Metal Painting and Coating Operations

Table of Contents  Background  Regulatory Overview  Planning P2 Programs  Overview of P2  Surface Preparation
Alternatives to Solvent-Borne Coatings  Application Techniques  Curing Methods  Equipment Cleaning

Appendix F Glossary

Acrylic : A resin resulting from the polymerization of derivatives of acrylic acids, including esters of acrylic acid, methacrylic acid, acrylonitrile, and their copolymers. Acrylics are also used in powder coatings in their thermoplastic form.

Active solvent: A liquid which dissolves a binder.

Additives: Any substance added in small quantities to another substance, usually to improve properties. Examples of additives include plasticizers, fungicides, and dryers.

Adhesive: A substance capable of holding materials together by surface attachment. Various descriptive adjectives are used with the term adhesive to indicate certain characteristics: physical (liquid adhesive, tape adhesive), chemical type (silicate adhesive, resin adhesive), materials bonded (paper adhesive), and conditions of use (hot-set adhesive).

Air-assisted airless spray: Paint spray application system using fluid pressure to atomize the paint and low pressure air to adjust the shape of the fan pattern.

Air-bearings: A stream of air used to support a spinning shaft. Air bearings have limited load carrying capacity but require no lubricants.

Air-dried coatings: Coatings which are not heated above 194oF (90oC) for coating or drying. In the South Coast Air Quality Management District, curing also must be done below (rather than at or below) 194oF (90oC) to qualify as air dried. Air-dried coatings also include forced-air dried coatings.

Air-dryers: Used to remove moisture from compressed air. Dryers have three basic styles of operation: 1. deliquescent types have disposable drying agents and tend to be marginally effective for painting; 2. refrigerated dryers cool the air to condense and remove the water. Most paint systems use this type; 3. desiccant types have a double bed dryer and are able to achieve the lowest dew point air. The beds are alternately on-stream and back-flushed to regenerate their moisture absorbing qualities. Some plants with critical finish requirements use this style of dryer to reach dew points of -40oF.

Air knife: A slotted jet of compressed air quickly blows superfluous water from parts, often before they enter a dryoff oven.

Airless spray: A paint spray application system using high fluid pressure to atomize paint by forcing it through a small orifice.

Air spray: A paint spray application system using air at high velocity and pressure to atomize the paint.

Air turbine: 1. Electric motor driven fans that create volumes of relatively low-pressure atomizing air for spraying. Their output is referred to as turbine air; 2. An air-driven precision fan that is used to spin a paint atomizing disk or bell head.

Aliphatic solvent: A solvent comprised primarily of straight chain hydrocarbons, including mineral spirits, kerosene, and hexane. These solvents are characterized as volatile organic compounds.

Alkali: Any substance that neutralizes acids. Alkalis are helpful in aqueous cleaning to speed soil removal and suspension. Alkali is synonymous with caustic.

Alkyd: A binder based on resins formed by the condensation of polyhydric alcohols with polybasic acids. They may be regarded as complex polyesters (Thermoset).

Amino resins: Resins used to crosslink polyesters, epoxies, acrylics, and alkyds to enhance their durability.

Amperes (AMPS): An electrodynamic unit of measure for the quantity of current in a steady electric flow.

Anode: The electrode at which chemical oxidation takes place. In electrodeposition (E-coating) the anode is indicated on diagrams by the positive (+) marking.

Anoltye: The water used to flush solubilizer molecules that form inside an electrocoating anode box. If used to flush a cathode box, it is termed catholyte.

Aromatic solvents: Hydrocarbon solvents which contain an unsaturated ring of carbon atoms, including benzene, naphthalene, anthracene and their derivatives. Toluene (toluol) and xylene (xylol) are commonly used aromatics. These solvents are characterized as volatile organic compounds.

Atomization: The formation of tiny liquid droplets during the spraying of coatings.

Autodeposition: Dip coating application method which depends on a chemical reaction to plate out the coating film.

Autodeposition (autophoretic): A precipitation reaction of an organic resin that occurs by the action of an acid etching a metallic substrate. The ions of the oxidized metal codeposit with the vinyl emulsion resin.

