The Language of Finishing: Paint and Coatings Glossary U–V

This installment of the AirSprayTech.com Paint and Coatings Industry Glossary covers terms beginning with U and V. These definitions address coating cure, ultrasonic inspection, urethane coatings, ultraviolet exposure, ventilation, viscosity measurement, volatile organic compounds, volume solids, and electrostatic application.

Coating terminology can have different meanings depending on the industry, specification, manufacturer, or application process. Always consult the governing specification, approved product data sheet, safety data sheet, equipment manual, and complete referenced standard before performing coating work.

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U

UHP Waterjetting — Ultra-High-Pressure Waterjetting

Ultra-high-pressure waterjetting uses water at extremely high pressure to remove coatings, corrosion products, salts, and other contamination from a surface.

UHP waterjetting can remove deteriorated coatings while substantially reducing airborne abrasive compared with dry abrasive blasting. It does not normally create the same new anchor profile produced by abrasive blasting, although it can expose an existing profile.

The process can leave the surface wet and may produce flash rust on steel. The acceptable surface condition, flash-rust level, water quality, and time before coating must be established by the project specification and coating manufacturer.

High-pressure waterjets can cause severe injury. Operators require specialized training, equipment inspection, protective clothing, controlled work zones, and adherence to the equipment manufacturer’s safety procedures.

UL — UL Solutions

UL is commonly associated with UL Solutions, an organization that develops safety standards and evaluates products, materials, systems, and assemblies.

Coating professionals may encounter UL listings or classifications for fire-resistive coatings, intumescent systems, roof assemblies, spray booths, ovens, electrical equipment, and building materials.

A UL designation generally applies to the complete listed system or assembly—not simply to one coating product. The substrate, primer, film thickness, topcoat, construction details, and application conditions may all be part of the listing.

Additional information is available from UL Solutions.

Ultrasonic Cleaning

Ultrasonic cleaning uses high-frequency sound energy transmitted through a liquid to create microscopic cavitation bubbles. Their formation and collapse help loosen contamination from immersed parts.

The process is used for precision components, spray-gun parts, automotive components, aerospace parts, medical equipment, and manufactured products.

Cleaning performance depends on the solution, frequency, temperature, exposure time, part geometry, and type of contamination. Sensitive spray-gun components, seals, air caps, and finished surfaces should not be placed in an ultrasonic cleaner unless the equipment manufacturer approves the procedure.

Ultrasonic Coating-Thickness Gauge

An ultrasonic coating-thickness gauge uses high-frequency sound pulses to measure coating thickness. It can be particularly useful where magnetic-induction or eddy-current gauges are unsuitable.

Ultrasonic instruments may measure organic coatings over concrete, wood, plastic, composites, wallboard, or other substrates when the coating and substrate provide a suitable acoustic interface.

Readings may be affected by surface roughness, coating texture, porosity, multilayer construction, substrate properties, probe pressure, couplant, and instrument setup.

A commonly referenced procedure is ASTM D6132, Nondestructive Measurement of Dry-Film Thickness Using an Ultrasonic Coating-Thickness Gauge.

Ultrasonic Inspection

Ultrasonic inspection uses high-frequency sound waves to evaluate material thickness, internal discontinuities, bond lines, or coating thickness.

In corrosion-control work, ultrasonic instruments are often used to measure remaining steel thickness without cutting into the structure. This can help determine whether corrosion has reduced a tank, pipe, vessel, or structural component below its required thickness.

Ultrasonic substrate-thickness inspection and ultrasonic coating-thickness measurement are different applications and may require different instruments, probes, settings, and procedures.

Ultraviolet Cure — UV Cure

Ultraviolet cure uses controlled ultraviolet radiation to initiate or accelerate a chemical reaction that turns a liquid or powder material into a cured film.

UV-curable coatings are used on wood products, flooring, plastic components, electronics, automotive parts, metal products, printing, and other high-speed manufacturing operations.

