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Paint and Coatings Defects Dictionary: S-Z
Last Updated: 08/23/2026

Paint and Coatings Defects Dictionary: S-Z

Definitions, Causes, Prevention, and Corrective Action

The S-Z installment completes the liquid paint and coatings dictionary. It covers wet-film movement, chemical attack, contamination, spray-pattern failures, trapped solvent, corrosion spread, water exposure, wrinkling, and color change.

Do not diagnose by name alone. Sagging and runs are related but differ in shape. Solvent popping, pinholing, and porosity can resemble one another. Staining may originate on the surface, within the coating, or below it. Examine the defect and the complete finishing process before selecting a repair.

Safety reminder: Follow coating and equipment manufacturers' instructions. Use required ventilation, grounding, respiratory protection, personal protective equipment, and pressure-relief procedures. Treat high-pressure fluid injection as a medical emergency.

Sagging

Definition:
Sagging is the downward movement of wet coating under gravity, producing a broad, uneven area of excessive film thickness.

What it looks like:
The finish forms waves, curtains, or heavy drooping sections. A sag is generally broader than a narrow run.

Common causes:

Excessive wet-film thickness
Gun too close, travel too slow, or overlap too heavy
Viscosity too low or too much slow reducer
Fluid delivery or tip size too large
Cold surface, poor flash time, or uneven geometry

Prevention:
Control viscosity, tip or nozzle size, fluid delivery, gun distance, speed, overlap, and flash time. Measure wet-film thickness on critical work.

Corrective action:
Follow the manufacturer's wet-film repair instructions when applicable. After cure, level the sag by sanding and polish or recoat as required.

Related defects: Runs, curtaining, excessive film thickness, and wrinkling.

Saponification

Definition:
Saponification is alkaline chemical attack on certain oil-based or alkyd coatings, producing soap-like compounds and loss of film integrity.

What it looks like:
The coating may become soft, sticky, discolored, chalky, or poorly adhered on alkaline concrete, masonry, plaster, or contaminated surfaces.

Common causes:

Applying alkyd or oil-based coating to a highly alkaline substrate
Moisture transporting alkaline compounds to the coating
Coating uncured masonry, concrete, or plaster
Inadequate testing, sealing, or substrate preparation

Prevention:
Test substrate condition and pH as required, correct moisture sources, allow adequate cure, and select an alkali-resistant primer and topcoat system.

Corrective action:
Remove affected coating, correct moisture and alkalinity problems, clean and prepare the substrate, and apply a compatible alkali-resistant system.

Related defects: Adhesion failure, chalking, soft film, and peeling.

Seediness

Definition:
Seediness is a granular surface defect caused by small gelled particles, pigment agglomerates, skins, or partially cured coating within the material.

What it looks like:
Numerous small, often uniform particles appear throughout the finish rather than as a few isolated dirt inclusions.

Common causes:

Old, contaminated, frozen, overheated, or improperly stored coating
Material used beyond pot life or shelf life
Incompatible products, reducers, or components
Inadequate mixing or pigment dispersion
Dried coating introduced from containers or equipment

Prevention:
Store materials correctly, verify shelf life, mix compatible components, observe pot life, strain as recommended, and keep containers and equipment clean.

Corrective action:
Stop using suspect material and confirm its condition. Sand or remove the affected finish and recoat with properly prepared material.

Related defects: Bittiness, dirt inclusions, nibs, and gel particles.

Settling

Definition:
Settling is the separation and accumulation of pigments or other solids at the bottom of a coating container during storage or use.

What it looks like:
A dense layer forms at the bottom while the upper liquid becomes clearer or different in color. Applied coating may show poor hiding, color shift, or gloss variation.

Common causes:

Long storage without agitation
High pigment density or inadequate formulation stability
Improper storage temperature or vibration
Insufficient mixing before or during use

Prevention:
Store material as directed, rotate inventory, mix from the bottom until uniform, and maintain appropriate agitation without entraining air.

Corrective action:
Determine whether the settled material can be fully redispersed. Do not use material containing hard, irreversible settlement unless the manufacturer approves it.

