Crack, Joint, and Spall Repair for a Flat, Durable Epoxy Floor

Why defect repair matters before epoxy

Epoxy is a thin, rigid film compared to concrete. If the slab has voids, loose edges, or moving cracks, the coating will mirror (telegraph) those defects or fail at the weak spots. The goal of repair is not “make it pretty,” but to create a sound, flat, and stable surface so the coating can bond and stay bonded.

Categorize defects: what you’re looking at

Hairline cracks (typically non-moving)

Very narrow cracks that do not show differential height (one side higher than the other) and do not open/close noticeably. These are often shrinkage-related and can usually be filled with rigid or semi-rigid materials.

Moving cracks

Cracks that change width with temperature/season, show repeated re-cracking, or have slight vertical movement. If a crack is moving, a rigid filler can split or debond, and the crack may telegraph through the coating.

Control joints (saw cuts / tooled joints)

Intentional joints designed to create a “weak plane” so the slab cracks in a controlled location. They may be clean and straight, or they may have spalls along the edges. Your coating plan must decide whether to fill them or honor them.

Spalls

Broken-out areas, usually at joints, cracks, or high-traffic zones. Spalls often have fractured edges and weak concrete beneath the surface. They must be removed back to sound concrete and rebuilt.

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Pop-outs

Small, cone-shaped breakouts (often 1/4 in to 2 in wide) caused by reactive aggregate or near-surface inclusions. They can be repaired like small spalls, but you must remove loose material completely.

Pitted surfaces

Widespread small voids and pinholes. Pitting can be cosmetic, but deep or widespread pitting can trap air and create outgassing bubbles in coatings, and it can reduce film build in low spots.

Tools and materials you’ll commonly use

• Crack chaser blade (V-groove) on an angle grinder for opening cracks
• Diamond cup wheel or grinder for flush grinding and re-profiling
• HEPA vacuum with a crevice tool for dust removal
• Epoxy crack filler (rigid) for non-moving cracks and small voids
• Polyurea or polyurethane joint/crack filler (semi-rigid to flexible) for movement-prone cracks and joints
• Patch mortar / epoxy mortar for spalls and rebuilds
• Margin trowel, putty knives, mixing pails
• Broadcast sand (for epoxy mortar mixes and to improve sanding/feathering characteristics)

Step-by-step: crack repair by type

Method A: Hairline or non-moving cracks (chase, fill, grind flush)

Use when: crack is stable, no vertical displacement, no signs of repeated movement.

1. Chase the crack. Use a crack chaser blade to open the crack into a clean V-groove. Aim for a consistent channel so filler can bond to solid sidewalls. Avoid making the groove excessively wide; you want enough width/depth for a durable fill, not a trench.
2. Vacuum thoroughly. Vacuum the groove and surrounding area. Dust left in the groove is a common cause of filler debonding.
3. Optional: blow out fine dust. If allowed by your jobsite conditions, use clean, dry compressed air after vacuuming, then vacuum again. The goal is “no visible dust.”
4. Fill with epoxy crack filler. Mix and dispense the epoxy filler into the groove. Slightly overfill so you can grind it flush later. Work it in with a putty knife to wet the sidewalls and avoid trapped air.
5. Allow proper cure. Follow the product’s recoat/grind window. Grinding too early can smear soft filler; too late can make grinding harder.
6. Grind flush. Use a grinder to bring the filler level with the slab. Feather the edges so there is no ridge that will show through the coating.

Practical tip: If the crack is very tight and won’t accept filler, chasing is not optional. A surface “wipe” of epoxy over a tight crack often fails because it bridges dust and doesn’t create sidewall bond.

Method B: Moving cracks (route, fill with polyurea, grind flush)

Use when: crack shows seasonal opening/closing, repeated re-cracking, or is near joints/columns where movement is common.

1. Chase/route the crack. Open it with a crack chaser to create a consistent reservoir. Movement cracks need enough depth for the filler to flex without tearing at the surface.
2. Vacuum clean. Remove all dust and debris from the routed crack.
3. Fill with polyurea (or polyurethane) suited to movement. Dispense the semi-rigid/flexible filler. Slightly overfill.
4. Let it cure to a shave/grind state. Many polyureas cure fast; timing matters. If you grind too late, some products become rubbery and can tear instead of grind cleanly.
5. Shave or grind flush. Use a sharp scraper for shaving if the product allows, then finish with a grinder for flatness.

Important limitation: No filler “stops” a moving crack. The goal is to accommodate movement and reduce telegraphing risk, not to permanently lock the slab.

Control joints: strategy options and what they mean

Option 1: Honor joints (recommended when movement is expected)

What it means: You keep the joint as a joint through the coating system. After coating, you re-cut or keep the joint line open and seal it with an appropriate joint sealant.

• Pros: Best at handling slab movement; reduces random cracking in the coating; easier to maintain in facilities that expect movement.
• Cons: Visible joint lines remain; joints can collect dirt if not sealed properly; requires careful detailing so coating edges don’t chip.

How it impacts telegraphing: You accept a visible line but reduce the chance of a random crack showing elsewhere.

