cracks show on the surface.
Stage 1: Look the slab over
What happens. Check for oil, old coatings, sealers, cracks, crumbling spots and damp patches. What you find sets the plan.
Why it matters. A sealed slab needs a different start than a bare one.
Try it yourself. Pour a little water on a bare spot. If it soaks in, there is likely no oil or sealer on top. If it beads up, something is sitting on the surface.
Ask your installer. What did you find on my slab, and what does it change?
Stage 2: Take off what should not be there
What happens. Old paint, sealer and loose coating come off. Oil stains are cleaned with a degreaser.
Why it matters. Oil that has soaked in can work its way back up through a coating.
Ask your installer. What happens to old paint, sealer or oil? (Question 4 below.)
Stage 3: Fill the cracks and divots
What happens. Each crack and divot gets filled first, so no primer or coating goes over an open flaw.
Why it matters. The floor does not follow the slab’s flaws. Hairline cracks are common in garage slabs and are fine. A crack that keeps widening, or a slab where one side has stepped higher, needs a closer look. Five signs a garage floor needs replacing covers when.
Ask your installer. What happens to cracks and divots? (Question 3 below.)
Stage 4: Grind the surface
What happens. A diamond grinder with a vacuum attachment takes off the weak top layer and opens the concrete.
Why it matters. The coating locks into that rough surface. This is the stage that decides how long the floor holds. Installers check the roughness against ICRI’s CSP 1 to CSP 10 samples, so ask which number the coating maker calls for.
Ask your installer. How will you open the surface? (Question 1 below.)
Stage 5: Do the edges, the door strip and the dust
What happens. A big grinder cannot reach the wall, so the edges get their own pass. The strip at the garage door takes every tire that rolls in, so it gets the same care as the middle.
Why it matters. Then every trace of dust is vacuumed, because dust left behind is sealed under the epoxy. Grinding concrete also makes silica dust, so the vacuum protects people as well as the floor.
Ask your installer. How are the edges and the door strip done? (Question 5 below.)
Stage 6: Check that the slab is dry
What happens. A slab that is still releasing water is not ready, however well it was ground.
Why it matters. Three tests are used and they answer different questions: a plastic sheet (yes or no), a calcium chloride test (a number) and an in-slab probe (humidity inside the slab). The moisture section above shows what each one proves.
Ask your installer. How will you check for moisture, and what if the slab fails? (Question 2 below.)
Grinding or acid etching: what the industry guide says
Read a few pages and you get two answers. Some tell you to etch the floor with acid. Others say grind it. The International Concrete Repair Institute settles it by roughness. Its guideline, ICRI 310.2R, ranks prep methods by how rough they leave the concrete. Installers check their work against ICRI’s ten rubber profile samples, numbered CSP 1 to CSP 10. Ask which number the coating maker calls for.
| Method | What it does | What it cannot do |
|---|---|---|
| Wash or degreaser only | Lifts dirt and oil off the top. | Does not roughen the surface at all. |
| Acid etch | Dissolves the surface lightly. It sits at the mild end of the ICRI scale and suits sealers and thin coatings. | Does not strip paint, sealer or an old coating, and does not level high spots. It needs rinsing, neutralizing and a long dry, which puts water in the slab. |
| Diamond grinding | Removes the weak top layer, high spots, old coatings and surface contaminants in one pass. Our method. | Needs dust control, and cannot fix a wet slab or deep damage on its own. |
| Shot blasting | Leaves a rougher surface than either, and covers big floors fast. | Needs heavy machines, so it is mostly used on commercial floors. |
Is the slab dry? What each moisture test can tell you
Water moving up through concrete pushes a coating off from below. Three tests are used, and they do not answer the same question.
| Test | What it tells you | What it cannot tell you |
|---|---|---|
| Plastic sheet (ASTM D4263) | A sheet is taped to the bare slab and left. Water under it shows that moisture is present. | It is a yes or no check, with no number. A dry sheet does not prove the slab is dry. |
| Calcium chloride (ASTM F1869) | A number: pounds of moisture leaving 1,000 square feet of slab in 24 hours. | It reads the condition on the day of the test, at the surface. ASTM says it is not for concrete made with lightweight aggregate, or for floors with patching or leveling compounds over them. |
| In-slab probe (ASTM F2170) | A probe in a drilled hole reads the percent humidity inside the slab. | The reading is for that day and those spots only. |
The coating maker sets the limit, so ask what your system allows. When a slab runs damp, the answer is a moisture barrier, not more grinding. Our Better and Best packages include one, and a floor below ground, like a basement, needs one.
