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Glossary of Terms

We have compiled a glossary of terminology about coatings as a service to our customers and industry colleagues.

Glossary of Coatings Terms

Standard Definitions of Terms Relating to COATINGS:


Abrasion resistance – the ability of a coating to resist being worn away and to maintain its original appearance and structure when subjected to rubbing, scraping or wear.

Acid number – the number of milligrams of potassium hydroxide required to neutralize the free acids of 1 gram of an oil resin, varnish or other substance – generally reported on the nonvolatile content.

Additive – a substance added in small quantities to another substance, usually to improve specific properties.

Adhesion promoter – a material built into a binder or to form primary bonds to either the substrate or the previously applied coating, with the specific aim of improving dry or wet adhesion, or both.

Architectural coating – an organic coating intended for onsite application to interior or exterior surfaces of residential, commercial, institutional or industrial buildings, in contrast to industrial coatings.


Bleeding – the diffusion of coloring matter through a coating from the substrate; also, the discoloration arising from such diffusions.  In the case of printing ink, the spreading or running of a pigment color by the action of a solvent such as water or alcohol.

Blister – a cone-shaped defect caused by the formation of a gas or liquid under a coating film which results in a localized loss of adhesion and lifting of the coating (film) from the substrate.

Blocking – the sticking of a coated surface to an adjacent surface when the two surfaces have been in contact for an extended period of time.

Blotching – see mottling.


Caulking compound – a soft, plastic material consisting of pigment and vehicle, used for sealing joints in buildings and other structures where normal structural movement may occur.

Coating – a liquid, liquefiable or mastic composition that is converted by evaporation, cross-linking or cooling to a solid or semi-solid protective, decorating or functional adherent layer after application … or the solid or semi-solid layer resulting from application of the composition.


Density – the mass per unit volume of a substrate at a specified temperature and pressure, usually expressed in g/mL … k.g.L. … g/cm3 … g/L … kg/m3 … or lb/gal.

Durability – a relative term indicating the degree of permanency.  It may be applied to individual protective, decorative or functional properties – for example, “the durability of gloss,” but if used in a general way – for example, “the excellent durability of paint,” describes the ability of the described coating to retain, to the indicated degree, all the properties required for the continued service of the coating.


Enamel – a paint that is characterized by its ability to form an especially smooth film.

Ester gum – under resin, synthetic, see ester gum.

Extended pigments – organic pigments diluted with an extender – for example, alumina trihydrate, blanc fixe or calcium carbonate.


Filler – a pigmented composition for filling pores or irregularities in a surface preparatory to application of other finishes.

Finish – a final coat in a paint system, at the termination of cure or drying.  Finish sometimes refers to the entire coating system: the texture, color and smoothness of a surface, plus other properties affecting appearance.

Fire-retardant – a descriptive term implying that the described product, under accepted test methods, will significantly reduce the rate of flame spread on the surface of a material to which it has been applied … resist ignition when exposed to high temperatures … or insulate a substrate to which it has been
applied and prolong the time required to reach its ignition, melting or structural-weakening temperature.

Fire-retardant coating – a coating that will do one or more of the following: reduce the flame spread on the substrate over which the coating is applied, sometimes at the sacrificed (see intumescent coating); resist ignition of the substrate when exposed to high temperatures; or insulate the substrate to which the coating is applied, thereby prolonging the time required to reach its ignition, melting or structural-weakening temperature.

Flatting agent – a material added to paints, varnishes and other coating materials to reduce the gloss of the dried film.

Fossil resin – under resin, natural, see fossil resin.

Fungus resistance – see mildew resistance.


Gallon – a volume equal to 231 in.3.  For paint, varnish, lacquer and related products this is measured at 77˚F (25˚C).

Grit – coarse, foreign particles in coatings, often of irregular shape, that are hard, abrasive and resistant to disintegration.


Hydroxyl number – the number of milligrams of potassium hydroxide equivalent to the hydroxyl content of 1 gram of sample.


Intumescent coating – a fire-retardant coating that when heated forms a foam produced by nonflammable gases such as carbon dioxide and ammonia.  This results in a thick, highly insulating layer of carbon, about 50 times as thick as the original coating, that serves to protect the coated substrate from fire.


Lacquer – a coating composition that is based on synthetic thermoplastic film-forming material dissolved in organic solvent that dries primarily by solvent evaporation.  Typical lacquers include those based on nitrocellulose, other cellulose derivatives, vinyl resins and acrylic resins, among others.

Leveling – the process whereby a film of liquid coating flows out after application so as to minimize any surface irregularities such as brush marks, orange peel, peaks or crafters that have been produced by the mechanical process of application.  Also, a measure or rating of the leveling ability of a coating.


