Paint and Coatings Defects Dictionary: I-M
Definitions, Causes, Prevention, and Corrective Action
The I-M installment covers defects involving impact, incomplete cure, contamination between coats, poor adhesion, application boundaries, film thickness, appearance, biological growth, and severe shrinkage cracking.
Similar symptoms can have different causes. A soft film may be incompletely cured, chemically attacked, or formulated to remain flexible. A visible repair edge may be lifting, mapping, or gloss variation. Diagnose the substrate, coating system, application record, and service exposure before selecting a repair.
Safety reminder: Follow the coating and equipment manufacturers' instructions. Use required ventilation, grounding, respiratory protection, personal protective equipment, and pressure-relief procedures. Evaluate existing coatings and biological contamination before disturbing them.
Impact Damage
Definition:
Impact damage is cracking, chipping, denting, or detachment caused when a coated surface is struck by a tool, object, abrasive, or moving load.
What it looks like:
The coating may show a central dent or chip with radial cracks, lifted edges, exposed substrate, or crushed layers around the point of contact.
Common causes:
- Handling, assembly, dropped tools, collisions, or material movement
- Coating too brittle, too thick, or insufficiently cured
- Poor adhesion or inadequate surface preparation
- Coating system not suited to the expected mechanical service
Prevention:
Select a coating with suitable impact resistance and flexibility, prepare the surface correctly, allow full cure before handling, and protect coated parts during transport and assembly.
Corrective action:
Remove loose material, inspect for substrate deformation or corrosion, feather sound edges, restore surface preparation, and apply the specified repair system.
Related defects: Chipping, cracking, delamination, and marring.
Incomplete Cure
Definition:
Incomplete cure occurs when a coating does not develop its intended hardness, chemical resistance, adhesion, or physical properties within the expected time.
What it looks like:
The film may remain soft, tacky, easily marked, solvent-sensitive, weak, or poorly adhered. Some failures become apparent only after topcoating or entering service.
Common causes:
- Incorrect component ratio or inadequate mixing
- Omitted hardener, catalyst, induction time, or required bake
- Low temperature or insufficient curing time
- Material used beyond pot life or shelf life
- Excessive film thickness, incompatible thinner, moisture, or contamination
Prevention:
Measure components accurately, mix thoroughly, observe induction and pot life, apply the specified thickness, and maintain the required time and temperature throughout cure.
Corrective action:
Consult the coating manufacturer. Additional time or controlled heat may complete some cures, but incorrectly mixed or contaminated material usually requires removal and replacement.
Related defects: Soft film, tackiness, adhesion failure, and solvent sensitivity.
Intercoat Contamination
Definition:
Intercoat contamination is unwanted material trapped between coating layers that interferes with adhesion, appearance, or long-term protection.
What it looks like:
The next coat may fisheye, crater, peel, blister, delaminate, or show dirt, staining, gloss differences, or localized texture.
Common causes:
- Dust, overspray, oil, grease, silicone, wax, or moisture
- Amine blush or soluble salts left on the previous coat
- Dirty compressed air, gloves, rags, tools, or handling equipment
- Condensation during storage or between shifts
- Failure to clean after sanding or environmental exposure
Prevention:
Inspect and clean every coat before recoating. Protect parts from airborne contamination, maintain clean compressed air, observe dew-point limits, and follow the manufacturer's intercoat-cleaning procedure.
Corrective action:
Determine the contaminant and plane of failure. Remove affected layers, clean and prepare the sound surface, verify cleanliness, and recoat within the approved procedure.
Related defects: Amine blush, cratering, delamination, fisheyes, and holidays.
Intercoat Delamination
Definition:
Intercoat delamination is separation specifically at the boundary between two coating layers.
What it looks like:
The upper coat peels or lifts while the lower coat remains attached. The back of the separated film may carry little or none of the underlying coating.
Common causes:
- Missed recoat window
- Contamination, gloss, or inadequate abrasion on the previous coat
- Incompatible coating chemistries
- Insufficient cure or excessive cure before recoating
- Moisture, incorrect mixing, or improper application conditions
Prevention:
Confirm compatibility, observe recoat windows, remove contamination, abrade cured surfaces when required, and document environmental conditions.
