Most coating failures are not caused by bad coating. They are caused by undetected concrete defects under a coating applied as if the defect were not there. Six defects account for almost all of them. This is the field handbook for detecting and treating each one — before the primer goes down, never after.
1. Cement Laitance
A thin, weak surface layer of cement and fines that floats to the top during finishing. Looks like sound concrete from above but cohesively fails under any pull stress.
How to detect:
- Visual — chalky or dusty appearance, sometimes with a slight gloss.
- Scratch test — drag a coin or screwdriver tip across the surface. Soft, easy mark = laitance.
- Water-drop test — water beads, then absorbs unevenly with a darker patch around each drop (the dense glassy laitance does not wet uniformly).
- Pull-off test — any laitance-bonded coating fails below 0.5 MPa with surface cohesive mode.
Why it matters: Friable surface layer 0.3–1 mm thick. Any coating bonded to it will delaminate. The most common reason for a CSP-perfect floor failing the pull-off test.
Treatment: Mechanical removal only — shot blast or diamond grind to expose sound aggregate. Acid will not fix it; chemical etch leaves laitance in place and weakens it further. Verify with a fresh water-drop test after removal: water should absorb uniformly within 30 seconds.
2. Curing-Compound Residue
Membrane-forming chemicals applied to fresh concrete to retain moisture during curing. Standard practice on commercial slabs. The membrane seals the surface and remains as a thin glossy film that blocks any coating from bonding.
How to detect:
- Water-drop test — water beads up, does not absorb at all, sometimes runs off in droplets.
- Slight gloss or sheen on the surface in raking light.
- History — ask the slab contractor. If a curing membrane was applied, it must be assumed present until proven removed.
Why it matters: Seals the concrete pores. Primer cannot wet the substrate; coating cannot bond. Failure mode is universal: full sheet delamination within months of installation.
Treatment: Mechanical removal — shot blast or diamond grind until water absorbs uniformly. Some non-membrane sodium-silicate cures (densifiers) can be coated over but still demand mechanical prep. The water-drop test is the only reliable acceptance check; do not trust visual inspection alone.
3. Oil and Grease Contamination
Vehicle drips, hydraulic oil, kitchen fats, lubricant spills. Penetrates the slab over time and can be 5–20 mm deep by the time it is visible from the surface.
How to detect:
- Visual stain — most commonly dark grey or brown discoloration.
- Water-drop test — water beads, then absorbs around the contaminated area but not within it.
- UV lamp — many machine oils fluoresce under UV-A (365 nm); useful in mechanical workshops and parking decks for mapping the contamination extent.
Why it matters: Coating fish-eyes, cratering, total adhesion loss. Hydrocarbons migrate to the coating-substrate interface during cure and destroy the bond from the inside out.
Treatment, in strict order:
- Degrease with alkaline cleaner (sodium-metasilicate or potassium-hydroxide based) plus scrub plus rinse. Repeat until water-drop test is uniform.
- If staining persists, cover with a bentonite or sawdust poultice (24–48 hours), then re-test.
- Only after the degrease passes the water-drop test, proceed to mechanical prep — shot blast or grind.
- If contamination is structural (transformer-oil-soaked slab, multi-decade kitchen fat), the coating is not feasible — break out and recast.
Critical mistake to avoid: Shot blasting before degreasing pulls the oil deeper into the slab and guarantees future adhesion failure. The sequence is degrease first, mechanical second — never the reverse.
4. Hollow and Delaminated Areas
Loss of bond between the top wear-course and the base slab, or internal voids in monolithic slabs. The coating spans an unsupported area and cracks on first loading.
How to detect:
- Chain drag test per ASTM D4580 — drag a chain across the slab, listen. Ring = sound, thud or drum = hollow. Mark hollows with chalk or lumber crayon.
- Sounding hammer — tap with a steel rod or geologist's hammer. Sound bonded concrete rings; hollow areas drum.
- Ground-penetrating radar (GPR) — for large commercial slabs with suspected structural delamination; expensive but quantitative.
Why it matters: Coating thickness alone cannot bridge an unsupported area. The first forklift or pallet truck transit cracks the coating; cracking propagates from the hollow edges. Hollow areas under microcement or terrazzo overlay tiles delaminate the tile itself.
Treatment:
- Cut out the unsound zone to sound concrete, repair with a polymer-modified cementitious mortar (SikaTop, Mapei Planitop family, R3 or R4 per EN 1504-3 class), let cure to full strength before profiling.