Related defects: Color variation, floating, flooding, and poor hiding.

Skinning

Definition:
Skinning is the formation of a partially cured film across the surface of liquid coating in a container.

What it looks like:
A flexible, rubbery, or brittle sheet forms over the material. Broken pieces can contaminate the coating and spray equipment.

Common causes:

Air exposure in a partially filled or poorly sealed container
Repeated opening, heat, or long storage
Oxidative-curing coating chemistry
Failure to follow storage or preservation instructions

Prevention:
Keep containers tightly sealed, minimize headspace and exposure when approved, use clean lids, and follow storage and inventory recommendations.

Corrective action:
Do not mix the skin back into the coating. Remove it carefully and strain only when the manufacturer permits continued use. Discard degraded material.

Related defects: Bittiness, nibs, seediness, and clogged spray equipment.

Soft Film

Definition:
Soft film is a coating that fails to develop or retain its expected hardness and resistance.

What it looks like:
The surface scratches, dents, imprints, blocks, or remains tacky under normal handling.

Common causes:

Incorrect component ratio or incomplete mixing
Low temperature, insufficient cure, or omitted catalyst
Excessive or incompatible reducer
Material used beyond pot life or shelf life
Plasticizer, solvent, chemical, or moisture attack

Prevention:
Mix accurately, observe induction and pot life, apply correct thickness, and provide the specified cure time and temperature.

Corrective action:
Consult the manufacturer. Additional cure may help correctly mixed material; incorrectly mixed, contaminated, or chemically damaged coating normally requires removal.

Related defects: Incomplete cure, blocking, marring, and solvent sensitivity.

Solvent Popping

Definition:
Solvent popping occurs when trapped solvent vapor breaks through a coating surface after the film has begun to set.

What it looks like:
Small craters, bubbles, pinholes, or ruptured openings appear during flash, bake, or early cure.

Common causes:

Excessive film thickness
Insufficient flash time between coats or before bake
Reducer too fast or oven temperature rising too quickly
High material or booth temperature
Surface skinning that traps solvent below

Prevention:
Apply specified film thickness, use the correct reducer, observe flash time, control temperature, and follow the recommended bake schedule.

Corrective action:
Allow full cure, sand or remove the affected film, and recoat using corrected thickness, flash, solvent, and temperature controls.

Related defects: Bubbling, cratering, pinholing, and porosity.

Spitting

Definition:
Spitting is the intermittent discharge of large droplets or blobs from a spray gun instead of a stable atomized pattern.

What it looks like:
Large droplets, spots, or splashes appear within or around the spray pattern.

Common causes:

Loose, worn, dirty, or damaged fluid nozzle, tip, needle, or packing
Air entering the fluid passage or suction line
Low material level, pump cavitation, or pulsation
Material buildup at the air cap or tip
Incorrect gun adjustment or unstable pressure

Prevention:
Maintain the gun and pump, tighten suction connections, keep adequate material level, clean the spray head, and stabilize air and fluid delivery.

Corrective action:
Stop spraying, relieve pressure, inspect and clean components, correct air leaks or pump problems, and test the pattern before returning to production.

Related defects: Poor atomization, pulsation, tails, and surface spots.

Spray Tails

Definition:
Spray tails are heavy bands or fingers at the outer edges of a spray pattern caused by incomplete atomization or poor pattern balance.

What it looks like:
The test pattern has dense material at one or both ends instead of an even distribution with controlled edges.

Common causes:

Fluid pressure or atomizing energy too low
Viscosity too high or material too cold
Tip or nozzle too large, worn, damaged, or partially blocked
Fluid delivery outside the setup's operating range
Restricted air-cap passages or inadequate air supply

Prevention:
Match the tip, nozzle, and air cap to the coating and flow requirement. Maintain viscosity, temperature, air capacity, and clean equipment.

Corrective action:
Clean and inspect the spray setup, verify viscosity and pressure while spraying, and replace worn or damaged parts. Do not exceed equipment ratings.

Related defects: Poor atomization, uneven pattern, spitting, and orange peel.