Corrective action:
Remove every poorly bonded layer to a sound coating, prepare it according to the approved repair procedure, and rebuild the system.
Related defects: Adhesion failure, delamination, lifting, and peeling.
Lap Marks
Definition:
Lap marks are visible bands, edges, or sheen differences where one application area overlaps another after the first area has begun to set.
What it looks like:
Dark, light, glossy, dull, or thicker stripes follow roller, brush, or spray-pass boundaries.
Common causes:
- Failing to maintain a wet edge
- Uneven overlap, gun distance, travel speed, or material delivery
- High temperature, low humidity, wind, or fast solvent evaporation
- Stopping and restarting in the middle of a visible surface
- Unevenly mixed flattening agents or pigments
Prevention:
Plan natural break points, maintain a wet edge, use consistent overlap and application technique, mix material uniformly, and select reducers or conditions that provide adequate working time.
Corrective action:
Allow the coating to cure, prepare the complete affected area uniformly, and recoat from one natural break to another.
Related defects: Gloss variation, striping, mottling, and color variation.
Leaching or Surfactant Leaching
Definition:
Surfactant leaching is the migration of water-soluble coating ingredients to the surface, commonly from waterborne coatings exposed to cool, damp, or humid conditions during early cure.
What it looks like:
Glossy, sticky, oily-looking, brownish, or translucent streaks and spots appear on the surface. The film below may remain otherwise sound.
Common causes:
- High humidity, condensation, rain, or low temperature during early cure
- Applying too heavily or without adequate drying time
- Poor ventilation in bathrooms, enclosed rooms, or damp areas
- Moisture repeatedly running over the fresh coating
Prevention:
Apply within recommended temperature and humidity limits, provide airflow, avoid condensation and early water exposure, and allow adequate drying between coats.
Corrective action:
After the coating has cured, wash residue using the manufacturer's approved method. Repeated cleaning may be needed. Recoat only if staining or film damage remains.
Related defects: Blooming, staining, streaking, and water spotting.
Lifting
Definition:
Lifting is the swelling, wrinkling, softening, or separation of an existing coating after a new coating or solvent is applied over it.
What it looks like:
The underlying finish puckers, curls, wrinkles, or pulls away, often shortly after the new wet coat is applied.
Common causes:
- Strong solvent attacking a sensitive or uncured undercoat
- Incompatible coating systems
- Recoating at the wrong stage of cure
- Excessive wet-film thickness or slow solvent release
- Applying over a weak or poorly bonded existing finish
Prevention:
Identify the existing coating, verify compatibility, observe recoat windows, use approved sealers when required, and apply controlled coats with proper flash time.
Corrective action:
Allow the area to stabilize, then remove all lifted and softened material. Prepare a sound surface and rebuild it with a compatible system.
Related defects: Delamination, featheredge splitting, wrinkling, and solvent attack.
Loss of Gloss
Definition:
Loss of gloss is an unintended reduction in surface reflectance during drying, curing, weathering, cleaning, or service.
What it looks like:
A finish intended to be glossy becomes dull, hazy, patchy, or less reflective.
Common causes:
- Ultraviolet exposure, weathering, chalking, or chemical attack
- Dry spray, poor flow, or excessive atomizing pressure
- Porous undercoat, solvent absorption, or die-back
- Moisture, low temperature, contamination, or incomplete cure
- Abrasive cleaning, polishing, or normal wear
Prevention:
Select a durable coating, seal porous surfaces, apply a uniform wet film, use proper atomization and reducer, and maintain specified curing conditions.
Corrective action:
Clean and evaluate the fully cured surface. Approved polishing may restore minor gloss loss. Degraded or internally dull films generally require preparation and recoating.
Related defects: Chalking, die-back, dry spray, gloss variation, and haze.
Low Film Thickness
Definition:
Low film thickness occurs when the applied coating does not reach the specified wet- or dry-film thickness needed for appearance, coverage, or protection.
