EpoxyGrindDIY guide
How to Repair Concrete Cracks Before Epoxy Coating
A crack filled the right way disappears under the coating. Filled the wrong way, it telegraphs right back through — sometimes worse than before.
Cracks need to be addressed before you profile the slab — filling after grinding means you can't level the patch flush with the surrounding concrete in the same pass, and you'll feel (and eventually see) the seam. The bigger judgment call is whether a crack is cosmetic (surface-level, not moving) or structural (wide, uneven on either side, or actively growing) — those get different treatment, and coating over a structural crack without addressing why it's there just hides a problem instead of fixing it.
Assess the crack
Hairline cracks (thinner than about 1/8in, stable, no vertical offset between the two sides) are cosmetic and straightforward to fill. Wider cracks, cracks with one side higher than the other, or cracks near a slab edge that seem to be actively opening are worth a closer look — they can indicate settling or a structural issue that a surface filler won't solve. When in doubt on anything that looks structural, consult a foundation or concrete professional before coating over it.
Clean out the crack
Use a wire brush, shop vac, or compressed air to remove loose debris, dust, and any old failed filler from inside the crack. A filler applied over debris bonds to the debris, not the concrete, and tends to pop back out.
Rout wider cracks for a better key
For cracks wider than a hairline, widening the top slightly with a crack chaser or grinder blade (creating a slight V or reverse-V profile) gives the filler more surface area to grip and resists the repair popping out under flex or vibration.
Fill with a rigid or semi-rigid epoxy or polyurea crack filler
Mix per the product's instructions and fill the crack, slightly overfilling since most fillers shrink a small amount as they cure. Polyurea and polyurethane fillers stay somewhat flexible, which suits cracks that see minor seasonal movement; rigid epoxy fillers are stronger but less forgiving of any ongoing movement — pick based on whether your crack is truly static.
Let the filler cure fully before grinding
Follow the product's cure time — grinding a filler that hasn't fully hardened smears it instead of leveling it. Once cured, grind the patch flush with the surrounding slab as part of your normal concrete prep pass.
Patch spalled or pitted areas the same way
For spalling (flaking, pitted, or crumbling surface concrete rather than a linear crack), use a concrete patching compound or a filled epoxy mortar rather than a thin crack filler, since spalled areas need more body to fill the irregular depth. Same rule applies: fully cure, then grind flush.
Mistakes to avoid
- Coating over a crack without filling it — the crack will telegraph through the coating, and it gives moisture and debris a path underneath the coating at the exact weak point.
- Filling before cleaning out the crack — filler bonds to whatever's in the crack, including old debris, and doesn't hold as well as filler applied to bare, clean concrete.
- Grinding before the filler fully cures — this smears soft filler instead of leveling it flush.
- Treating an actively moving or structural crack as purely cosmetic — a surface filler doesn't address why the crack is there, and it can reopen.
- Underfilling instead of slightly overfilling — most fillers shrink a bit as they cure, so a level-full fill often ends up as a shallow dip once cured.
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FAQ
How do I know if a crack is structural vs. cosmetic?
Width, vertical offset between the two sides, and whether it's actively growing are the main signals. A stable, hairline crack with both sides level is almost always cosmetic. Anything wider, offset, or near a wall/corner that seems to be moving is worth a professional look before you coat over it.
Can I use the same filler for cracks and for spalled/pitted areas?
A thin crack filler generally isn't the right product for a broader spalled area — you want something with more body, like a filled epoxy mortar or concrete patching compound, to fill the irregular depth properly.
Will a filled crack be completely invisible under the epoxy?
On a solid-color coat, a well-filled and leveled crack is usually very hard to see. Under a flake or metallic finish, the pattern itself helps hide it further, but a very close inspection can sometimes still reveal a faint line.
Do control joints (the intentional grooves in a garage floor) need to be filled too?
Control joints are usually left as-is or filled with a flexible joint filler rather than a rigid one — they're designed to allow controlled movement, and a rigid filler defeats that purpose and can crack again at the joint.