Paint and Coatings Defects Dictionary: N-R
Definitions, Causes, Prevention, and Corrective Action
The N-R installment covers contamination in the wet film, texture and atomization problems, overspray, adhesion loss, coverage irregularities, pinholes, porosity, runs, and corrosion appearing through a coating system.
Several defects can resemble one another. A rough finish may result from nibs, orange peel, dry spray, overspray, or poor atomization. A small opening may be a pinhole, crater, fisheye, solvent pop, or holiday. Examine the defect closely and review the complete application record before choosing 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.
Nibs
Definition:
Nibs are small raised particles or points in a coating film caused by dust, fibers, dried coating, or other solid contamination.
What it looks like:
Individual bumps interrupt an otherwise smooth finish. They may feel sharp or gritty and are often easier to see under low-angle lighting.
Common causes:
- Dust, lint, hair, abrasive residue, or clothing fibers
- Unstrained coating or contaminated containers
- Dried material breaking loose from lids, hoses, guns, pumps, or pressure pots
- Dirty booth, filters, racks, floors, or compressed air
- Static attraction on the part
Prevention:
Clean parts, equipment, and the spray area; strain coating as recommended; maintain booth filters and airflow; control static; and separate sanding and grinding from finishing.
Corrective action:
Remove the contamination source. After cure, nib-sand and polish minor defects if permitted. Severe contamination may require sanding and recoating.
Related defects: Bittiness, dirt inclusions, seediness, and overspray.
Orange Peel
Definition:
Orange peel is an uneven surface texture resembling the skin of an orange. Atomized droplets fail to level into a smooth, continuous film before the coating sets.
What it looks like:
The surface has rounded hills and valleys that distort reflected light. Texture may be fine or coarse and uniform or patchy.
Common causes:
- Viscosity too high or reducer too fast for the conditions
- Poor atomization, low atomizing air, or incorrect fluid pressure
- Wrong tip, fluid nozzle, air cap, or spray technology
- Gun too far away, travel speed too fast, or fluid delivery too low
- Insufficient wet-film thickness or flow time
- High temperature, excessive airflow, or cold material
Prevention:
Use the recommended viscosity and reducer, select the proper spray setup, maintain correct gun distance and speed, and balance fluid delivery with only enough atomizing energy to form an acceptable pattern.
Corrective action:
Correct viscosity, reducer, temperature, pressure, tip or nozzle selection, and technique. A cured defect may require sanding or polishing followed by recoating.
Related defects: Dry spray, poor atomization, poor flow, and rough finish.
Overspray
Definition:
Overspray is atomized coating that misses the intended target or settles onto adjacent surfaces as wet, partially dry, or dry particles.
What it looks like:
Nearby surfaces develop a dusty, gritty, dull, discolored, or rough deposit. Material use, booth loading, and filter loading may increase.
Common causes:
- Excessive atomizing air or fluid pressure
- Gun held too far from the workpiece
- Spray pattern too wide or poorly aimed
- Incorrect tip, nozzle, air cap, or application technology
- Poor part positioning, masking, booth airflow, or transfer efficiency
- Fast-drying material or excessive air movement
Prevention:
Use the lowest pressure that produces acceptable atomization, maintain proper gun distance and angle, match pattern size to the part, optimize part orientation, and maintain correct booth airflow.
Corrective action:
Correct the spray setup and airflow. Remove deposited material using an approved cleaning, sanding, or polishing method. Recoat if the finish is damaged.
Related defects: Dry spray, rough finish, material waste, and poor transfer efficiency.
Peeling
Definition:
Peeling is the separation of a coating from the substrate or another coating layer in strips, sheets, or curled sections.
What it looks like:
Large areas lift and detach, often with flexible edges that can be pulled away from the surface.
Common causes:
- Contamination, moisture, corrosion, or inadequate surface preparation
- Incompatible coatings or missed recoat window
- Applying over a glossy surface without required abrasion
- Coating a damp substrate or a surface near the dew point
- Poor cure, excessive thickness, or substrate movement
Prevention:
Verify cleanliness, dryness, profile, compatibility, dew-point separation, and recoat timing. Correct moisture entry before coating.
Corrective action:
Remove all poorly bonded coating to a sound layer or substrate, identify the plane and cause of failure, correct it, and rebuild the compatible coating system.
Related defects: Adhesion failure, delamination, flaking, and lifting.
Picture Framing
Definition:
Picture framing is a visible color, gloss, or film-build difference around the perimeter of a panel, wall, door, or other coated area.