Azeotrope: A liquid mixture that distills with out change in composition. Azeotropes are characterized by a constant minimum or maximum boiling point which is lower or higher than any of the components.

Baked coatings: Coatings that are cured or dried at or above an oven air temperature of 194oF (90oC).

Barytes: Colorless crystalline solids, which are a form of barium sulfate (also called barite). Barytes are used as an extender pigment in primers and coatings.

Bells: A rotating head that is shaped to deliver paint forward in a circular pattern. The bell may be directed at any angle and be moved on robots or reciprocators, just as spray guns are.

Bentonite: A type of clay derived from volcanic ash, which is often used as a paint pigment.

Binder: The solid (non-volatile) material in a coating that binds the pigment and additive particles together to form a film. In general, binders are resins.

Biocide: A chemical agent capable of killing organisms responsible for microbial degradation. Biocides are sometimes added to waterborne coatings.

Bituminous coating: An asphalt or tar compound used to provide a protective finish for a surface.

Bleeding: Discoloration which occurs when colorants from a lower coat diffuse into a surface coat.

Blistering: The formation of hollow bubbles in the paint film caused by air, moisture, or solvents trapped under the film.

Blocked isocyanates (blocking agent): Isocyanates, normally extremely reactive with water, can only be used in waterborne coatings if they can be prevented from reacting before the water is baked out of the paint film. This is done by capping or blocking the isocyanate group with a thermally decomposable chemical. In a bake oven, the water evaporates, the chemical cap decomposes and the isocyanate crosslinks the paint. Blocked isocyanates are often employed for E-coat curing.

Blocking: Undesirable sticking together of painted surfaces when pressed together under normal conditions. Sticking or blocking can be reduced by anti-block paint additives.

Blooming: Powder-like deposit forming on the surface of the film often resulting from partial dissolving and redepositing of pigment by a solvent component.

Blushing: Whitish, milky area which develops on the film and may be caused by absorption of water vapor by the drying film.

Bounce-off, bounceback: Paint droplets from air-atomized application that rebound or bounce away from the surface due to the blasting effect of the air.

Brush coating: Manual application of coatings using brushes and rollers.

Bulk coating: The painting of large masses of small unchangeable parts by a variety of possible techniques such as dip-spin and dipping.

Burn-off ovens: A paint stripping method accomplished by combustion of the coating in gas-fired, burn-off ovens in which high temperatures are controlled by injecting of water spray into the oven.

CARC: Chemical Agent Resistant Coatings. The polyurethane-based coatings are highly crosslinked to resist chemical attack. CARC is often used on military equipment that might become contaminated by nuclear, biological, or chemical substances.

Cathode: The cathode is defined as the electrode at which chemical reduction takes place. In electrodeposition (E-Coating) the cathode is indicated on diagrams by the negative (-) marking.

Caustic: A substance that neutralizes acids. Caustics are used in aqueous cleaning to speed soil removal and increase soil suspension. Caustic is synonymous with alkali.

Cellosolve: The generic term for the solvent family of mono-alkyl ethers of ethylene glycol. For example, a widely-used solvent is butyl cellosolve, which chemically is ethylene glycol monobutyl ether.

Centrifugal coater: see dip-spin coater

Chalking: The degradation of a paint film by gradual erosion of the binder, usually due to weathering.

Checking: Slight breaks in the film that do not penetrate to the substrate surface. If the substrate surface is exposed it is called cracking.

Chipping: Total or partial removal of a dried paint film in flakes by damage or wear during service.

Chlorinated solvents: Organic solvents that contain chlorine. Examples include 1,1,1-trichloroethane and methylene chloride. These solvents are characterized as volatile organic compounds. Their use is regulated and heavily restricted.

Coating: A liquid or mastic composition which is converted to a solid protective, decorative, or functional adherent film. The South Coast Air Quality Management District defines coatings as materials which are applied to a surface and which form a continuous film in order to beautify and/or protect the surface.

Coating line: Coating lines are all operations involved in the application, and/or drying of surface coatings. However, this definition does not specificly delineate what separates coating lines in a source, especially when a single oven may cure parts from multiple spray booths. For most rules, where the exemption level of the rule is not related to the volume of coating applied per coating line, this definition does not apply.