Option 2: Fill joints (used when a seamless look is required and movement is minimal)

What it means: You fill the joint flush so the floor appears monolithic.

• Pros: Smooth, seamless appearance; easier cleaning in some environments.
• Cons: Higher risk of telegraphing or re-cracking if the slab moves; may require periodic maintenance or re-filling; can create a “ghost line” under certain topcoats or lighting.

How it impacts maintenance: If the slab moves, you may need to re-open and re-seal joints later, which can disturb the coating along the joint line.

Practical decision guide

Step-by-step: control joint repair (fill strategy)

1. Square and clean the joint edges. If edges are ragged or spalled, sawcut or grind to remove weak concrete and create solid sidewalls.
2. Vacuum the joint. Use a crevice tool to remove dust from the full depth.
3. Install backer rod (when needed). For deeper joints, backer rod controls sealant depth and helps prevent three-sided adhesion. Use the correct diameter so it fits snugly.
4. Fill with semi-rigid polyurea/polyurethane joint filler. Slightly overfill to allow flush finishing.
5. Shave/grind flush. Make the joint level with the slab so it won’t show as a ridge under epoxy.

Note: If you plan to honor joints, you still repair spalled edges and weak concrete at the joint, but you do not create a fully flush, rigid bridge across a moving joint.

Spalls, pop-outs, and pitting: repair methods that last

Understand feather-edge limitations

Many cementitious patch mortars do not hold up well at a true “zero edge” (feather edge). Thin edges can curl, chip, or debond, especially under hard coatings. Epoxy mortars generally perform better at thin edges, but they still need sound concrete and proper profiling. When possible, create a defined repair boundary (slight sawcut or grind perimeter) so the patch has thickness at the edge.

Step-by-step: spall repair (durable rebuild)

1. Remove unsound concrete. Chip or grind out the spall until you reach solid, non-fractured concrete. If the edges are undercut or cracked, keep going—patching over weak edges leads to “ringing” and future pop-off.
2. Create a clean repair profile. Square up the perimeter with a sawcut or grinder when practical. A neat boundary helps the patch resist edge chipping.
3. Vacuum and inspect. The repair cavity should be dust-free with no loose aggregate. Tap around the area; hollow-sounding concrete should be removed.
4. Select the patch material.
   • Epoxy mortar (epoxy + graded sand) for high strength, fast return to service, and better thin-edge performance.
   • Patch mortar (cementitious) when compatible with the system and thickness requirements are met; verify it can be coated and that it meets minimum thickness at edges.
5. Place and compact. Pack the mortar firmly into the cavity to eliminate voids. Slightly overbuild so you can grind flat.
6. Cure appropriately. Respect cure times before grinding or coating. A patch that is still green or weak can tear out during grinding.
7. Grind flush and re-profile. Grind the patch level with the surrounding slab. Then re-profile the repaired area to match the surrounding surface texture so the epoxy system wets out evenly.

Step-by-step: pop-out repair (small, repeated defects)

1. Open each pop-out to sound concrete. Use a grinder to remove the cone and any loose edges.
2. Vacuum clean. Dust control is critical because these are small bond areas.
3. Fill with epoxy filler or epoxy mortar. For shallow pop-outs, a trowelable epoxy filler works well. For deeper ones, use epoxy mortar.
4. Grind flush. Make the repair disappear to the touch (flat) even if it remains slightly visible in color.

Step-by-step: pitted surfaces (skim and refine)

Goal: fill voids so you don’t trap air and you don’t end up with thin coating in low spots.

1. Identify severity. Light pinholes may be addressed with a tight skim coat; deeper pitting may require multiple passes.
2. Skim with an epoxy-based filler. Squeegee or trowel a thin layer across the surface, forcing material into pits.
3. Broadcast sand (optional but helpful). While the skim is wet, lightly broadcast sand to create a grindable surface and reduce gumming during sanding.
4. Grind/sand smooth. Remove high spots and leave filled pits. Re-skim if needed.

Common failure patterns (and how to avoid them)

• Filler debonds from crack/joint sidewalls: usually dust left behind or crack not chased deep enough. Fix by re-routing and re-filling.
• Patch edges chip: feather-edge too thin or perimeter left on weak concrete. Fix by removing to sound concrete and giving the patch a defined edge thickness.
• Telegraphing lines: expected when filling moving joints/cracks. Reduce by honoring joints or using appropriate semi-rigid fillers and accepting that some lines may still show.
• High ridges over repairs: overfill not ground flush. Ridges will show through epoxy and can become wear points.

Quality checklist before you coat

• Flatness: run a straightedge over repairs; no proud ridges, no dish-shaped low spots that will puddle primer.
• Hardness/soundness: patches and fillers are fully cured and resist gouging; no crumbly mortar; no soft or rubbery filler smearing when abraded.
• No loose edges: spall perimeters are solid; no flaking, hollow-sounding concrete, or feather edges that lift under scraping.
• Flush transitions: repaired cracks/joints are level with surrounding slab; joint lines are intentional (either cleanly honored or cleanly filled).
• Surface texture continuity: repaired areas are re-profiled to match adjacent concrete so the epoxy wets out consistently.