What your slab needs, by situation
Open the line that sounds like your floor. The answer appears right under it.
A new slab
What has to happen first. The concrete has to cure before it is coated. The data sheet for the coating gives the days, and about 28 is the usual rule. The smooth top layer is still ground off.
A job like it. Fenton new build: the floor went in first, before the cabinets and slatwall.
An older bare slab
What has to happen first. Clean it, grind it, and fill any cracks. Age matters less than condition.
A job like it. Oakville, 500 sq ft: ground, cracks repaired, two days.
Oil or grease stains
What has to happen first. Oil is cleaned and treated before the coating goes on. Heavy staining is the one case where the answer may be more than a grind, so ask for a look in person.
A job like it. Ask question 4 below.
Old paint, sealer or coating
What has to happen first. It comes off first. Grinding strips it, so the new coating grips concrete and not the old layer.
A job like it. Ask question 4 below.
Cracks or a crumbling surface
What has to happen first. Cracks and divots are filled before any product goes down. A slab that keeps moving needs a closer look first.
A job like it. Clayton, 200 sq ft: a crumbling slab fixed, then coated, in a single day.
A damp floor or a basement
What has to happen first. Test before anything else. A moisture barrier goes under the coating, and a floor below ground needs one.
A job like it. Ask question 2 below.
Tile or a patio
What has to happen first. Tile is ground so the coating can bond, and damaged spots are repaired. A patio is prepped like any concrete slab.
A job like it. University City: restaurant tile ground, repaired and coated. Also the Fenton garage and patio.
Who this is not for. Grinding and a coating cannot hold a failing slab together. If a slab has stepped or heaved, keeps widening its cracks, or crumbles deep below the surface, it may need repair or replacement before any coating goes on. A coating only sticks to concrete that is sound. If your floor looks like that, ask for a look in person before you plan around a coating.
Three checks you can do before the estimate
Three checks you can do before anyone visits
None needs a tool, and none replaces a proper test. They tell you what to mention at the estimate.
- 1. The water dropOn a bare, clean spot, pour a little water. Water that soaks in points to a slab with nothing on top. Water that beads up can mean oil or something sealed onto the surface.
- 2. The plastic sheetSeal a square of clear plastic over bare slab, taping every edge, and leave it for a day. Drops or fog underneath mean moisture is coming up. A dry sheet does not prove the slab is dry.
- 3. The crack walkWalk the floor and look for cracks wider than a hairline, cracks that have grown, and any spot that sits higher on one side than the other.
What did you see? Open the line that matches.
Water soaked in
The water soaked in. That points to bare concrete with nothing sitting on top. Grinding still opens the surface, and oil treatment may not be needed. Mention it at the estimate.
Water beaded up
The water beaded up. Oil, a sealer or an old coating may be on the surface, and it has to come off before anything is applied. Tell the installer before the visit. Stage 2 above and the old paint line in the slab list cover it.
The sheet stayed dry
The sheet stayed dry. No moisture showed up on that spot on that day. It does not prove the slab is dry, because the plastic sheet test only shows whether moisture is present. Ask which proper test the installer uses.
Drops formed under the sheet
Drops or fog formed under the sheet. Moisture is coming up through the slab. Ask what the installer will do about it, such as a moisture barrier, and read the damp floor line in the slab list.
Only hairline cracks
Only hairline cracks. Hairline cracks are common in garage slabs and are fine. They are filled before the coating goes on.
A crack that widens or steps
A crack that widens, or one side higher than the other. That needs a closer look in person before you plan around a coating. Read the who this is not for paragraph in the slab section, then ask for a visit.
Dust: why the vacuum matters
Grinding concrete makes fine dust, and some of it is silica. OSHA lists cutting, grinding and drilling concrete as a source of respirable crystalline silica. It links that dust to silicosis, lung cancer and kidney disease.