Cobwebbing: The tendency of spray paint to form strands rather than droplets as it leaves the spray gun. Cobwebbing may be caused by too volatile a solvent or too little air pressure.

Continuous coater: An enclosed automatic spray booth that recovers and reuses oversprayed paint. A continuous coater is suitable for coating large volumes of similarly-sized parts.

Conversion coating: A chemical or electrochemical treatment of a metal surface to convert it to another form, which provides an insulating barrier of exceedingly low solubility between the metal and its environment, and is an integral part of the metallic substrate. Examples are phosphate coating of steel and zinc and chromate anodizing of aluminum.

Cosolvents: Water-miscible organic solvents. Waterborne paints frequently require cosolvents in addition to water for easier manufacturing and improved application properties.

Cracking: The splitting of a dry paint film, usually the result of aging. This includes: hair cracking, checking, crazing, and alligatoring (crocodiling).

Cratering: Small round depressions in a paint film which may or may not expose the underlying surface.

Crawling: A defect in wet paint or varnish film where it recedes from small areas of the surface, leaving them apparently uncoated. Crawling is caused by an incompatible film on the surface.

Crazing: The formation of fine surface cracks, often as a network, which do not penetrate to the underlying surface.

Crosslinking: The setting up of chemical links between the molecular chains of a resin to form a three-dimensional network polymer system. Crosslinking generally toughens and stiffens coatings.

Cup gun: A spray gun used with a siphon cup.

Cure: Using heat, radiation, or reaction with chemical additives to change the properties of a polymeric system into a more stable, usable condition. For liquid coatings, it is the process by which the liquid is converted into a solid film.

Current density: A measure of the total electrical flow across a given area, frequently expressed in units of amps/square foot.

Cyclone separator: A funnel-bottomed enclosure that rapidly moves particulate-laden streams of air in a circular path. As the relatively high mass of particles are thrown to the sides of the enclosure, they slide down through the funnel into a container for reuse. Cyclone separators are commonly used for powder coating applications.

Deionized water: Water resulting from the removal of contaminants by a double-bed ion exchanger. The ion exchanger replaces positive impurity ions with H+ (hydrogen) ions and negative impurity ions and OH-(hydroxide) ions. The hydrogen ions and hydroxide ions then combine to form H0H (H20). Deionized water is comparable in purity to distilled water but is much less costly to produce.

Diluent: Liquids which increase the capacity of a solvent for the binder. Diluents cannot dissolve the binder themselves, but are used to control viscosity, flash time, or cost. While true solvents can be added in unlimited amounts to lower paint viscosity, it may be more economical to lower viscosity with less costly diluent solvents. When added to a prepared paint, a diluent will lower the viscosity just as effectively as a true solvent. However, if too much diluent is added, the resin will separate out of solution and the paint becomes unusable.

Dip coating: The process in which a substrate is immersed in a solution (or dispersion) containing the coating material and withdrawn.

Dip-spin coater: Bulk painting of small and unchangeable parts accomplished by dipping a mesh basket of parts, followed by rapid rotation of the basket to remove excess paint. Parts from the dip-spin coater are dumped onto a belt for curing.

Disks (discs): Rotating heads that deliver paint using a horizontal 360 degrees motion and an omega loop conveyer line. A disk is usually mounted horizontally on a vertical reciprocator.

Dispersion coating: A type of coating in which the binder molecules are present as colloidal particles and spread uniformly throughout the formulation as a stable mixture.

Doctor blade: Device used to prepare paint and varnish films of even and predetermined thicknesses.

Drier: An additive which accelerates the drying of coatings.

E-coating (electrodeposition): A dip coating application method where the paint solids are given an electrical charge opposite to the part being coated. In this method, which closely parallels electroplating, paint is deposited using direct electrical current. The electrochemical reactions that occur cause water-soluble resins to become insolubilized onto parts which are electrodes in the E-coating paint tank. Subsequent resin curing is required.

Eductor: Nozzles located along E-coat return headers and spaced laterally at intervals across the tank. These nozzles help to agitate the paint and prevent settling of pigments, which results in cleaner film deposits.

