The Language of Finishing: Paint and Coatings Glossary L–M

This installment of the AirSprayTech.com Paint and Coatings Industry Glossary covers terms beginning with L and M. These definitions address liquid and powder application, protective linings, industrial maintenance, surface preparation, film-thickness measurement, coating defects, spray equipment, worker safety, and quality control.

Coating terminology may 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 referenced standard before performing coating work.

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L

Lacquer

Lacquer is a fast-drying coating that generally forms a film primarily through solvent evaporation. Traditional lacquers include nitrocellulose and acrylic systems, although the word is sometimes used more broadly for clear or colored finishing materials.

Lacquer is used in automotive refinishing, furniture finishing, cabinet work, musical instruments, metal products, and specialty applications. Many lacquers can be recoated quickly and polished to a smooth finish. However, strong lacquer solvents may attack an existing coating that is not solvent-resistant.

Laitance

Industry areas: Concrete floors, secondary containment, waterproofing, linings, and concrete repair.

Laitance is a weak, powdery, or loosely bonded layer that can form at the surface of concrete. It commonly contains fine cement particles and other materials brought to the surface by excess water, finishing practices, or concrete settlement.

Coating over laitance can result in delamination because the coating bonds to the weak surface layer rather than to sound concrete. Laitance is commonly removed by shot blasting, grinding, abrasive blasting, scarifying, or another suitable mechanical preparation method.

Laminar Airflow

Laminar airflow is air moving in relatively smooth, parallel layers with limited turbulence. Controlled airflow is important in spray booths, clean rooms, automotive refinishing, aerospace finishing, and other operations where contamination and overspray movement must be controlled.

Actual spray-booth airflow is influenced by booth design, filters, part position, personnel, fixtures, open doors, exhaust loading, and the spray plume. A booth may be designed for uniform airflow while still developing local turbulence around the workpiece.

Lap

A lap is an area where one spray pass, roller pass, coating sheet, membrane, or application section overlaps another. Controlled overlap helps produce continuous coverage and uniform film thickness.

In spray application, approximately 50 percent overlap is a common starting technique, but the correct overlap depends on the spray pattern, gun speed, material, target thickness, and part geometry. Too little overlap may create thin bands, while excessive overlap can produce heavy film, runs, or color variation.

Lap Mark

A lap mark is a visible difference in color, sheen, texture, or film thickness where one application area overlaps another. Lap marks are particularly noticeable with stains, metallic coatings, low-sheen finishes, roof coatings, floor coatings, and large wall areas.

Common causes include allowing the previous section to dry before overlapping, inconsistent spray distance, uneven roller pressure, changes in application speed, and differences in film thickness. Maintaining a wet edge and using a planned application sequence can reduce lap marks.

Latex Coating

Latex coating is a common term for a waterborne coating in which polymer particles are dispersed in water. Modern latex coatings may be based on acrylic, vinyl-acrylic, styrene-acrylic, or other polymer technologies.

The word “latex” does not necessarily mean the coating contains natural rubber latex. Water evaporates after application, allowing the dispersed particles to come together and form a continuous film.

Temperature, humidity, ventilation, surface condition, and application thickness can affect film formation. Exposing a latex coating to cold, moisture, or washing before adequate cure may damage the developing film.

Lead-Based Paint

Lead-based paint is a coating containing lead at or above a threshold established by the applicable law, regulation, or project requirement. Lead may be present in older primers, industrial coatings, residential paint, bridges, tanks, machinery, marine structures, and other previously coated surfaces.

Sanding, grinding, abrasive blasting, torch cutting, welding, or disturbing a lead-containing coating can generate hazardous dust or fumes. The appearance, age, or color of a coating cannot reliably establish whether lead is present.

Before disturbing a potentially lead-containing coating, the responsible parties should arrange appropriate evaluation and establish the required exposure controls, containment, hygiene, respiratory protection, waste handling, and regulatory compliance.

Additional information is available from the OSHA Lead Safety and Health Topics page and the United States Environmental Protection Agency.

Leakage

Leakage is the unintended escape of coating, solvent, compressed air, hydraulic fluid, or another material from a hose, seal, fitting, valve, pump, pressure vessel, or applicator.