What it looks like:
The finish may show poor hiding, early corrosion, visible substrate, weak edge coverage, uneven color, or premature wear. The defect may be invisible without measurement.
Common causes:
- Insufficient passes, excessive gun speed, or poor overlap
- Low fluid delivery or incorrect tip or nozzle size
- Over-reduction or misunderstanding volume solids
- Difficult geometry, sharp edges, or poor access
- Incorrect wet-film or dry-film measurement
Prevention:
Calculate required wet-film thickness from volume solids, verify output and technique, stripe-coat difficult areas, and measure film thickness with appropriate calibrated tools.
Corrective action:
Confirm compatibility and recoat timing, prepare the surface as required, and apply enough additional coating to meet the specification without exceeding maximum thickness.
Related defects: Edge failure, grinning, holidays, and rust breakthrough.
Mapping
Definition:
Mapping is the visible reproduction of repair edges, sanding patterns, substrate variations, or underlying defects through the final finish.
What it looks like:
Outlines, rings, patches, scratches, or changes in texture and gloss reveal where repairs or different materials lie beneath the topcoat.
Common causes:
- Insufficient filling, feathering, priming, or sanding
- Porosity or solvent absorption differences
- Repair materials not fully cured
- Topcoat applied too wet or with aggressive solvent
- Film build insufficient to level the surface
Prevention:
Complete repairs with compatible materials, allow full cure, feather and sand uniformly, apply adequate primer or sealer, and inspect under proper lighting before topcoating.
Corrective action:
Allow full cure, sand the affected area level, reseal or reprime when required, and reapply the finish uniformly.
Related defects: Die-back, featheredge splitting, sinkage, and telegraphing.
Marring
Definition:
Marring is visible surface damage caused by rubbing, scuffing, pressure, abrasion, handling, or contact without necessarily penetrating the coating.
What it looks like:
Scuffs, shiny spots, dull patches, rub marks, scratches, or impressions appear on the finish.
Common causes:
- Handling or packaging before full cure
- Insufficient hardness or abrasion resistance
- Dirty pads, racks, conveyors, clothing, or cleaning tools
- Parts rubbing during transport or assembly
- Improper cleaners or abrasive polishing
Prevention:
Allow adequate cure, use clean padded handling equipment, separate parts, select a coating suited to service, and use approved cleaning products.
Corrective action:
Clean and evaluate the surface. Polishing may correct light marks if approved. Deeper damage may require sanding and recoating.
Related defects: Abrasion, impact damage, scratching, and gloss variation.
Metamerism
Definition:
Metamerism occurs when two colors match under one light source but appear different under another because their spectral reflectance differs.
What it looks like:
Adjacent parts appear to match in daylight but differ under fluorescent, LED, incandescent, or other lighting.
Common causes:
- Different pigment combinations used to produce the same apparent color
- Different coating batches, suppliers, substrates, or application methods
- Repair color formulated under only one light source
- Film thickness, orientation, texture, or metallic-flake differences
Prevention:
Use approved color standards, consistent materials and processes, and evaluate matches under multiple specified light sources with controlled viewing conditions.
Corrective action:
Confirm the mismatch under standardized lighting. Re-formulate or recoat using a color match with acceptable spectral performance.
Related defects: Color mismatch, mottling, shading, and texture-related color variation.
Microblistering
Definition:
Microblistering is the formation of very small blisters within or beneath a coating, often associated with moisture, soluble contamination, or osmotic pressure.
What it looks like:
Numerous tiny raised spots create a fine pebbled or pimply surface. They may be most visible after humidity, immersion, or temperature changes.
Common causes:
- Moisture or soluble salts trapped beneath the coating
- Coating a damp or porous substrate
- Contaminated blast media, rinse water, or compressed air
- Insufficient cure before moisture exposure
- Permeability or film thickness unsuitable for service
Prevention:
Control soluble contamination and moisture, prepare and dry the substrate, maintain dew-point separation, cure completely, and select a system suited to immersion or humidity exposure.