Electrostatic spray: Method of spray application of coating where an electrostatic potential is created between the part to be coated and the paint particles.

Emulsion: A two-phase liquid system in which small droplets of one liquid (the internal phase) are immiscible in, and are dispersed uniformly throughout, a second continuous liquid phase (the external phase). This contrasts with latex, which consists of solids dispersed in a liquid.

Emulsion paint: A coating comprised of an emulsion of a resin binder in water.

Enamels: Topcoats which are characterized by their ability to form a smooth surface; originally associated with a high gloss, but may also include a lower degree of gloss. Also a class of substances having similar composition to glass with the addition of stannic oxide, or other infusible substances to render the enamel opaque. Can be used to describe a coating which forms a film through chemical union of its component molecules during curing. In shop terminology can be used to describe paint which is no longer a lacquer. All paints, powder or liquid, that form crosslinking chemical bonds during curing are considered enamels. The majority of industrial finishes fall into this category.

Epoxies: Binders based on epoxy resins. Epoxy crosslinking is based on the reaction of the epoxide groups with other materials such as amines, alcohols, phenols, carboxylic acids, and unsaturated compounds. Also used as a thermoset powder coating.

Etching: A chemical solution used to remove a layer of base metal to prepare a surface for coating or binding.

Etching filler: Coatings that contain less than 23% solids by weight, and at least 0.5% acid by weight, and are used instead of applying a pretreatment coating followed by a primer.

Exempt compounds: Hydrocarbon compounds excluded from the definition of volatile organic compound, as defined by the U. S. Environmental Protection Agency, on the basis that these compounds have negligible contribution to tropospheric ozone formation. Acetone is an exempt compound.

Extender (pigments): White powders used to give body to the coating.

Fading: The loss of color in a pigmented coating film, over time, following exposure to light, heat, etc.

Faraday cage: Electrostatic application causes paint particles to be attracted to the nearest grounded object. This attraction force is often strong enough to pull paint particles out of their intended flight direction. Recessed areas on parts often receive insufficient paint coverage since they require a slightly longer path for paint particles. As a result, these Faraday Cage areas may need touch-up painting with non-electrostatic spray.

Faraday cage effect: The phenomenon by which charged particles are prevented from entering recessed areas during the electrostatic application of coatings.

Fatty edge: An excess bead of paint that forms on the bottom edges of parts when they are in the drippage zone following dip or flow coating.

Film: One or more layers of coating covering an object or surface.

Fisheye: A paint defect resulting in a pattern of small surface depressions or craters in the wet film, often caused by surface contamination such as oil or silicone materials.

Flash point: The lowest temperature of a liquid at which it gives off sufficient vapor to form an ignitable mixture with air.

Flash-off time: The time required between application of wet-on-wet coatings or between application and baking to allow the bulk of the solvents to evaporate. In baked coatings, the flash-off time helps to prevent solvent boil off and film blistering.

Flat coatings: Coatings with a gloss reading of less than 15 on an 85-degree meter or less than 5 on a 60-degree meter. This definition is usually found in architectural coating rules.

Flocculation: The formation of loose clusters of dispersed pigment particles in liquid coatings.

Flooding, floating, or mottle: Tendency of pigment particles to separate and concentrate in an area such as the surface.

Flow coating: A coating application system where paint flows over the part and the excess coating drains back into a collection system.

Fluidized bed: Finely divided powders can be made into a fluid-like state by passing air through the porous plate bottom of a powder hopper. This permits powder particles to be used in dip tanks and to be transported in a manner similar to liquids.

Flushable electrode: An anode in cathodic E-coating placed inside a semi-permeable membrane enclosure. Excess solubilizer generated at the anode can be continuously removed by water pumped into the bottom of the enclosure. Flushable electrodes in anodic E-coating can also be used (but rarely are needed) for the cathode.

Free radical polymerization: Reactive electrons that chemically bond to adjacent molecules and produce a cured paint film. Certain organic compounds will form highly reactive electron configurations by the action of UV light (or other activation sources). These reactive species are called free radicals because, to an extent, `free' electrons are available for bonding.