Leakage can create fire, health, environmental, housekeeping, and injection hazards. It can also cause pressure loss, erratic spray patterns, material waste, pump cycling, finish defects, and production interruptions.

A leaking high-pressure system should be depressurized according to the manufacturer’s procedure before inspection or repair. Never attempt to locate a high-pressure fluid leak with a hand or finger.

Leveling

Leveling is a liquid coating’s ability to flow after application and reduce brush marks, roller stipple, orange peel, spray texture, and other surface irregularities before the film sets.

Leveling is influenced by viscosity, solvent evaporation, wet-film thickness, surface temperature, material temperature, atomization, airflow, reducer selection, and application technique.

Too little flow may leave texture or dry spray. Excessive flow can produce sags, runs, edge pullback, or poor film build on vertical surfaces.

Lifting

Lifting is a coating defect in which an existing film wrinkles, swells, softens, curls, or separates after a subsequent coating or solvent is applied.

Common causes include applying a strong solvent over a sensitive coating, recoating outside the approved window, applying excessive wet-film thickness, coating over an incompletely cured layer, or using incompatible coating systems.

Lifting is different from ordinary loss of adhesion because it frequently involves a visible solvent reaction. A test patch should be performed when the identity or solvent resistance of an existing coating is uncertain.

Light Reflectance Value — LRV

Light Reflectance Value is a numerical indication of the amount of visible light a surface reflects. It is commonly expressed on a scale from low reflectance for dark colors to high reflectance for light colors.

LRV is used in architectural coatings, safety design, facility lighting, accessibility planning, roofing, and energy-management decisions. It should not be confused with gloss, which describes the reflection of light at a specified angle.

Line Speed

Industry areas: OEM finishing, powder coating, automated liquid finishing, E-coat, coil coating, and conveyorized systems.

Line speed is the rate at which parts move through a production or finishing process. It affects pretreatment contact time, flash-off time, spray exposure, film build, oven dwell time, cooling time, and total production capacity.

Increasing line speed without adjusting other process variables may reduce coating thickness or oven dwell time. Decreasing it can create excessive film build, longer heat exposure, or production imbalance.

Line Striping

Line striping is the application of painted lines, symbols, numbers, curbs, arrows, or other markings on pavement, concrete, athletic fields, factory floors, parking areas, airports, and roadways.

Airless striping machines are commonly used because they can apply sharply defined lines without atomizing air. Line quality depends on tip condition, pressure, material viscosity, gun height, machine speed, surface cleanliness, and the alignment of the gun and tip.

Traffic should not be allowed onto the coating until the material has dried or cured sufficiently for the service conditions.

Lining

A lining is a protective coating system applied to the interior surface of a tank, vessel, pipe, containment area, duct, or process equipment. Linings protect the substrate from water, chemicals, fuels, wastewater, abrasion, corrosion, or contamination of the stored product.

Lining service is frequently more demanding than atmospheric coating service. Proper selection requires knowledge of the stored material, concentration, operating temperature, cleaning process, pressure, immersion cycle, and possibility of chemical mixtures.

Surface preparation, stripe coating, film thickness, continuity, cure, and holiday testing are particularly important. A lining should not be placed into service until it has achieved the cure required by the manufacturer and project specification.

Lining Holiday

A lining holiday is a pinhole, void, crack, thin spot, or discontinuity that exposes the substrate or otherwise interrupts the protective film. A defect may be too small to see during ordinary visual inspection.

Holiday detection uses electrical test equipment selected according to the coating type, thickness, substrate, and governing procedure. Detected holidays are marked, repaired, allowed to cure, and retested when required.

Liquid Coating

A liquid coating is applied in fluid form and becomes a continuous film through solvent or water evaporation, chemical reaction, cooling, exposure to moisture, ultraviolet energy, or a combination of mechanisms.

Liquid coatings include primers, enamels, lacquers, stains, clear coats, epoxies, polyurethanes, roof coatings, floor coatings, tank linings, automotive finishes, and many other products.