What it looks like:
A darker, lighter, glossier, duller, or heavier border surrounds the central area like a picture frame.
Common causes:
- Brush or roller cutting-in followed by a different application method
- Double coating or excessive overlap around edges
- Different film thickness, texture, or drying rate at the perimeter
- Failure to maintain a wet edge
- Spray passes concentrated along borders or recesses
Prevention:
Coordinate cutting-in and field application, maintain a wet edge, use compatible tools and consistent film thickness, and plan spray passes to avoid excessive perimeter buildup.
Corrective action:
Allow the coating to cure, prepare the entire visible area uniformly, and recoat from one natural break to another.
Related defects: Lap marks, gloss variation, mottling, and striping.
Pinholing
Definition:
Pinholing is the formation of very small holes that extend into or through a coating film because air, gas, solvent, or moisture escaped after the surface began to set.
What it looks like:
Tiny circular openings appear individually or in groups. They may expose the layer below and can create holidays in protective coatings.
Common causes:
- Air escaping from porous concrete, wood, castings, welds, or filler
- Entrained air from mixing, pumping, or application
- Excessive film thickness or rapid surface cure
- Solvent or moisture trapped beneath the surface
- Incorrect reducer, temperature, or application over a warming substrate
Prevention:
Seal porous surfaces when specified, apply while substrate temperature is stable or falling when appropriate, control mixing and pumping, use the correct viscosity and film thickness, and allow required flash time.
Corrective action:
Determine the source of escaping gas or liquid. Sand or remove the affected film, fill or seal porosity when required, and recoat under controlled conditions.
Related defects: Bubbling, cratering, holidays, porosity, and solvent popping.
Poor Atomization
Definition:
Poor atomization occurs when a spray system does not break the coating into droplets appropriate for the required finish and application rate.
What it looks like:
The pattern may contain coarse droplets, heavy center or edges, tails, spitting, pulsation, uneven texture, or incomplete breakup.
Common causes:
- Viscosity too high or material too cold
- Insufficient or excessive atomizing energy
- Worn, damaged, dirty, or mismatched tip, nozzle, needle, or air cap
- Fluid delivery outside the spray setup's operating range
- Restricted air passages, inadequate compressor capacity, or pressure loss
- Pump pulsation, air in the fluid, or unstable fluid pressure
Prevention:
Match the spray technology and setup to the coating, maintain equipment, verify air and fluid capacity under flow, control viscosity and temperature, and use only the pressure needed for an acceptable pattern.
Corrective action:
Test the pattern, inspect and clean the gun, verify tip or nozzle condition, measure pressures while spraying, confirm viscosity and temperature, and correct unstable fluid delivery.
Related defects: Dry spray, orange peel, overspray, spitting, and tails.
Poor Flow and Leveling
Definition:
Poor flow and leveling occur when a wet coating does not smooth out sufficiently before it sets.
What it looks like:
Brush marks, roller texture, spray texture, overlap lines, or surface irregularities remain visible after drying.
Common causes:
- Viscosity too high or incorrect reducer
- Coating or substrate too cold
- Film too thin to level or too thick to release solvent correctly
- Fast evaporation, high airflow, or high temperature
- Incorrect application tool, spray setup, or technique
- Coating beginning to gel or exceeding pot life
Prevention:
Maintain recommended material temperature and viscosity, use the correct reducer and application equipment, apply the specified film thickness, and control airflow and drying conditions.
Corrective action:
Correct the material, equipment, or environmental cause. Sand or polish cured defects when allowed, or prepare and recoat.
Related defects: Orange peel, brush marks, lap marks, and dry spray.
Poor Hiding
Definition:
Poor hiding is insufficient opacity that allows the substrate, primer, repair, or previous color to remain visible through the finish.
What it looks like:
Underlying color, repairs, stripes, edges, or variations show through the topcoat.
Common causes:
- Insufficient film thickness or too few coats
- Over-reduction or low material solids
- Pigment settlement or inadequate mixing
- Incorrect primer or ground-coat shade
- Uneven overlap, fluid delivery, or spray technique
Prevention:
Mix thoroughly, use the recommended primer shade, apply uniform coverage and specified film thickness, and avoid over-reduction.
Corrective action:
Apply additional compatible coating within the manufacturer's recoat instructions. Uneven or cured work may require preparation before recoating.
Related defects: Grinning, low film thickness, holidays, and color variation.