- Small isolated hollows in monolithic slabs (no overlay) can be salvaged by epoxy injection — drill 4+ ports around the hollow, inject low-viscosity epoxy (Sikadur-52 or Mapei Epojet LV) until resin weeps from adjacent ports.
5. Surface Dusting and Efflorescence
White residue rubs off the surface or appears as crystalline bloom. Sometimes both at once.
How to detect:
- Hand swipe — white powder transfers to the palm. Quantity correlates to severity.
- Visual bloom — patches of white crystalline deposits, often radiating from cracks, joints, or moisture entry points.
Why it matters: Surface dusting is a symptom of curing failure — concrete that did not retain enough moisture during the first 7–28 days. Efflorescence is a symptom of moisture migration carrying soluble salts (calcium hydroxide, sodium sulfate) to the surface. Both indicate an unstable, friable, soluble surface that will not bond a coating, and both can be accompanied by an active moisture path that will undermine any coating installed on top.
Treatment:
- Grind to sound, non-dusting concrete. Verify with the hand-swipe test — no powder should transfer after grinding.
- Address the root-cause moisture path before coating — install a vapor barrier underneath if missing, ground-water control around the slab, or drainage detail. Treating only the surface guarantees the efflorescence returns under the coating.
- If the moisture cannot be eliminated structurally, specify a chloride-and-moisture-barrier primer such as Sikafloor-EpoCem or MasterSeal 7000 CR before the topcoat.
6. Rebar Exposure and Spalling
Loss of concrete cover over reinforcing steel; rust expansion has cracked or detached the cover. Common on coastal slabs over 20 years old, or any slab where chlorides have reached the rebar.
How to detect:
- Visual — rust staining, exposed steel, cracked or spalled patches typically aligned along rebar lines.
- Cover meter — ferrous detection instrument confirms rebar location and remaining cover depth where the surface is still intact.
- Chain drag — spalled or about-to-spall zones drum even when the surface is not yet broken.
Why it matters: Loss of structural cover plus chloride or carbonation contamination at the rebar. Continued rebar corrosion under a new coating cracks the coating from below within 1–3 years. This is a structural defect, not just a coating defect — coating it over without repair is malpractice.
Treatment (per EN 1504 family):
- Mechanically remove all loose and chloride-contaminated concrete to at least 20 mm behind the rebar.
- Abrasive-clean the rebar to SA 2½ (near-white metal) per ISO 8501-1.
- Prime the rebar with Sika MonoTop-1010 (cementitious anti-corrosion primer) or Sikadur-32 Hi-Mod epoxy primer per the system spec.
- Reinstate cover with an R3 or R4 polymer-modified cementitious mortar (SikaTop-122 PLUS, Mapei Planitop 400 or HDM).
- Cure to the mortar TDS strength, then prep the repaired surface to the appropriate CSP for the floor coating system.
The Field Sequence — How to Run a Defect Survey
Before quoting any seamless-floor job, the substrate survey must include:
- Visual walkover — staining, cracks, spalls, rebar exposure, joint condition. Mark with chalk and photograph.
- Water-drop test — detects laitance, curing residue, oil contamination, and dense glassy surfaces. Fast, free, conclusive on the first three defects.
- Hand-swipe test — surface dusting indicator. Reveals curing failure and efflorescence at a glance.
- Chain drag — hollow and delaminated areas. ASTM D4580 procedure on slabs over 200 m²; sounding hammer for smaller slabs.
- UV lamp (workshops, parking decks) — fluorescent oils otherwise invisible.
- Pull-off test — quantitative confirmation that the substrate is coatable per EN 1504-2 thresholds.
Each defect found on the survey adjusts the bid. The slab that looks identical to the one next door but has hidden laitance and a curing membrane on top is a different job — and pricing it as if it were not is the contractor's first warranty claim.
Read next: Mechanical preparation methods compared · ICRI CSP 1–10: which profile for which coating · Pull-off test per ASTM D7234 and EN 1542.
Sources
- ASTM D4580 — Standard Practice for Measuring Delaminations in Concrete Bridge Decks by Sounding.
- EN 1504 family — Products and systems for the protection and repair of concrete structures.
- Evolving Elements — Removing Cement Laitance from New Concrete.
- Brick'n'Bolt — Concrete Laitance: Causes, Effects and Removal.
- ChemCo Systems — Preparation of Surfaces technical bulletin.
- Graco — Removing Oil, Grease, and Dirt Before Blasting.
- For Construction Pros — Sounding Concrete: Detect Damage Before Repair.