Common application methods include brush, roller, conventional air spray, HVLP, airless, air-assisted airless, electrostatic spray, dip, flow coat, and plural-component spray.

Liquid-to-Air Ratio

Liquid-to-air ratio describes the relationship between coating flow and atomizing air in an air-operated spray process. The correct balance helps produce the required atomization, pattern shape, transfer efficiency, and film build.

Too much material for the available atomizing air may produce coarse droplets, heavy edges, or poor breakup. Too much atomizing air may create excessive overspray, dry spray, bounce-back, and material waste.

Lower Explosive Limit — LEL

Also called: Lower flammable limit, or LFL.

The lower explosive limit is the lowest concentration of a flammable gas or vapor in air that can support combustion when an ignition source is present. Below this concentration, the mixture is generally too lean to burn.

Coating operations must not treat the LEL as an acceptable operating target. Spray areas, ovens, flash-off zones, mixing rooms, and confined spaces require properly designed ventilation, electrical classification, monitoring, and operating controls.

LEL instruments must be appropriate for the atmosphere, maintained, calibrated, and used by trained personnel. Oxygen concentration, sensor limitations, temperature, humidity, and the specific solvent mixture can affect the reading.

Lot Number

A lot number is a manufacturer-assigned code identifying material produced or packaged under defined production conditions. It provides traceability if a question arises about product age, quality, color, performance, or manufacturing history.

Inspectors and applicators should record lot or batch numbers for each coating component used. Multi-component products may have separate numbers for the base, activator, catalyst, and other components.

Low-Pressure Spray

Low-pressure spray is a broad description for coating application performed with relatively low atomizing-air or fluid pressure. The phrase does not identify one specific spray technology.

HVLP, compliant spray, conventional air spray, LVLP, and certain low-pressure fluid-delivery systems may all be described as low-pressure equipment in different contexts. The actual technology and pressure-measurement location should be identified before comparing equipment.

Low-Solids Coating

A low-solids coating contains a relatively small percentage of film-forming material compared with the amount of volatile material present. More of the applied wet film is lost through evaporation during drying or cure.

A low-solids coating generally requires a greater wet-film thickness to achieve a specified dry-film thickness than a higher-solids product. Volume solids, not a general label such as “high solids” or “low solids,” should be used for wet-to-dry film calculations.

Low-Temperature Cure

Low-temperature cure describes a coating’s ability to develop specified properties at a relatively low substrate or ambient temperature. The permitted minimum temperature varies significantly among products.

“Low-temperature” does not mean that environmental restrictions can be ignored. Surface temperature, dew point, humidity, ventilation, condensation, induction time, and extended cure time may still control when the coating can be applied or placed into service.

Low-VOC Coating

A low-VOC coating is formulated to contain a reduced amount of regulated volatile organic compounds compared with conventional products in the same category. The applicable VOC limit may depend on product category, location, application method, and current regulation.

Low VOC does not mean odorless, nonflammable, or harmless. Applicators must still review the safety data sheet, provide suitable ventilation, use required protective equipment, and follow applicable environmental and occupational requirements.

LVLP — Low Volume Low Pressure

LVLP is a term used for certain air spray guns designed to atomize coatings using less compressed-air volume than many conventional or HVLP guns. It is often selected where compressor capacity is limited or where smaller components are being finished.

LVLP is not defined as consistently across the industry as HVLP. Air consumption, inlet pressure, air-cap pressure, fluid delivery, pattern size, and transfer efficiency should be evaluated from the actual equipment data rather than from the LVLP label alone.

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M

Magnetic Dry-Film Thickness Gauge

A magnetic dry-film thickness gauge measures the thickness of a nonmagnetic coating over a ferrous substrate, such as paint over carbon steel. Instruments may use magnetic pull-off or electronic magnetic-induction principles.

Readings can be affected by surface profile, substrate thickness, curvature, edge proximity, probe position, temperature, and instrument condition. The gauge should be adjusted or verified on the prepared substrate or representative reference standards according to the governing procedure.

A commonly referenced method is ASTM D7091, Nondestructive Measurement of Dry-Film Thickness.