Staining

Definition:
Staining is localized discoloration caused by contamination, chemical exposure, moisture, corrosion products, substrate extractives, or material migration.

What it looks like:
Spots, streaks, rings, drips, or patches differ from the intended color and may remain after ordinary cleaning.

Common causes:

Water, chemicals, oils, smoke, dirt, or biological material
Rust, tannins, dyes, asphalt, or other substances migrating through the film
Porous or inadequately sealed substrate
Cleaning products or service chemicals attacking the coating

Prevention:
Remove contamination, seal stain-producing substrates, select chemical-resistant coatings, correct leaks, and test the complete system.

Corrective action:
Identify whether the stain is on, within, or beneath the coating. Clean when possible; otherwise isolate the source with an approved sealer or remove and rebuild the affected system.

Related defects: Bleeding, discoloration, rust bleed-through, and water spotting.

Streaking

Definition:
Streaking is the appearance of elongated lines or bands that differ in color, gloss, texture, or film thickness from the surrounding finish.

What it looks like:
Visible brush, roller, spray, drain, or contamination lines run across or down the surface.

Common causes:

Uneven application, overlap, pressure, or fluid delivery
Inadequate mixing or pigment separation
Water, surfactant, cleaner, or contamination running over the film
Brush or roller marks that do not level
Uneven substrate absorption or drying

Prevention:
Mix consistently, maintain uniform technique and film thickness, seal porous surfaces, control drainage and condensation, and allow adequate flow and leveling.

Corrective action:
Determine whether the streak is residue, color variation, or film texture. Clean when appropriate; otherwise prepare and recoat the complete visible area.

Related defects: Lap marks, mottling, surfactant leaching, and water spotting.

Surface Contamination

Definition:
Surface contamination is any unwanted material on a substrate or coating that interferes with wetting, adhesion, cure, appearance, or protection.

What it looks like:
Contamination may be visible or invisible and can produce fisheyes, craters, dirt, staining, delamination, blistering, or incomplete cure.

Common causes:

Oil, grease, silicone, wax, dust, moisture, salts, or corrosion
Dirty compressed air, tools, gloves, rags, containers, or equipment
Residue from cleaning, sanding, blasting, or handling
Airborne material from nearby production

Prevention:
Define and verify a cleaning standard, maintain clean air and equipment, separate contaminating operations, protect prepared surfaces, and inspect before every coat.

Corrective action:
Identify the contaminant, remove affected coating when necessary, clean by an approved method, verify cleanliness, and recoat.

Related defects: Adhesion failure, cissing, cratering, fisheyes, and intercoat contamination.

Telegraphing

Definition:
Telegraphing is the visible reproduction of substrate joints, repairs, sanding marks, texture, fasteners, or coating boundaries through the finish.

What it looks like:
Lines, outlines, patches, scratches, or texture from underlying construction remain visible after topcoating.

Common causes:

Insufficient filling, sanding, sealing, or primer build
Different porosity or movement among underlying materials
Repair products not fully cured
Topcoat shrinkage or aggressive solvents
Film thickness insufficient to level the surface

Prevention:
Stabilize and level the substrate, complete compatible repairs, allow full cure, seal porosity differences, and inspect under proper lighting before finishing.

Corrective action:
Sand the affected area level, repair movement or porosity, reprime or reseal, and reapply the finish.

Related defects: Mapping, featheredge splitting, sinkage, and picture framing.

Undercutting

Definition:
Undercutting is the lateral spread of corrosion or deterioration beneath a coating from a scratch, edge, holiday, fastener, or other break.

What it looks like:
Coating lifts, blisters, or loses adhesion outward from the damaged area while corrosion advances underneath.

Common causes:

Moisture and contaminants entering through coating damage
Inadequate surface preparation or soluble salts
Poor edge coverage or stripe coating
Coating system with insufficient corrosion resistance
Failure to repair damage promptly

Prevention:
Prepare the substrate correctly, control salts, protect edges, achieve continuous film thickness, select a suitable system, and repair breaks quickly.

Corrective action:
Remove coating beyond all loss of adhesion and corrosion, restore surface preparation, feather sound edges, and apply the approved repair system.