A grinder with a vacuum attachment pulls the dust at the machine. It also keeps the dust out of your floor, because dust left on the slab is sealed under the epoxy. We grind with a vacuum attachment, and you can see the hose on the Clayton slab above. If you are thinking of doing your own, rental counters rarely carry industrial grinders, and this is the part to take seriously.
Reading a failed floor: which stage was missed
A failed floor shows its cause months later, and the symptom points back to a stage. Use this table to work backward from what you see.
| What you see | Stage to check | Why |
|---|---|---|
| Coating peeling off in sheets | Stage 4: Grind the surface | It was holding to a weak top layer, an old sealer or old paint, not to sound concrete. Stage 2, taking the old layer off, matters here too. |
| Bubbles or blisters | Stage 6: Check that it is dry | Moisture rising through the slab lifts the coating from below. Moisture is the cause most often named for bubbling and blistering. |
| Dark, oily patches coming back through | Stage 2: Strip it | Oil that soaked into the concrete can work its way back up through a coating. |
| Lines or ridges where the cracks were | Stage 3: Fill it | Cracks and divots that were not filled first leave the floor following the slab’s flaws. |
| A floor that holds in the middle but lifts at the walls or the door strip | Stage 5: Edges and dust | A big grinder cannot reach the wall, and the door strip takes every tire. Both need their own pass. |
Five of our floors, and what prep each one needed
- Clayton, July 2024The slab was crumbling. The concrete was fixed first, then the coating went on the same day, so the listing stayed on schedule. See the Clayton job
- Oakville, December 2025, 500 square feetThe garage was cleared, the concrete diamond ground and the cracks repaired. Base coat and flakes went down the first day, the topcoat the next. See the Oakville job
- Fenton, April 2026, garage and screened patioThe job covered 700 square feet. Both spaces were diamond ground and the cracks repaired, then one system of epoxy base coat, vinyl flake and polyaspartic topcoat was laid across them. Two days. See the Fenton patio job
- University City, February 2026, restaurant restroomsThe tile, 800 square feet of it, was diamond ground so the coating could bond. Damaged areas were repaired, then the flake system and topcoat went on. Two days. See the Blueberry Hill job
- Fenton, June 2025, a new buildThe epoxy floor in this 700 square foot garage went in first, so it could cure before the cabinets, workbench and slatwall went up. Four days for the whole garage. See the Fenton new build
Six questions to ask before you sign
| Ask | A good answer | A warning sign |
|---|---|---|
| 1. How will you open the surface? | Diamond grinding or shot blasting, with a vacuum on the machine. | A wash or an acid etch and nothing more, with no word about old paint or sealer. |
| 2. How will you check for moisture, and what if the slab fails? | They name a test, say what limit the coating allows, and have a plan, such as a moisture barrier. | “It looks dry.” |
| 3. What happens to cracks and divots? | They are filled before any coating goes down, and a moving crack is pointed out. | “The epoxy will fill them.” |
| 4. What happens to old paint, sealer or oil? | Each is named and dealt with before the coating. | “We coat right over it.” |
| 5. How are the edges and the door strip done? | They get their own pass, not just the middle of the floor. | Nothing specific. |
| 6. What does the written warranty cover? | Peeling, delamination and blistering, in writing. Ours is a limited lifetime warranty. | A promise that is not in writing. |
How long preparation takes
Most residential garages take two to three days from start to finish. Poor concrete, a lot of crack repair or a three-car garage can add time. Light foot traffic is safe at 24 hours and vehicles at 48 to 72 hours. The coats that go on after preparation are covered in what installers do, step by step, and cure times in polyaspartic coating cost and cure time.
Common questions about epoxy floor preparation
Watch Brandon answer these
Short clips from Brandon on this exact topic. Each one opens in a window, so you keep your place.
How do you prepare concrete for epoxy?
Read the full answer
Can you epoxy over old concrete?
Read the full answer
Can you install flooring over cracked concrete?
Read the full answer
What to do next
To see what your slab needs, call 314-308-7780, or schedule an on-site estimate. We look at the slab and tell you what preparation it needs before you decide.
See epoxy garage floor installation for the systems and packages. Fenton, 772 Merus Ct., Monday through Friday, 8 AM to 5 PM. Garage Designs has been remodeling St. Louis garages since 2002.