Corrective action:
Determine the blister depth and contents, remove affected material, correct moisture or salt contamination, restore surface preparation, and recoat.
Related defects: Blistering, osmotic blistering, pinholing, and underfilm corrosion.
Mildew
Definition:
Mildew is visible fungal growth on a coating surface, usually where moisture, nutrients, warmth, and limited sunlight or airflow support growth.
What it looks like:
Black, gray, green, brown, or whitish spots and patches appear on the surface. Dirt and other biological growth can resemble mildew.
Common causes:
- Persistent dampness, condensation, leaks, or poor ventilation
- Shaded surfaces and limited airflow
- Dirt, oils, or organic residue providing nutrients
- Coating not designed with suitable mildew resistance
Prevention:
Correct moisture and ventilation problems, keep surfaces clean, improve drainage and airflow, and select a coating appropriate for mildew-prone service.
Corrective action:
Identify the growth and follow approved cleaning, containment, and personal-protection procedures. Correct the moisture source, allow the surface to dry, and recoat if the existing film is damaged.
Related defects: Algae growth, staining, biological contamination, and moisture damage.
Mottling
Definition:
Mottling is an irregular patchy appearance caused by uneven color, metallic-flake orientation, pigment distribution, film thickness, or application.
What it looks like:
Light and dark patches, clouds, stripes, or uneven metallic appearance are visible across the finish.
Common causes:
- Uneven gun distance, angle, speed, overlap, or fluid delivery
- Incorrect atomizing pressure, viscosity, or reducer
- Inconsistent metallic orientation or pigment distribution
- Uneven film thickness, flash time, or substrate color
- Material settling or inadequate agitation
Prevention:
Mix and agitate consistently, standardize spray technique and setup, maintain uniform film thickness and flash, and use the recommended orientation or control coat when specified.
Corrective action:
Allow the coating to reach the required stage, prepare the surface as directed, and reapply under uniform conditions. Metallic finishes may require recoating the entire visible panel or part.
Related defects: Floating, flooding, lap marks, and color variation.
Mud Cracking
Definition:
Mud cracking is a pattern of deep, irregular cracks caused by severe shrinkage in an excessively thick or rapidly surface-dried coating film.
What it looks like:
Wide cracks divide the film into islands resembling dried mud. Cracks can extend through the applied coat and expose the layer below.
Common causes:
- Excessive wet- or dry-film thickness
- Heavy application in corners, depressions, or overlapping passes
- Rapid surface drying over wet material
- Incorrect thinner, temperature, airflow, or curing condition
- Applying a high-solids coating beyond its thickness limits
Prevention:
Use the correct tip or nozzle, pressure, distance, speed, and overlap. Measure wet-film thickness, avoid heavy passes, and follow maximum thickness and environmental limits.
Corrective action:
Remove the cracked coating to a sound layer or substrate. Applying another coat will not repair the internal stress. Prepare the surface and reapply at the specified thickness.
Related defects: Alligatoring, checking, cracking, crazing, and wrinkling.
I-M Troubleshooting Summary
Incomplete cure, intercoat contamination, and intercoat delamination require different responses even when the final symptom is poor adhesion. Verify mixing and cure before blaming surface preparation. Locate the precise plane of separation before rebuilding a coating system.
Application records are essential. Document coating batch, mix ratio, induction time, pot life, viscosity, spray setup, film thickness, temperature, humidity, dew point, flash time, and cure. For appearance defects, also record substrate color, primer shade, lighting, gun technique, and material agitation.
Continue the Defects Dictionary
- A-B: Adhesion failure through bubbling
- C-D: Chalking through dry spray
- E-H: Edge failure through holidays
- I-M: Impact damage through mud cracking
- N-R: Nibs through runs
- S-Z: Sagging through wrinkling
- Powder Coating Defects: A separate guide to powder application and curing defects
Need Help Troubleshooting a Coating Defect?
Azimuth Spray Systems can help evaluate spray equipment, fluid delivery, compressed-air requirements, application settings, and finishing-process problems. Call 817-490-1777 or email [email protected].
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