Maintenance Coating

A maintenance coating is applied to an existing structure, facility, machine, or previously coated surface to restore protection, appearance, identification, cleanability, or another required function.

Maintenance painting may involve spot repair, overcoating, complete removal and replacement, or a combination of methods. The existing coating must be evaluated for adhesion, compatibility, contamination, deterioration, and the possible presence of hazardous materials.

Manufacturer’s Product Data Sheet — PDS

A product data sheet is the coating manufacturer’s technical document describing a product’s intended uses, properties, mixing instructions, application requirements, film thickness, coverage, cure schedule, environmental limits, recoat intervals, recommended thinners, and other technical information.

The current data sheet for the exact product should be used. Similar product names, regional formulations, colors, and revised versions may have different requirements.

A product data sheet addresses technical use. A safety data sheet addresses hazards, handling, exposure, first aid, storage, and emergency information. Both documents are important, but they serve different purposes.

Masking

Masking is the temporary covering of areas that must be protected from coating, abrasive, dust, overspray, chemicals, or surface-preparation damage.

Masking materials may include tape, paper, plastic film, plugs, caps, reusable fixtures, liquid masking compounds, and heat-resistant materials for powder coating.

The masking product must be compatible with the substrate, coating, solvent, curing temperature, and required edge quality. Removing tape too early may disturb the wet coating, while removing it too late may tear or lift the cured film.

Mass Flow

Mass flow is the amount of material moving through a system during a specified period, commonly expressed as mass per unit of time. It differs from volumetric flow, which measures volume per unit of time.

Mass flow is useful where material density changes or where accurate coating consumption, powder delivery, mixing, or process control is required. Automated finishing systems may monitor flow to help maintain consistent film build.

Material Compatibility

Material compatibility is the ability of a coating, solvent, hose, seal, pump, spray gun, liner, filter, or another component to remain functional when exposed to the material being handled.

Incompatible wetted parts may swell, soften, dissolve, corrode, contaminate the coating, or fail prematurely. Chemical compatibility should be confirmed with the equipment and coating manufacturers, especially when handling aggressive solvents, moisture-sensitive materials, acids, plural-component products, or heated coatings.

Material Pressure

Material pressure is the fluid pressure used to move coating through a hose, regulator, manifold, valve, and spray gun. The required pressure depends on the coating viscosity, hose length and diameter, production rate, tip or nozzle size, elevation, filters, and application technology.

Excessive material pressure can increase fluid delivery, wear, leakage, overspray, and finish problems. Insufficient pressure can produce poor atomization, an unstable pattern, inadequate flow, or pressure drop when the gun is triggered.

Material Safety Data Sheet — MSDS

Material Safety Data Sheet is the former name commonly used for the hazard-information document now called a Safety Data Sheet, or SDS, under the modern hazard communication system.

Older project files and product labels may still use the abbreviation MSDS. Current workplace documents should use the standardized SDS format where required. The document provides information about hazards, exposure controls, protective equipment, first aid, fire response, spills, storage, and disposal considerations.

Maximum Recoat Time

Maximum recoat time is the longest period allowed between coating layers under specified conditions without additional preparation. After this period, the existing film may become too hard, smooth, or fully cured to develop adequate intercoat adhesion.

If the maximum recoat interval is exceeded, cleaning, abrading, solvent wiping, applying a tie coat, or another manufacturer-approved procedure may be required. Recoat times are often affected by surface temperature and should not be evaluated only by the number of hours shown on a clock.

Mechanical Cleaning

Mechanical cleaning removes rust, mill scale, coating, concrete laitance, contamination, or surface irregularities using physical force. Common methods include abrasive blasting, grinding, sanding, needle scaling, scarifying, shot blasting, and power-tool cleaning.

The selected method must produce the cleanliness and surface profile required by the coating system without causing unacceptable damage. Mechanical cleaning may spread oil or grease if those contaminants are not removed before the work begins.

Mechanical Mixer

A mechanical mixer uses a powered blade, paddle, or impeller to combine coating components and produce a uniform material. It is generally more reliable than hand stirring for settled pigments, zinc-rich coatings, multi-component products, and large containers.