Related defects: Filiform corrosion, rust creep, edge failure, and delamination.

Uneven Color

Definition:
Uneven color is unintended variation in hue, value, chroma, or visual uniformity across a coated surface or among parts.

What it looks like:
Light and dark areas, shade shifts, bands, patches, or inconsistent metallic appearance are visible.

Common causes:

Inadequate mixing, agitation, or pigment settlement
Uneven film thickness, overlap, gun distance, or atomization
Different primer shades, substrates, batches, or cure conditions
Floating, flooding, mottling, or poor hiding
Inconsistent reducer, viscosity, or application method

Prevention:
Standardize materials, mixing, substrate color, spray technique, film thickness, flash, and cure. Maintain agitation when required.

Corrective action:
Identify the source of variation. Prepare and recoat the complete visible panel or part under uniform conditions when spot repair would remain visible.

Related defects: Floating, flooding, metamerism, mottling, and poor hiding.

Water Spotting

Definition:
Water spotting is visible marking caused when droplets or standing water deposit minerals, extract coating components, or disturb a coating before full cure.

What it looks like:
Circular rings, cloudy spots, stains, gloss changes, or etched areas remain after water evaporates.

Common causes:

Water exposure before the coating develops resistance
Hard-water mineral deposits
Acid rain, contaminated rinse water, or chemicals in droplets
Standing water, condensation, or poor drainage
Heat or sunlight accelerating droplet evaporation

Prevention:
Protect fresh coating from water, provide drainage, use suitable rinse water, dry parts, and select a coating with the required water and chemical resistance.

Corrective action:
Clean using an approved method. Polishing may remove minor spots. Etched, stained, or softened films may require sanding and recoating.

Related defects: Staining, surfactant leaching, blushing, and gloss variation.

Wrinkling

Definition:
Wrinkling is the formation of ridges and folds when a coating surface cures or skins while material beneath remains wet, soft, or mobile.

What it looks like:
The surface develops ripples, puckers, or skin-like folds. The film below may remain soft.

Common causes:

Excessive film thickness
Heavy topcoat over an uncured layer
Rapid surface cure caused by heat or airflow
Incorrect thinner, drier, catalyst, or mixing ratio
Recoating at an improper stage or solvent attack

Prevention:
Apply specified thickness, observe flash and recoat times, mix accurately, and maintain the recommended temperature and airflow.

Corrective action:
Allow the coating to stabilize, then remove wrinkled and soft material to a sound layer. Correct the cause and reapply the system.

Related defects: Lifting, mud cracking, sagging, and incomplete cure.

Yellowing

Definition:
Yellowing is an unintended shift toward a yellow or amber color during cure, aging, or service.

What it looks like:
White, clear, or light-colored finishes become cream, amber, or unevenly yellow. Areas with heat, darkness, or chemical exposure may change differently.

Common causes:

Natural oxidation of certain alkyd, oil, or resin systems
Ultraviolet, heat, smoke, chemical, or nitrogen-oxide exposure
Overbake or excessive curing temperature
Migration from substrate, sealer, adhesive, or underlying coating
Coating chemistry not color-stable for the service

Prevention:
Select a non-yellowing or color-stable system, control bake and cure, isolate migrating materials, and reduce smoke, heat, and chemical exposure.

Corrective action:
Identify whether yellowing is inherent, exposure-related, or caused by migration. Permanent color change generally requires preparation and recoating with a more suitable system.

Related defects: Bleeding, discoloration, fading, and staining.

S-Z Troubleshooting Summary

Wet-film defects such as sagging, spitting, spray tails, and solvent popping require attention to material condition, spray equipment, film thickness, and timing. Service defects such as saponification, undercutting, water spotting, and yellowing require equal attention to substrate chemistry and exposure.

Record coating batch, mixing ratio, induction, pot life, viscosity, material temperature, tip or nozzle, air cap, pressures while spraying, gun technique, wet- and dry-film thickness, temperature, humidity, dew point, flash, and cure. A reliable record separates recurring process failures from isolated contamination or handling damage.