Fusion: The melting of a powder coating into a solid film.

Grain refiners: Agents used in water rinses prior to zinc phosphating or in the zinc phosphatizing bath itself to produce smaller crystals. Finer grain zinc phosphate crystals provide superior corrosion resistance and paint adhesion.

Ground (electrical ground): An object so massive that it can lose or gain overwhelmingly large numbers of electrons without becoming perceptibly charged.

Halogenated hydrocarbons (halogenated solvents): Formed by substituting one of the halogen elements (chlorine, bromine, or fluorine) into a chemical compound to change both the physical and chemical nature of the compound.

Heat-resistant coatings: Designed to resist degradation upon continuous or intermittent exposures to a predetermined elevated temperature. A San Diego Air Pollution Control District rule stipulates that the coating must withstand temperatures of 400oF during normal use as determined by ASTM Method D-2485.

High boilers: Solvents with a boiling point above 212oF (tail-end solvents). These solvents usually evaporate during baking.

High-solids: Solvent-borne coatings that contain greater than 50% solids by volume or greater than 62%(69% for baked coatings) solids by weight.

High temperature coatings: Coatings certified to withstand a temperature of 1000oF for 24 hours.

High volume low pressure spray: Spray equipment used to apply coating by means of a gun which operates between 0.1 and 10.0 psig air pressure. The high volume of air is produced by a turbine.

Hot water curing: A curing procedure which involves immersing parts in 180oF water. Hot water curing is faster than oven curing for parts that act as a large heat sink, but is normally not used since it results in reduced corrosion resistance.

Hydrocarbon solvent: An organic compound consisting exclusively of the elements carbon and hydrogen. They are principally derived from petroleum and coal tar, and include aliphatic, aromatic, and napthenic solvents.

Hydroxides: The chemical opposites of acids. Also known as caustics and alkalis. Examples are sodium hydroxide and potassium hydroxide.

Hygroscopic: A material property defined by the ability of a substance to readily absorb moisture from the air. Hygroscopic materials, such as silica gel and calcium chloride, are used as desiccants. Thinly spread deposits of hygroscopic materials can absorb enough water to completely dissolve.

Inhibitor: A chemical additive that retards undesired chemical reactions such as corrosion, oxidation, drying, and skinning.

Initiator: A chemical used to help start a chemical reaction such as polymerization. Its action is similar to that of a catalyst, except that it is usually consumed in the reaction.

Inorganic polymers: Substances whose principal structural features are made up of homopolar interlinkages between multivalent elements other than carbon. This does not preclude the presence of carbon-containing groups in the side branches, or in interlinkages between principal structural members. Examples of such polymers are ethyl and butyl silicates, mica, clays, and talc.

Ionized air cloud: A cloud of air molecules that have picked up excess electrons around the tip of an operating electrostatic spray gun . The electrons from the power pack flow off the end of the needle electrode at the gun tip. When paint droplets pass through the ionized air cloud they accumulate electrons that enable electrostatic attraction of the droplets to parts being coated.

Isocyanate: A compound containing the functional group -N=C=O. Isocyanates are crosslinked with hydroxyls to form polyurethanes.

Kick-out: The portion of binder that comes out of solution as small lumps.

Lacquer: Coating composition based on synthetic thermoplastic film-forming material dissolved in organic solvent and dried primarily by solvent evaporation. Typical lacquers include those based on nitrocellulose, other cellulose derivatives, vinyl resins, and acrylic resins.

Latent solvent: A liquid which cannot itself dissolve a binder but which increases the tolerance of the coating for a diluent.

Latex: Stable dispersion of polymeric solids in an aqueous medium.

MEQ (milliequivalents): The concentration of E-coat solubilizer in the bath.

MHO: Unit of conductance equal to the reciprocal of the ohm.

Molecule: The smallest particle of a substance that retains all the properties of that substance and is composed of one or more atoms. Water, for example, consists of molecules having 2 hydrogen atoms and 1 oxygen atom. The chemical formula, H2O, indicates the composition of a water molecule. Organic polymers often have many thousands of atoms per molecule.