The mixer should be suitable for the container and material. Excessive mixing speed can draw air into the coating, create foam, increase heat, or produce a hazardous static or ignition condition.

MEK Rub Test

An MEK rub test uses a solvent-wetted cloth to evaluate the cure or solvent resistance of certain coatings. MEK refers to methyl ethyl ketone, also known as 2-butanone.

The procedure generally involves rubbing the coating with a controlled back-and-forth motion and observing softening, color transfer, film removal, or another specified endpoint. Results depend on the coating type, solvent, cloth, pressure, stroke length, number of double rubs, and operator technique.

An MEK rub result is not a universal measurement of coating cure. It must be interpreted according to the specified procedure and coating manufacturer’s requirements.

Relevant standards include ASTM D4752 for solvent resistance of ethyl-silicate inorganic zinc-rich coatings and ASTM D5402 for assessing solvent resistance of organic coatings.

Membrane Coating

A membrane coating forms a continuous, flexible barrier intended to resist water penetration, chemicals, cracking, or movement. Membrane systems are used on roofs, decks, walls, floors, foundations, secondary containment, and waterproofing applications.

Performance depends on substrate preparation, continuity, film thickness, reinforcement, joint treatment, penetration details, drainage, cure, and resistance to expected movement. Small pinholes or unsealed transitions can defeat an otherwise continuous membrane.

Metallic Coating

Metallic coating may refer to paint containing metallic-effect pigments or to a protective metallic layer applied by thermal spray, galvanizing, plating, or another process. The intended meaning should be established from the application.

In decorative liquid coatings, metallic particles reflect light and create a metallic appearance. Gun angle, distance, overlap, airflow, film thickness, orientation coat, and flash time can affect color and mottling.

In corrosion protection, a metallic coating may provide barrier or sacrificial protection. Examples include thermal-sprayed zinc or aluminum and galvanized zinc coatings.

Metallic Pigment Orientation

Metallic pigment orientation describes how metallic-effect flakes align within a coating film. Proper alignment helps produce uniform color, brightness, flop, and appearance.

Uneven orientation can cause mottling, striping, dark and light patches, or color differences when the surface is viewed from different angles. Atomization, gun distance, overlap, reducer selection, application technique, flash time, and film thickness all influence orientation.

Metering

Metering is the controlled measurement and delivery of a liquid, powder, catalyst, hardener, or other component. Accurate metering is essential in plural-component coating systems, proportioning pumps, powder feed systems, and automated finishing lines.

A metering error can change the mix ratio, viscosity, cure, color, film build, spray pattern, or application rate. Metering equipment should be checked and maintained according to the manufacturer’s instructions.

Micron — µm

A micron, properly called a micrometre, is one-millionth of a metre. It is commonly abbreviated as µm and is used internationally to express coating thickness and surface profile.

One mil equals approximately 25.4 microns. Conversely, one micron equals approximately 0.0394 mil. The required unit should be confirmed before recording or converting measurements.

Mil

A mil is one-thousandth of an inch, or 0.001 inch. It is commonly used in the United States to express wet-film thickness, dry-film thickness, lining thickness, and surface-profile depth.

A mil is not the same as a millimetre. One mil equals approximately 25.4 microns, while one millimetre equals approximately 39.37 mils.

Mil Gauge

Mil gauge is an informal term that may refer to a wet-film thickness gauge, dry-film thickness gauge, or another instrument displaying thickness in mils. The phrase alone does not identify the instrument or measurement principle.

Inspection records should identify the actual gauge type, manufacturer, model, serial number, measurement range, and verification status when required.

Mill Scale

Mill scale is a hard oxide layer formed on steel during hot rolling and fabrication. It is typically dark gray or blue-black and may initially appear tightly bonded.

Mill scale and exposed steel can create different electrochemical conditions. As the scale cracks or loosens, corrosion may develop beneath or around it and cause coating failure.

Protective coating specifications often require removal of mill scale by abrasive blasting. The required degree of removal depends on the surface-preparation standard and coating system.