Cure depends on UV wavelength, intensity, exposure time, lamp condition, distance, coating thickness, pigment, part geometry, and line speed. Areas shielded from the light may remain uncured unless the system is designed for shadowed regions.

Ultraviolet Degradation

Ultraviolet degradation is the deterioration of a coating or material caused by exposure to ultraviolet radiation, primarily from sunlight or an artificial source.

Possible effects include chalking, fading, yellowing, gloss loss, cracking, embrittlement, erosion, and reduced adhesion.

UV resistance varies substantially among coating chemistries. A coating with excellent chemical or immersion resistance may have poor exterior color and gloss retention unless protected by a weather-resistant topcoat.

Ultraviolet Exposure Testing

Ultraviolet exposure testing evaluates how a coating changes when subjected to controlled UV radiation, heat, moisture, condensation, or water spray.

Testing may be used to compare color change, gloss loss, chalking, cracking, adhesion, and other properties. Results depend on the lamp type, irradiance, temperature, moisture cycle, specimen preparation, and evaluation method.

Accelerated UV hours should not be converted directly into outdoor years using a universal formula. Correlation depends on the material, climate, exposure, and property being measured.

Underbake

Underbake occurs when a coating or coated part receives insufficient heat exposure to achieve the required cure. It may result from inadequate oven temperature, insufficient time at temperature, excessive line speed, heavy part mass, poor airflow, or inaccurate controls.

Possible symptoms include softness, poor adhesion, low hardness, reduced chemical resistance, poor impact performance, gloss variation, blocking, or coating transfer during handling.

Oven air temperature alone does not prove that the coating has cured. Actual part-metal temperature and time at temperature are commonly the controlling values.

Undercoat

An undercoat is a coating layer applied beneath a finish coat. Depending on the system, it may function as a primer, surfacer, sealer, intermediate coat, sound-deadening layer, corrosion-resistant layer, or color base.

The term is widely used in automotive, wood, architectural, marine, and industrial finishing, but its exact meaning varies. The product designation and coating-system schedule should identify its intended function.

Undercure

Undercure means a coating has not completed the physical or chemical development required for its intended performance.

Causes include insufficient temperature or cure time, incorrect mix ratio, inadequate mixing, expired material, low humidity for moisture-cure products, poor ventilation, excessive film thickness, or contamination.

An undercured film may remain soft, tacky, brittle, weakly adhered, solvent-sensitive, or chemically vulnerable. A dry surface appearance does not prove complete cure.

Undercutting

Undercutting is the lateral spread of corrosion or coating separation beneath an otherwise intact film. It often begins at a scratch, scribe, edge, holiday, crack, or mechanically damaged area.

The coating may remain attached above the corrosion products while the affected area expands underneath. Undercutting can be promoted by salts, moisture, poor surface preparation, inadequate adhesion, and insufficient edge protection.

Underfilm Corrosion

Underfilm corrosion is corrosion occurring beneath a coating. It may begin at a holiday, damaged area, poorly prepared substrate, contaminated region, edge, weld, or area where moisture penetrates the system.

Warning signs include blistering, staining, rust breakthrough, swelling, cracking, and coating disbondment. The visible defect may be smaller than the corroded area beneath the film.

Underfilm Staining

Underfilm staining is visible discoloration originating beneath a clear or translucent coating. It may result from corrosion, moisture, wood extractives, adhesive, filler, chemical residue, or migration from the substrate.

Correction requires identifying and addressing the source. Applying another clear or colored coat over the stain may not prevent it from returning.

Underthinning

Underthinning occurs when a coating receives less approved thinner or reducer than needed for the specified application conditions and equipment.

Possible effects include excessive viscosity, poor atomization, orange peel, dry spray, poor flow, an uneven pattern, and excessive pressure requirements.

Many coatings are designed for application without thinning. Material should never be thinned automatically; the product data sheet and actual application conditions should control.