Porosity
Definition:
Porosity is the presence of microscopic or visible voids, pores, channels, or discontinuities within a coating film.
What it looks like:
The film may appear rough, sponge-like, pinholed, or normal to the eye while still allowing air, moisture, chemicals, or electrical current to pass through.
Common causes:
- Air, solvent, gas, or moisture trapped during cure
- Porous substrate releasing air
- Foaming, poor atomization, dry spray, or insufficient flow
- Incorrect film thickness, mixing, reducer, or cure
- Contamination or an inherently porous application method
Prevention:
Prepare and seal porous substrates when required, control entrained air and solvent release, apply at specified thickness, and inspect protective films using approved methods.
Corrective action:
Determine the depth and cause. Sand, remove, fill, or seal the affected area as specified and apply a continuous compatible repair coat. Retest when required.
Related defects: Holidays, pinholing, bubbling, and solvent popping.
Runs
Definition:
Runs are narrow downward movements of wet coating caused by gravity before the film develops enough body to remain in place.
What it looks like:
Elongated beads, trails, or heavy curtains extend downward from an overloaded area. A run is generally narrower and more localized than broad sagging.
Common causes:
- Excessive wet-film thickness or slow gun travel
- Gun held too close or pattern overlap too heavy
- Viscosity too low or too much slow reducer
- Fluid delivery too high or tip or nozzle too large
- Cold surface, poor flash, or inadequate air movement
- Irregular geometry collecting excess coating
Prevention:
Use the proper viscosity, tip or nozzle, fluid delivery, gun distance, speed, and overlap. Apply controlled coats and observe flash times.
Corrective action:
If permitted, correct a wet run using the coating manufacturer's procedure. After cure, level the defect by sanding and polish or recoat as required.
Related defects: Sagging, curtaining, excessive film thickness, and wrinkling.
Rust Bleed-Through
Definition:
Rust bleed-through is reddish-brown staining that migrates from corrosion beneath or adjacent to a coating and becomes visible at the surface.
What it looks like:
Rust-colored spots, streaks, or stains appear through the finish, commonly at pits, seams, fasteners, welds, and edges.
Common causes:
- Residual corrosion left in pits, seams, or inaccessible areas
- Moisture and oxygen reaching incompletely protected steel
- Soluble iron contamination or rust staining carried by water
- Insufficient primer coverage or incompatible stain-blocking ability
- Corrosion products not removed before coating
Prevention:
Remove corrosion and contamination to the specified standard, clean pits and seams, dry the surface, stripe-coat difficult areas, and use a compatible corrosion-control system.
Corrective action:
Remove the coating and corrosion around the stain, identify moisture entry, restore surface preparation, and rebuild the specified primer and topcoat system.
Related defects: Flash rusting, rust breakthrough, staining, and underfilm corrosion.
Rust Breakthrough
Definition:
Rust breakthrough is visible corrosion of steel through a coating system after the protective barrier has failed or become too thin.
What it looks like:
Rust spots, lines, or larger corroded areas appear at holidays, scratches, edges, thin films, damaged areas, or beneath blistered and peeling coating.
Common causes:
- Holidays, pinholes, low film thickness, or mechanical damage
- Inadequate surface preparation or soluble-salt contamination
- Edge failure, poor stripe coating, or difficult geometry
- Coating system unsuitable for the environment
- Normal aging beyond the maintenance interval
Prevention:
Prepare steel to specification, control soluble salts, stripe-coat edges and welds, verify film thickness and continuity, select the correct system, and repair damage promptly.
Corrective action:
Determine the extent of underfilm corrosion, remove failed coating and rust, restore the specified surface preparation, feather sound edges, and apply the approved repair system.
Related defects: Edge failure, holidays, rust bleed-through, undercutting, and blistering.
N-R Troubleshooting Summary
Begin by separating surface texture from film discontinuity. Orange peel and poor flow leave a continuous but uneven film. Pinholes and porosity create openings or internal voids. Holidays describe any unacceptable discontinuity and may result from pinholes, skips, thin edges, contamination, or damage.
For spray defects, record coating viscosity and temperature, reducer, tip or fluid-nozzle size, air cap, air and fluid pressures while spraying, gun distance, travel speed, overlap, booth airflow, and wet-film thickness. For corrosion failures, document surface preparation, soluble-salt control, stripe coating, dry-film thickness, holiday testing, exposure, and damage history.
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 rust breakthrough
- 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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