Quick Answer
Before applying epoxy, grind the concrete to a Concrete Surface Profile (CSP) 2–3: a fine, uniform scratch that looks like 80–120 grit sandpaper. The standard process is: (1) solve moisture and cracks, (2) remove old coatings with PCD tooling if needed, (3) grind at 30/40 grit to open laitance, (4) follow with 60/80 grit, and (5) finish at 120 grit for a clean profile. Run a HEPA vacuum the whole time. Stop at CSP 2–3—not CSP 5+—because epoxy is thin and will not fill deep pits. Tooling such as WerkMaster PCD Tooling (JOY-WM-PCD1R1) removes old coatings, while Diamond Trapezoid Grinding Disc (JOY-TP-R3) opens the concrete to the right profile.
What CSP Does Epoxy Need?
The International Concrete Repair Institute defines CSP 1 through 10 by roughness. For epoxy self-levelers and moisture-tolerant epoxies:
| Coating type | Required CSP |
|---|
| Thin epoxy sealer (<10 mil) | CSP 1–2 |
| Self-leveling epoxy (1–3 mm) | CSP 2–3 |
| Epoxy mortar / broadcast systems | CSP 3–5 |
| Polyurethane toppings | CSP 3–4 |
Most residential and garage epoxy jobs land at CSP 2–3. Too rough and the coating looks uneven; too smooth and it delaminates.
Step-by-Step Grinding for Epoxy Prep
- Inspect and repair. Patch active cracks with epoxy paste, sawcut and fill control joints, and repair spalled areas. Moisture test (RH < 75% for standard epoxy).
- Remove old coatings. If there is old paint, glue, sealer or curing compound, strip it first with PCD tooling like WerkMaster PCD Tooling (JOY-WM-PCD1R1) or a PCD cup wheel on edges.
- Open with 30/40 grit. A metal-bond trapezoid on a planetary grinder removes laitance and high spots. Take 0.1–0.3 mm per pass, overlapping by a third.
- Refine at 60/80 grit. Removes the coarse scratch and opens uniform texture. A disc like Diamond Trapezoid Grinding Disc (JOY-TP-R3) works well on medium concrete.
- Finish at 120 grit. Produces the final CSP 2–3 profile. Do not go finer—epoxy needs tooth.
- Vacuum and tack. HEPA vacuum, then a microfiber tack mop with 70% water / 30% denatured alcohol. No dust must remain.
- Coat within 24 hours. Freshly ground concrete re-dusts; do not let it sit.
Why Not Shot Blasting or Acid Etching?
- Acid etching is outdated. It leaves salts, does not open hard concrete uniformly, and creates environmental issues. Most epoxy makers no longer accept it as a prep method.
- Shot blasting is fast but produces CSP 3–5, too rough for thin epoxy. It also can micro-fracture soft or young concrete.
- Grinding gives a controlled CSP 2–3, flat floor, and removes laitance uniformly. It is the preferred method for epoxy prep on interior slabs.
Moisture and Temperature Checks
Epoxy fails on wet floors more often than on rough floors. Before coating:
- Concrete moisture RH below 75% (70% for sensitive systems).
- Surface temperature above 10°C and at least 3°C above dew point.
- Ambient RH below 60% during cure.
- No active water leaks from above or below.
If moisture is high, use a moisture-tolerant epoxy or install a vapor barrier system instead of forcing standard epoxy.
Common Mistakes
- Skipping laitance removal: trowel-weak surface will delaminate with the epoxy. Grinding removes it; a quick buff does not.
- Going too fine: 200+ grit produces a surface epoxy cannot grip. Stop at 120.
- Grinding but not patching: cracks and holes telegraph through epoxy. Patch and feather first.
- Dusting on prep day: oil, grease, tire marks must be degreased before grinding; grinding just spreads them.
- Coating days later: freshly ground concrete adsorbs dust and oils; coat within 24 h.
Frequently Asked Questions
Q: What CSP do I need for epoxy?
A: CSP 2–3 for most self-leveling and broadcast epoxy systems. That equals an 80–120 grit scratch. Check the product data sheet; some mortars require CSP 3–5.
Q: Can I grind with an angle grinder for epoxy prep?
A: Only for edges, corners and patches. Open floors need a walk-behind planetary grinder for a flat, uniform profile. Hand grinding leaves swirl marks and an uneven scratch pattern.
Q: Do I need to densify before epoxy?
A: No. Densifier is for polished concrete. Applying it before epoxy seals the surface and weakens adhesion. Save densifier for polished floors.
Q: How soon after grinding can I apply epoxy?
A: Within 24 hours, ideally the same day. Vacuum thoroughly just before the coat. If more than a day passes, give the floor a light 120-grit pass and re-vacuum.
Conclusion
Epoxy prep is a controlled grind, not a remove-as-much-as-possible job. Strip old coatings with PCD like WerkMaster PCD Tooling (JOY-WM-PCD1R1), open the slab with 30/40 grit, refine through 60/80 and finish at 120 grit with discs like Diamond Trapezoid Grinding Disc (JOY-TP-R3), then coat within 24 hours. The result is a CSP 2–3 profile that bonds for decades instead of delaminating after the first winter.