Mildew resistance – also known as fungus resistance, the ability of a coating to resist fungus growth that can cause discoloration and the ultimate decomposition of a coating’s binding medium.

Mildewstat – a chemical agent that inhibits the growth of mildew.

Mottling – the presence on the surface of a film of irregularly shaped, randomly distributed areas that vary in color, gloss or sheen, causing the film to be non-uniform in appearance.  Also known as blotching.

Mud-cracking – an irregular broken network of cracks in the film which occurs due to volatile loss while drying or curing.


Nonvolatile content – the portion of a coating that does not evaporate during drying or curing under specified conditions, comprising the binder and, if present, the pigment.  The percent volatile content is obtained by subtracting the nonvolatile content from 100.)

Nonvolatile vehicle – the liquid portion of a paint excepting its volatile thinner and water.


OEM coatings – coatings from original equipment manufacturers that include automotive, marine, furniture, appliance and many other miscellaneous consumer and industrial applications.

Opacity – the degree of obstruction to the transmission of visible light.  (In this sense, “opacity” is a relative term, considering that given a film being sufficiently thin, there is no “absolutely opaque” substance.)

Open time – the length of time a coating remains wet enough to allow for brushing-in at the laps.  Also known as wet edge time.


Paint (noun) – a classification sometimes employed to distinguish pigmented drying oil coatings (paints) from synthetic enamels and lacquers.  Enamel paint – a paint, the vehicle of which is an emulsion of binder in water – the binder being oil, oleoresinous varnish, resin or some other emulsifiable binder.  

Latex paint – a paint containing a stable aqueous dispersion of synthetic resin, produced by emulsion polymerization as the principal constituent of the binder (modifying resins may also be present).  Oil paint – a paint that contains drying oil or oil varnish as the basic vehicle ingredient.  Paste paint – a paint in which the pigment is sufficiently concentrated to permit a substantial reduction with vehicle before use.  Water paint – a paint, the vehicle of which is a water emulsion, water dispersion, or ingredients that react chemically with water.

Paint (verb) – to apply a thin layer of a coating to a substrate by brush, spray, roller, immersion or any other suitable means.

Pigment – fine solid particles used in the preparation of paint or printing ink and substantially insoluble in the vehicle.  Asphaltic materials are not pigments except when they contain substances substantially insoluble in the vehicle in which they are used.

Pigment volume – the percent by volume of pigment in the nonvolatile portion of a paint or printing ink, as calculated from bulking value and composition data.  The letters “PV” are commonly used as an abbreviation.

Pinholes – small pore-like flaws in a coating that extend entirely through the applied film and have the general appearance of pin pricks when viewed by reflected light.

Plasticizer – a substance added to varnishes, lacquers or paints to impart flexibility.

Primer – the first of two or more coats of a paint, varnish or lacquer system.

Printing ink – a colored or pigmented liquid or paste composition that dries to a solid film after application as a thin layer by printing machinery.


Resin, natural – a solid organic substance originating in the secretion of certain plants or insects, which is thermoplastic, flammable, nonconductive of electricity, breaks with a conchoidal fracture when hard, and dissolves in certain specific organic solvents but not water. 

Fossil resin – a natural resin of ancient origin usually found in the earth.

Resin, synthetic – a synthetic substance physically similar to natural resin. 

Acrylic resin – a synthetic resin made from derivatives of acrylic acid. 

Alkyd resin – a synthetic resin made from polyhydric alcohols and polybasic acids; generally modified with resins, fatty oils or fatty acids. 

Epoxy resins - a class of polymeric materials characterized by the presence of more than one three-membered ring known as the epoxy, epoxide, oxirane or ethyoxyline group. 

Ester gum – a resin made from rosin or rosin acids and a polyhydric alcohol such as glycerine or pentaerythritol. 

Maleic resin – a resin made from a natural resin and maleic anhydride or maleic acid. 

Melamine resin – a synthetic resin made form melamine and aldehyde. 

Penta resin – ester gum made from rosin and pentaerythritol. 

Phenolic resin – a synthetic resin made from phenols and aldehydes. 

Styrene resin – a synthetic resin made from vinyl benzene. 

Urea resin – a synthetic resin made from urea and an aldehyde. 

Vinyl resin – a synthetic resin made from vinyl compounds.

Rust – a reddish material, primarily hydrated iron oxide that is formed on iron or its alloys, resulting from exposure to humid atmosphere or chemical attack.  


Sag or sagging – non-uniform downward flow of a wet paint film that occurs between the time of application and setting, resulting in an uneven coating having a thick lower edge.

Sealer – a liquid composition to prevent excessive absorption of finish coats into porous surfaces.  Also, a composition to prevent bleeding (see also size).