Uniformity

Uniformity is the consistency of a coating’s thickness, color, gloss, texture, cure, and appearance across the workpiece or coated area.

Uniformity is affected by surface preparation, mixing, spray pattern, overlap, gun speed, distance, part geometry, booth airflow, oven performance, grounding, and material condition.

A visually uniform coating may still contain unacceptable variations in dry-film thickness or cure. Appearance and measured performance should be evaluated separately.

Use-By Date

A use-by date is the manufacturer’s stated date by which a properly stored product or coating component should be used.

It differs from pot life, which begins after components are mixed. It may also differ from a manufacturing date or retest date.

Expired material should not be used merely because it appears normal. Written manufacturer authorization and appropriate testing may be required.

Urethane Coating

Urethane coating is a commonly used name for polyurethane coating. These coatings are selected for properties such as abrasion resistance, flexibility, chemical resistance, gloss retention, color retention, and weathering.

Urethane coatings may be two-component, moisture-cure, waterborne, solvent-borne, clear, pigmented, or elastomeric. Their performance and safety requirements vary substantially.

Many two-component polyurethane products contain an isocyanate-functional hardener. Application requires appropriate exposure assessment, ventilation, protective clothing, eye protection, and respiratory protection. Consult the product SDS and OSHA’s Isocyanates Safety and Health information.

Urethane Reducer

Urethane reducer is a solvent blend specifically designed to adjust the viscosity, flow, atomization, or evaporation rate of a compatible urethane coating.

Reducers may be offered in slow, medium, fast, or temperature-related grades. Selection depends on coating temperature, part size, airflow, application method, and environmental conditions.

Using an unapproved solvent can cause moisture contamination, poor flow, solvent pop, lifting, cure problems, or loss of coating performance.

UV — Ultraviolet

UV is the abbreviation for ultraviolet radiation, a region of electromagnetic energy beyond visible violet light.

In the coatings industry, UV may refer to sunlight exposure, accelerated-weathering testing, or an intentional curing process. The same term therefore can describe either a cause of coating degradation or a controlled source of curing energy.

UV Resistance

UV resistance is a coating’s ability to withstand ultraviolet exposure without unacceptable chalking, fading, yellowing, cracking, gloss loss, erosion, or embrittlement.

UV resistance is especially important for automotive, aerospace, rail, marine, roofing, architectural, and exterior industrial applications.

An interior coating or immersion-grade lining should not be assumed suitable for direct sunlight unless the manufacturer specifically approves the exposure.

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V

V-Packings

V-packings are sealing elements with a V-shaped cross section used in pumps, hydraulic equipment, and high-pressure fluid systems. Multiple packing rings may be installed together to seal around a moving rod or piston.

Packing material must be compatible with the coating, solvent, pressure, temperature, and equipment design. Worn or damaged packings can cause leakage, pressure loss, rapid pump cycling, contamination, and inconsistent fluid delivery.

Vacuum Blasting

Vacuum blasting is an abrasive-blast process that uses a shroud and vacuum system around the blasting head to collect abrasive, dust, coating debris, and corrosion products near the point of impact.

It can reduce airborne dust and simplify cleanup in localized maintenance work. The efficiency depends on shroud contact, surface geometry, vacuum airflow, abrasive recovery, equipment condition, and operator technique.

Vacuum blasting does not eliminate the need to evaluate worker exposure, hazardous coatings, containment, waste, noise, and the prepared surface.

Vacuum Recovery

Vacuum recovery uses suction to collect abrasive, dust, powder, liquid, or coating debris. It may be integrated with blasting equipment or used during cleanup and material recycling.

The vacuum, filters, hoses, collection vessel, grounding, and disposal method must be suitable for the collected material. Ordinary shop vacuums may be inappropriate for combustible dust, flammable liquids, hazardous materials, or fine toxic particles.

