Delamination and peeling
Adhesive failure (clean separation at an interface) points to contamination, inadequate profile, blush or a missed recoat window. Cohesive failure (a split inside one layer) points to a weak or under-cured layer, or to concrete laitance. A pull-off test such as ASTM D4541 or D7234 shows which you have.
Blistering
- Osmotic blistering: soluble salts left on the substrate draw water through the film, building pressure. Common in immersion service. Prevent with salt testing and proper cleaning.
- Moisture vapor drive: vapor from a damp slab pushes up under impermeable coatings.
- Solvent entrapment: retained solvent vaporizes when the coating heats up.
- Cathodic disbondment: on cathodically protected steel, alkali generated at defects undercuts the coating.
Cracking and checking
Brittle films crack when the substrate moves or temperatures swing. Causes include excessive film build, over-hardening with age, too little flexibility for the substrate (for example rigid epoxy over moving cracks) and shrinkage during cure.
Chalking, fading and yellowing
UV breaks down the binder at the surface, freeing pigment as a powdery chalk. Aromatic epoxies, polyurethanes and polyureas chalk and yellow far faster than aliphatic topcoats. See UV degradation & weathering.
Rust and undercutting
Rust bleeding through or creeping from damage points to low film thickness, missed edges and welds, holidays, or inadequate surface preparation. Stripe-coating edges and checking DFT (ASTM D7091) are the usual cures.
Surface defects
| Defect | Typical cause |
|---|---|
| Fish eyes and craters | Silicone or oil contamination, overdosed silicone additives |
| Pinholes | Outgassing, CO₂ from moisture, solvent popping |
| Orange peel | Poor atomization, wrong thinner or viscosity |
| Sags and runs | Excess film build, too much thinner, low thixotropy |
| Greasy or hazy epoxy | Amine blush |