Molten salt bath: A mixture of inorganic salts melted at temperatures between 650o and 900oF. Painted items immersed in these are rapidly stripped by combustion of the paint.

Nitrocellulose: A binder (resin) based on a polymer from cotton cellulose. Nitrocelluloses were primarily used in lacquers, and were widely used from the 1920's to the 50's on automobiles.

OHM: A standard unit of resistance to electrical flow.

Ohmeter: A device that measures (in units of ohms) electrical resistance in a circuit.

Oil base: Coatings which form films through crosslinking of unsaturated plant oil (drying oils) in the presence of oxygen.

Omega loop: The conveyor for rotating disk paint applicators that is shaped to produce a circular path around the vertically oriented disk to deliver paint from all 360 degrees of its circumference. The term was derived because the shape of the conveyor resembles the capitalized form of the Greek letter.

Orange peel: An irregularity in the surface of a paint film resulting from the inability of the wet film to level out after being applied.

Overbake or overcure: Exposure of the coating to a temperature higher or for a longer period of time, or both, than recommended for optimal curing; the condition may adversely affect the appearance and properties of the coating.

Overspray: Any portion of a spray-applied coating which does not land on a part.

Oxygenated solvents: Volatile organic compounds which contain oxygen in addition to carbon and hydrogen. Includes alcohols, esters, ketones, and ether-alcohols.

Peeling: Failure of a coating film to maintain adhesion with its substrate. Sheets or ribbons of the film detach from the substrate. The condition results from contaminated surfaces or excessive differences in polarity and thermal expansion characteristics between the surface and the film.

Permeate: The output from ultrafiltration, also called ultrafiltrate.

pH: The measure of the acidity or alkalinity of a solution and defined as the logarithm of the reciprocal of the hydrogen-ion concentration of a solution. The scale ranges from 2 for highly acidic solutions to 14 for highly basic or alkaline solutions. Neutral solutions have a pH of 7. Because the scale is logarithmic, the intervals are exponential.

Phenolic resins: Resins formed by condensation of phenols and aldehydes.

Phosphating: A pretreatment for steel or certain other metal surfaces by chemical solutions containing metal phosphates and phosphoric acid as the main ingredients. A thin, inert adherent, corrosion-inhibiting phosphate layer forms which serves as a good base for subsequent paint coats.

Pigment: Finely ground insoluble particles dispersed in coatings to influence properties such as color, corrosion resistance, mechanical strength, hardness, durability, etc. Particles may be natural or synthetic, and inorganic or organic.

Polar: Descriptive of molecules where the atoms and their electrons and nuclei are so arranged that one end of the molecule has a positive electrical charge and the other end of the molecule has a negative electrical charge. The greater the distance between the two charged ends, the higher the polarity. Polar molecules ionize in solution and impart electrical conductivity.

Polyester: A polymer in which the monomer units are linked by the functional group -COO-. Polyester has been used as thermoplastic powder coating, and in the following thermosetting powder coatings: epoxy polyester hybrid powder, urethane polyester powder, and polyester TGIC powder.

Polyethylenes: Thermoplastic resins composed of polymers of ethylene (CH2CH2). Polyethylenes are normally translucent, tough, waxy solids that are unaffected by water and a large range of chemicals. Frequently used in powder coatings.

Polymers: A high molecular weight organic compound, natural or synthetic, with a structure that can be represented by a repeated small unit, or mer.

Polypropylenes: Tough lightweight thermoplastic resins composed of polymers of propylene (CH3CHCH2). They are commonly used in powder coating.

Popping: Eruptions in a coating film after it has become partially set, causing craters to remain in the film.

Pot life: The length of time a coating material is useable after the original package is opened or after a catalyst or other ingredient is added.

Powder coatings: Any coating applied as a dry (without solvent or other carrier), finely divided solid which adheres to the substrate as a continuous film when melted and fused.

Power-and-free conveyor: A separate pusher chain unattached to paint hooks and riding freely on a separate support beam (as distinguished from a continuous power conveyor). This conveyor allows parts spacing to vary and parts to be held stationary even when the pusher chain is moving.