Valve

A valve controls, directs, starts, stops, or regulates the flow of air, liquid coating, powder, solvent, water, or another material.

Finishing equipment may use ball valves, needle valves, check valves, relief valves, circulation valves, triggered fluid valves, dump valves, and pressure-control valves.

The valve must be pressure-rated and chemically compatible with the material. A valve should not be used as the only means of protecting a system from overpressure unless it is specifically designed and approved for that function.

Vapor Blasting

Vapor blasting is a term used for certain wet-abrasive blasting processes in which water and abrasive are combined at or near the blasting nozzle.

The process can reduce airborne dust and abrasive rebound compared with dry blasting. It may also help wash contaminants from the surface.

The prepared surface must be evaluated for flash rust, residual abrasive, soluble salts, moisture, profile, and cleanliness before coating.

Vapor Barrier

A vapor barrier is a material or system designed to restrict the movement of water vapor through a building assembly, floor, wall, roof, or substrate.

The term is often used broadly, although building-science documents may distinguish between vapor barriers and vapor retarders according to measured permeance.

A surface coating should not be assumed to function as a code-compliant vapor barrier without appropriate test data and assembly design.

Vapor Pressure

Vapor pressure is the pressure produced by a vapor in equilibrium with its liquid at a given temperature. A liquid with higher vapor pressure generally has a greater tendency to evaporate.

Vapor pressure influences solvent evaporation, flammability, worker exposure, container pressure, spray-booth ventilation, and material behavior at changing temperatures.

Vapor Transmission

Vapor transmission is the movement of vapor through a coating, membrane, substrate, or building assembly. Water-vapor transmission is particularly important in concrete floors, walls, roofs, masonry, and containment systems.

Excessive moisture-vapor transmission can contribute to blistering, loss of adhesion, staining, and coating failure. Different moisture tests measure different properties and should not be treated as interchangeable.

Vehicle

In traditional coating terminology, the vehicle is the liquid portion that carries the pigments and helps form the coating film. It generally includes the binder and liquid carrier.

The term should not be confused with an automobile or other transportation vehicle. Its intended meaning is usually clear from the technical context.

Ventilation

Ventilation is the controlled movement of air used to supply fresh air and remove vapors, mist, dust, heat, overspray, powder, or other airborne contaminants.

Coating operations may use general ventilation, local exhaust, spray-booth ventilation, confined-space ventilation, oven exhaust, or supplied breathing air.

Ventilation must be designed for the material, process, enclosure, contaminant, fire hazard, and worker exposure. Moving air with an ordinary fan is not automatically adequate or safe.

Ventilation Rate

Ventilation rate is the quantity of air supplied to or exhausted from an area during a specified period. It may be expressed in cubic feet per minute, cubic metres per minute, air changes per hour, or another defined unit.

The required rate depends on booth or enclosure size, contaminant generation, solvent characteristics, powder concentration, process equipment, worker location, and applicable codes.

Air volume alone does not establish effective control. Air distribution, capture velocity, replacement air, short-circuiting, filter loading, and exhaust location must also be considered.

Vermiculite

Vermiculite is a lightweight mineral that expands when heated. It may be used in fireproofing, insulation, textured materials, construction products, and certain protective systems.

In fire-protective coatings, lightweight mineral fillers may contribute to insulation and char structure. Performance must be based on the tested and listed coating or fireproofing system rather than on the presence of vermiculite alone.

Vinyl Coating

Vinyl coating is a broad term for coatings based on vinyl resins. They have been used for chemical resistance, water resistance, corrosion protection, flexibility, and rapid drying.

Vinyl systems may be found on industrial equipment, marine structures, concrete, tanks, and previously coated assets. Strong solvent content and narrow compatibility requirements can make repair or overcoating difficult.

The exact vinyl resin, existing film condition, solvent sensitivity, and recoat requirements should be identified before maintenance work.