Size – usually a liquid composition to prevent excessive absorption of all paints into plaster, old wall paint and similar porous surfaces.  Also a liquid composition used as a first coat on metal to improve adhesion of succeeding coats (the latter usage is limited to the metal decorating).  The terms sealer and size are nearly synonymous, but usage has established certain differences.  A sealer is ordinarily a thin varnish or clear lacquer and is usually applied on wood and metal surfaces.  Ordinary painter’s size is a then solution of glue, starch or other water-soluble substance and is usually applied on plaster surfaces.

Solid – pertaining to flammability regulations, a substance that has a viscosity greater than 1 x 103 St (1 x 10-1 m2s-1) at 104˚F (40˚C) or an equivalent viscosity at an agreed-upon temperature.  (This includes powders and granular materials.)

Specific gravity – the ratio of the weight of a given volume of substance to the weight of an equal volume of water.  Also, the ratio of the density of a substance at a specified temperature to the density of water.  (See also Density.)


Thinner – the portion of a varnish, lacquer, paint or printing ink (or related product) that volatizes during the drying process.

Tint (noun) – a color produced by the mixture of white pigment or paint in predominating amount with a colored pigment or paint, not white.  The tint of a color is much lighter and much less saturated than the color itself.

Tint (verb) – to mix a white paint with a colorant, or to mix a colored paint with a white colorant.  Also, to adjust the color of a test specimen to be a closer match to a standard.

Toner – an organic pigment that does not contain inorganic pigment or an inorganic carrying base.


Urethane coatings – coatings based on vehicles containing a minimum of 10% by weight (nonvolatile vehicle basis) of a polyisocyanate monomer reacted in such a manner as to yield polymers containing any ratio, proportion or combination of urethane linkages, active isocyanate groups or polyisocyanate monomer.  The reaction products may contain excess isocyanate groups available for further reaction at the time of application, or may contain essentially no free isocyanate as supplied. 

Type 1, one-package pre-reacted – urethane coatings characteristic by the absence of any significant quantity of fee isocyanate groups.  They are usually the reaction product of a polyisocyanate and a polyhydric alcohol ester of vegetable oil acids and are hardened with the aid or metallic soap dryers. 

Type II, one-package moisture-cured – urethane coatings characterized by the presence of free isocyanate groups and capable of conversion to useful films by the reaction of these isocyanate groups and ambient moisture. 

Type III, one-package heat-cured – urethane coatings that dry on cure by thermal release of blocking agents and regeneration of active isocyanate groups that subsequently react with substances containing active hydrogen groups. 

Type IV, two-package catalyst – urethane coatings that comprise systems wherein one package contains a prepolymer or adduct having free isocyanate groups capable of forming useful films by combining with a relatively small quantity of catalyst, accelerator or crosslinking agent such as a monomeric polyol or polyamine contained in a second package (this type has limited pot-life after the two components are mixed). 

Type V, two-package polyol – urethane coatings that comprise systems therein one package contains a prepolymer or adduct or other polyisocyanate capable of forming useful films by combining with a substantial quantity of a second package containing a resin having active hydrogen groups with or without the benefit of a catalyst (this type has limited pot-life after the two components are mixed). 

Type VI, one-package nonreactive lacquer – urethane solution coatings characterized by the absence of any significant quantity of free isocyanate or other functional groups.  Such coatings convert to solid films primarily by solvent evaporation.


Varnish – a liquid composition that is converted to a transparent or translucent solid film after application as a thin layer.  Bituminous varnish – a dark-colored varnish containing bituminous ingredients (the varnish may be either of the oil or spirit type).  Oil varnish – a varnish that contains resin and drying
oil as the basic film-forming ingredients and is converted to a solid film primarily by chemical reaction.  Shellac varnish – a solution or “cut” of a specified type and grade or dry lac resin in a suitable alcohol.  Spar varnish – a varnish for exterior surfaces, the naming originating from its use on the spars of ships.  Spirit varnish – a varnish that is converted to a solid film, primarily by solvent evaporation.

Vehicle – the liquid portion of a paint or printing ink.  Anything that is dissolved in the liquid portion of a paint or printing ink is part of the vehicle.

Volume percent solids – the portion of a coating that remains as part of the dry film, expressed as percent by volume.


Waterborne coating – a coating in which the principal volatile constituent is water.  (See also water-reducible coating.)

Water-reducible coating – a coating that can be reduced in viscosity by the addition of water.

Water vapor transmission rate – often abbreviated as WVT, the steady state rate of water vapor movement through a free film under specific conditions of temperature and humidity at each surface, customarily expressed in grains per square foot per hour (gr/ft.2 per hour) or grams per square meter per day (g/m2 per day).

Water percent solids – the portion of a coating that remains as part of the dry film, expressed as weight.

Wet adhesion – the ability of a coating film to adhere tightly to the substrate directly beneath it, under wet conditions such as rain, dew, cleaning and washing, etc.

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