Minimum Recoat Time

Minimum recoat time is the shortest permitted interval before another coat can be applied under specified conditions. Recoating too soon may trap solvent or water, disturb the underlying film, cause wrinkling, interfere with cure, or create intercoat defects.

The stated time usually depends on temperature, ventilation, humidity, film thickness, and coating chemistry. A coating that feels dry to the touch may not yet be ready for recoating.

Minimum Film-Forming Temperature — MFFT

Minimum film-forming temperature is the lowest temperature at which a waterborne coating or polymer dispersion can form a continuous, coherent film under defined conditions.

Application below the MFFT can result in cracking, powdering, poor adhesion, reduced water resistance, or an uneven appearance. Substrate temperature is often more important than air temperature when determining whether conditions are suitable.

Mist Coat

A mist coat is a light, thin application of coating deposited before a fuller coat. It may be used to improve wetting, reduce bubbling over porous surfaces, limit solvent attack, control metallic orientation, or help manage application over inorganic zinc.

A mist coat is not automatically a complete coat and may not provide the film thickness or protection required by the specification. Its purpose and required procedure should be confirmed with the coating manufacturer.

Mix Ratio

Mix ratio is the specified proportion in which the components of a multi-component coating must be combined. It may be stated by volume, weight, or another defined method.

A ratio of four parts base to one part activator by volume is not necessarily equivalent to the same ratio by weight. Density differences can make unauthorized conversion inaccurate.

Incorrect proportions can cause incomplete cure, softness, brittleness, poor adhesion, color variation, chemical weakness, gloss change, or shortened service life. Never estimate component quantities by eye.

Mixing

Mixing is the controlled process of making a coating uniform and, when applicable, combining its separate components in the specified proportions.

Proper mixing may involve pre-mixing the base, accurately measuring each component, combining them in the required order, mechanically mixing for a specified time, observing induction time, and straining the material before application.

Scraping material from the sides and bottom of the container may be necessary. Mixing should avoid unnecessary air entrainment, contamination, excessive heat, and unsafe ignition sources.

Mixing Station

A mixing station is a designated area where coatings are opened, measured, mixed, reduced, strained, and transferred to application equipment.

A properly arranged station helps control contamination, spills, waste, ignition hazards, and mixing errors. It should include suitable ventilation, lighting, grounding where required, approved containers, measuring equipment, spill controls, labels, and access to current product and safety information.

Moisture-Blistering

Moisture blistering is the formation of raised areas caused by water or water vapor acting within or beneath a coating system. The blisters may contain liquid, vapor, corrosion products, or softened coating.

Possible causes include coating a damp substrate, osmotic pressure from soluble contamination, moisture transmission through concrete, water intrusion behind the coating, inadequate cure before immersion, or permeability differences between coating layers.

The presence of water inside a blister does not, by itself, identify the original failure mechanism. Investigation should consider the blister location, pattern, substrate, contamination, exposure, and coating history.

Moisture Content

Moisture content is the amount of water present in wood, concrete, masonry, abrasive, compressed air, or another material. Excessive moisture can interfere with coating adhesion, cure, appearance, and long-term performance.

Moisture meters use different operating principles and may report different units. A reading obtained from wood, concrete, or masonry should be interpreted using the correct instrument, substrate mode, test procedure, and acceptance criteria.

Moisture-Cure Coating

A moisture-cure coating uses moisture from the air or substrate environment as part of its chemical curing reaction. Moisture-cure urethanes are common examples in industrial and protective coating work.

Curing speed can be affected by humidity, temperature, film thickness, and ventilation. Very dry conditions may slow cure, while excessive moisture can cause bubbling, foaming, or other defects in moisture-sensitive materials.

Containers should be protected from atmospheric moisture during storage and use. Introducing moisture into the container can cause skinning, gelling, pressure buildup, or premature reaction.

Moisture Vapor Emission

Moisture vapor emission is the movement of water vapor through concrete, masonry, roofing, or another substrate. It can contribute to blistering, loss of adhesion, staining, and failure of floor or membrane coatings.

Testing should be selected for the substrate and coating system. Different methods measure different properties and should not be treated as interchangeable. The coating manufacturer’s moisture limits and specified test method must be followed.