Virgin Powder

Virgin powder is new powder-coating material that has not passed through the spray system or been recovered from overspray.

Virgin powder may be used alone or blended with reclaimed powder. The permitted reclaim ratio depends on the product, color, appearance requirements, contamination control, and powder supplier’s recommendations.

Viscometer

A viscometer is an instrument used to measure a fluid’s resistance to flow. Different viscometers use different measurement principles and report different units.

Examples include efflux cups, rotational viscometers, Stormer viscometers, falling-ball instruments, and capillary viscometers.

A viscosity result is meaningful only when the instrument, procedure, temperature, spindle or cup, speed, and units are reported.

Viscosity

Viscosity is a fluid’s resistance to flow. In simple terms, it describes how easily or reluctantly a liquid moves under specified conditions.

Viscosity affects pumping, circulation, atomization, fluid delivery, leveling, sag resistance, transfer efficiency, and finish quality.

Temperature strongly affects coating viscosity. A measurement without the test temperature, instrument, and method may have little value.

Viscosity Cup

A viscosity cup measures flow time through a calibrated opening. The cup is filled or immersed, the opening is released, and the time required for the liquid stream to reach the specified endpoint is recorded.

Common types include Ford, Zahn, DIN, ISO, and other manufacturer-specific cups. Seconds measured with one cup type or orifice should not be treated as equivalent to another.

ASTM D1200 addresses the Ford viscosity cup, while ASTM D4212 addresses certain dip-type viscosity cups.

Viscosity-Cup Efflux Time

Efflux time is the measured number of seconds required for a coating to flow through a specified viscosity-cup orifice to the procedure’s defined endpoint.

The report should identify the cup type, cup number or orifice, coating temperature, test procedure, and result in seconds.

A difference of a few degrees in material temperature can change the result. The cup must also be clean, undamaged, level where required, and suitable for the viscosity range.

Visual Inspection

Visual inspection is the examination of a surface, coating, component, or process using normal or corrected vision, appropriate lighting, and specified visual standards where applicable.

It can identify contamination, rust, mill scale, missed areas, runs, sags, blistering, cracking, holidays large enough to see, overspray, color differences, and other visible conditions.

Visual inspection does not replace instrumental testing for film thickness, soluble salts, dew point, cure, adhesion, or microscopic discontinuities.

VOC — Volatile Organic Compound

A volatile organic compound is an organic chemical that participates in atmospheric photochemical reactions as defined by the applicable environmental regulation.

Coating VOC content may be expressed in grams per litre or pounds per gallon. The value may differ depending on whether water and exempt compounds are included or excluded and whether the coating is measured as supplied or as applied.

VOC limits can vary by product category, application, jurisdiction, facility, and regulation. “Low VOC” is not a universal value and does not mean the coating is odorless, nonflammable, or harmless.

Official information about determining VOC content is available through EPA Method 24 for Surface Coatings.

VOC Content as Applied

VOC content as applied is the VOC content of a coating after all components, catalysts, reducers, thinners, and other permitted additions have been included in the proportions actually used.

Adding thinner can increase the coating’s VOC content as applied even when the original packaged material complies with a stated limit. Accurate mixing and reduction records may be required to demonstrate compliance.

Void

A void is an empty space, cavity, pore, gap, or unfilled area within a substrate, coating, repair material, sealant, or lining.

Voids may form through trapped air, poor wetting, outgassing, incomplete filling, shrinkage, inadequate application pressure, or improper mixing.

Voids can reduce film continuity, adhesion, chemical resistance, impact strength, and corrosion protection.

Voltage

Voltage is the electrical potential difference between two points. In electrostatic finishing, voltage creates the electrical field used to charge liquid droplets or powder particles and attract them toward a grounded workpiece.

Electrostatic applicators commonly display voltage in kilovolts, abbreviated kV. Higher voltage can improve charging and wraparound under suitable conditions, but the highest setting is not always best.