Power conveyor (continuous): Electrically driven cables or chains mechanically attached to hoods which are used to hang parts to be painted. The conveyor is used to carry parts through the painting process. When the line is operating, all individual hooks on the line will continue to move and maintain their spacing.

Precursor: A chemical compound which is released into the atmosphere, undergoes a chemical change, and leads to a new (secondary) pollutant. Volatile organic compounds are precursors to ozone.

Pressure pot: Various-sized paint tanks containing delivery tubes which extend to the bottom of the tank. These tanks are pressurized with compressed air to force paint to the application device.

Primers: Coatings which are designed for application to a surface to provide a firm bond between the substrate and subsequent coatings.

Reactive diluent: A liquid which is a VOC during application, and through chemical reaction, such as polymerization, 20% or more of the VOC becomes an integral part of the finished coating.

Reciprocator: An automated device which moves a paint-applying tool in alternating directions along a straight or slightly curved horizontal or vertical path.

Resin: The polymer (plastic) component of a paint that cures to form a paint film. Also known as binder or vehicle.

Retarders: Solvents added to a coating to slow down a chemical or physical change, such as the rate of evaporation.

Reverse osmosis: In reverse osmosis, high pressures are applied to force water out of the concentrated solution, often to obtain pure (or purer) water. Solvent is driven through a semi-permeable membrane separating solutions of different concentrations.

Ringing: The occurrence of circular spots in a sprayed repair area (spotting).

Roll coating: Process by which a film is applied mechanically to sheet or strip material.

Rusting (face and/or scratch): The appearance of metal oxidation (corrosion) on the surface of damaged paint.

Sagging: The downward flow of a coating film as a result of the film being applied too heavily or fluid-like.

Sandscratch swelling: A paint defect where solvent from a repair coat soaks into scratches in the initial coat and causes paint swelling.

Sealers: A liquid coat applied to a porous substrate such as wood or plaster, to prevent the substrate from absorbing subsequent coatings.

Shelf life: The length of time a coating may normally be stored without losing any chemical/physical properties. Manufacturers typically specify the shelf life.

Silicone release: A coating which contains silicone resins and is intended to prevent food from sticking to metal surfaces such as baking pans.

Silicones: Resins consisting of silicon-oxygen linkages, unlike organic resins which contain carbon.

Silking: A surface defect which results in parallel flow lines in the paint film.

Siphon cup (suction cup): When a special air spray tip is employed, a partial vacuum is created by the atomizing air just outside the fluid orifice. As a result, atmospheric pressure on the paint in a container connected to the fluid line(such as a siphon cup) will force paint out of the container into the fluid line. The term siphon is actually a misnomer; suction is a more accurate description of the action.

Skinning: The formation of a surface skin on coating liquids formed by the coating reacting with air or rapidly loosing solvent.

Slitting: Cutting wide coils of roll-coated materials into narrower widths.

Solubilizer: Compound that forms polar polymer ions when mixed with water-insoluble resins. Since water is a polar solvent and resins are usually non-polar, the resins must be treated to increase their polarity if they are to be used in waterborne paints.

Solution paint: Resin molecules fully dissolved by solvents in the paint.

Solvency: The degree to which a solvent holds a resin or other paint binder in solution.

Solvent: The liquid or blend of liquids used to dissolve or disperse the film forming particles in a coating which evaporate during drying. A true solvent is a single liquid which can dissolve the coating. The term solvent is often used to describe terpenes, hydrocarbons, oxygenated, furans, nitroparaffins, and chlroinated solvents.

Solvent-borne: Coatings in which volatile organic compounds are the major solvent or dispersant.

Specific gravity: Weight of a given volume of any substance compared with the weight of an equal volume of water. Also known as relative density.

Static electricity (electrostatics): Electrons temporarily removed from various items can cause static charges. Whatever has excess electrons has a negative charge; the object from which electrons have been taken will be positively charged. Electrons will tend to jump from a negatively charged object to a positively charged. object.

Stencil coating: Ink or other coating which is rolled or brushed onto a template or stamp in order to add identifying letters and/or numbers to metal parts and products.