Mottling

Mottling is an uneven, blotchy, clouded, or patchy appearance in a coating. It is frequently associated with metallic and pearlescent automotive finishes but can occur in other decorative coatings.

Common causes include uneven spray overlap, inconsistent gun distance, poor atomization, excessive wetness, incorrect reducer, uneven flash time, varying film thickness, or improper metallic pigment orientation.

Consistent gun technique, controlled application conditions, and an approved orientation or control coat can help reduce mottling.

MPa — Megapascal

The megapascal is a metric unit of pressure or stress. One megapascal equals one million pascals. MPa is commonly used to report pull-off adhesion, tensile strength, pressure, and other engineering measurements.

For practical conversion, 1 MPa is approximately 145 psi. Reports should identify the original unit and avoid combining rounded conversions when evaluating compliance.

Mud Cracking

Mud cracking is a coating defect characterized by interconnected cracks resembling dried mud. It usually occurs when a coating is applied too thickly or when the surface of the film dries or cures more rapidly than the material beneath it.

Mud cracking is frequently associated with waterborne coatings, inorganic zinc-rich primers, heavily filled materials, roof coatings, and high-build products.

Correction generally requires removal of the defective material to a sound layer, preparation of the surface, and reapplication within the permitted film-thickness range. Applying another coat over the cracks may hide the defect temporarily without restoring coating integrity.

Multi-Coat System

A multi-coat system uses two or more coating layers to provide the required protection and appearance. A typical system may include a primer, intermediate coat, stripe coat, and finish coat.

Each layer performs a specific function. The primer may provide adhesion or corrosion protection, the intermediate coat may add barrier thickness, and the finish coat may provide weathering resistance, color, gloss, or cleanability.

The performance of the system depends on compatibility, correct thickness, cleanliness between coats, compliance with recoat windows, and proper cure of every layer.

Multi-Component Coating

A multi-component coating is supplied in two or more separate parts that must be combined before application. Common examples include epoxies, polyurethanes, polyureas, and certain polyester or specialty lining systems.

The separate components begin a chemical reaction when mixed. Accurate proportioning, thorough mixing, induction time where required, temperature control, and pot-life management are essential.

The term “two-component” does not necessarily mean a one-to-one ratio. The specified mix ratio may be very different and may be stated by either volume or weight.

Multi-Component Spray Equipment

Multi-component spray equipment stores coating components separately and combines them at a controlled ratio. The materials may be mixed before entering the gun, in a mix manifold, inside the gun, or immediately outside the spray tip, depending on the equipment design.

This equipment is used for fast-reacting or limited-pot-life materials such as plural-component epoxies, polyurethanes, polyureas, foams, and high-build linings.

Ratio assurance, temperature control, pressure balance, flow monitoring, proper flushing, and operator training are critical. Off-ratio material may appear acceptable during application but fail to cure or perform correctly.

MWFT — Minimum Wet-Film Thickness

Minimum wet-film thickness is the lowest wet-film thickness permitted during application to provide the required dry-film thickness after evaporation and cure.

The target wet-film thickness can be estimated using the coating’s stated volume solids. Allowances may be necessary for thinning, surface profile, application loss, and the manufacturer’s permitted variation.

Wet-film measurements allow the applicator to make adjustments while the coating is still being applied, before an insufficient dry film becomes a completed defect.

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Referenced Standards and Industry Resources

• OSHA Lead Safety and Health Topics
• United States Environmental Protection Agency Lead Information
• AMPP Standards and Technical Resources
• International Concrete Repair Institute
• ASTM D4417 — Field Measurement of Surface Profile of Blast-Cleaned Steel
• ASTM D4541 — Pull-Off Strength of Coatings
• ASTM D4752 — Solvent Resistance of Inorganic Zinc-Rich Coatings
• ASTM D5402 — Solvent Resistance of Organic Coatings
• ASTM D7091 — Nondestructive Measurement of Dry-Film 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 N through P.

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 L-M, industrial coating terms, protective lining terminology, maintenance coating terms, liquid coating glossary, material pressure, mil thickness measurement, moisture cure coatings, multi-component coatings