Excessive voltage can contribute to back-ionization, poor penetration into recesses, uneven film build, or difficulty coating areas affected by the Faraday cage effect.

Voltage Drop

Voltage drop is a reduction in electrical potential within an electrostatic application system. It may occur when the gun approaches a grounded part, current demand increases, resistance changes, or the equipment detects a condition requiring output reduction.

Voltage and current should be considered together. A displayed maximum kV setting does not guarantee that the same voltage is present at the electrode during spraying.

Voltmeter

A voltmeter is an instrument used to measure electrical potential difference. Specialized high-voltage meters or probes may be required to test electrostatic coating equipment.

An ordinary electrical meter should never be connected to electrostatic high-voltage equipment unless it is specifically designed and rated for that purpose. Testing should follow the equipment manufacturer’s procedures.

Volume

Volume is the amount of three-dimensional space occupied by a material. Coating quantities may be expressed in gallons, litres, fluid ounces, cubic centimetres, or other volume units.

Volume is important when measuring coating mix ratios, calculating coverage, determining container capacity, and estimating material requirements.

A mix ratio specified by volume should not be converted to weight without reliable density information and an approved calculation.

Volume Mixing

Volume mixing is the proportioning of coating components according to their measured volumes. For example, a four-to-one ratio by volume requires four equal-volume portions of one component for each equal-volume portion of the second component.

Graduated containers or accurate measuring equipment should be used. Estimating levels in an irregular container can produce an incorrect ratio.

Volume Solids

Volume solids is the percentage of a liquid coating’s volume expected to remain as dried or cured film under specified test conditions.

Volume solids is used to estimate wet-film thickness, dry-film thickness, theoretical coverage, and material requirements.

Required WFT = Required DFT ÷ Volume Solids as a Decimal

For example, a coating containing 65 percent volume solids theoretically requires approximately 9.2 wet mils to produce 6 dry mils before allowances for thinning, surface profile, or application loss.

Volume solids should not be confused with solids by weight. Because coating ingredients have different densities, the two values are not interchangeable.

Volumetric Flow Rate

Volumetric flow rate is the volume of material moving through a system during a specified period. Coating flow may be expressed in fluid ounces per minute, cubic centimetres per minute, litres per minute, or gallons per minute.

Flow rate affects production speed, atomization, film build, pattern performance, pump selection, hose sizing, and material consumption.

Volumetric flow rate differs from mass flow rate. The relationship between them depends on the material’s density.

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Referenced Standards and Industry Resources

• AMPP Standards and Technical Resources
• UL Solutions
• OSHA Isocyanates Safety and Health Information
• EPA Method 24 — Surface Coatings
• ASTM D1200 — Viscosity by Ford Viscosity Cup
• ASTM D4212 — Viscosity by Dip-Type Viscosity Cups
• ASTM D6132 — Ultrasonic Measurement of Dry-Film Thickness
• ASTM D7378 — Measurement of Applied Powder to Predict Cured Thickness

Continue Building Your Coatings Vocabulary

Understanding coatings terminology helps applicators, inspectors, equipment operators, specifiers, facility owners, and purchasing professionals communicate more accurately. Continue with the next installment of The Language of Finishing for definitions beginning with the letters W through Z.

For assistance selecting professional spray guns, pumps, pressure equipment, powder coating systems, electrostatic equipment, spray booths, ovens, or genuine replacement parts, contact AirSprayTech.com—The Finishing Authority® at 817.490.1777.

Copyright © 2026 Azimuth Spray Systems, LLC. All rights reserved. AirSprayTech.com—The Finishing Authority®. No portion of this glossary may be reproduced, distributed, or republished without prior written permission.

Tags: paint coatings glossary U-V, ultrasonic coating thickness, urethane coating terms, ultraviolet coating cure, ventilation requirements, viscosity measurement, VOC coating terminology, volume solids, electrostatic voltage terms