Surface tension: The energy required to expand a liquid surface by one unit area. Liquids reduce their surface area to bring intermolecular attractive forces into equilibrium. A low degree of surface tension is preferred for liquid coatings to maximize minimize wetting and spreading and minimize edge-pull and fish-eye effects.

Surfacer: Easily sanded coating used to fill surface irregularities.

Terpene solvents: Volatile organic compounds obtained from pine trees and are the oldest solvents used in coatings. Includes turpentine, dipentene, and pine oil.

TGIC (triglycidyl isocyanurate): A complex chemical used to crosslink paint, especially polyester powders, to increase exterior durability.

Thermoplastic: Resin capable of being repeatedly softened by heat and hardened by cooling. These materials, when heated, undergo a substantial physical, rather than chemical, change. Thermoplastic resins can be completely dissolved with appropriate solvents.

Thermoset: Resin that, when cured by application of heat or chemical means, changes into a substantially infusible and insoluble material. Thermosetting resins will soften but will not dissolve in any solvent.

Thinning: The process of adding volatile liquid to a coating to reduce its viscosity. The liquid may be a solvent, diluent or a mixture of both. Thinning may also be called reducing or "adding makeup solvent".

Thixotrope: Substance that temporarily causes high paint viscosities by forming loosely-held three-dimensional particle networks within paint fluids. Agitation of the paint by stirring, pumping, spraying, etc., quickly destroys the networks and viscostity drops sharply. When agitation is halted, the networks rapidly reform and paint viscosity rises again.

Thixotropy: The tendency for the viscosity of a liquid to be shear-rate dependent. When a liquid is rapidly shaken, brushed, or otherwise mechanically disturbed the viscosity decreases rapidly.

Throwing power: The ability of electro-deposited coatings to cover interior surfaces.

Topcoat: The final coating film or multiple layers of the same coating film applied to the surface.

Touch-up: The portion of the coating which is incidental to the main coating process but is necessary to cover minor imperfections.

Transfer efficiency: The ratio of solids adhering to a surface to the total amount of coating solids used in the application process, expressed as a percentage.

Underbake or undercure: Exposure of the coating to a temperature lower or for a shorter period of time, or both, than recommended for optimal curing; the condition may cause tackiness, softness, and inferior film durability.

Ultrafiltation: Ultrafiltration uses low-pressure membrane filtration to separate small molecules from large molecules and fine particulates. For example, E-coat rinse water is extracted from the paint bath by ultrafiltation.

Ultrafiltrate: The output from an ultrafiltration unit; also called permeate.

Ultrasonic cleaning: Vibrational frequencies slightly higher than those audible used to agitate immersion cleaning tanks. Microbubble formation in the liquid accelerates dislodgement of soils.

Undercoats: Coatings formulated and applied to substrates to provide a smooth surface for subsequent coats.

Urethanes: Materials based on resins made by the condensation of organic isocyanates with compounds or resins containing hydroxyl groups. Categories of polyurethane coatings include: single component prereacted-urethane coatings; single component moisture-cured urethane coatings; single component heat-cured urethane coatings; two component catalyst-urethane coatings; two component polyurethane coatings; and one component nonreactive lacquer-urethane solution coatings.

Vacuum metallizing: Process in which surfaces are thinly coated by exposing them to metal vapor under a vacuum.

Varnish: Clear or pigmented coatings formulated with various resins and designed to dry by chemical reaction on exposure to air. These coatings are intended to provide a durable transparent or translucent solid protective film.

Vehicle: The liquid portion of a coating in which the pigment is dispersed; it is composed of binder, solvent and diluent.

Vinyl chloride polymers: Polymers formed by the polymerization of vinyl chloride or copolymerization of vinyl chloride with other unsaturated compounds, the vinyl chloride being in greatest amount by weight. Can be used in thermoplastic powder coatings.

Vinyl resins: Resins which contain the unsaturated vinyl group, (CH2 = CH-) including polyvinyl acetate, polyvinyl chloride, copolymers of these, the acrylic and methacrylic resins, the polystyrene resins, etc.

Viscosity: The property of a fluid whereby it tends to resist relative motion within itself. A thick liquid such as syrup has a high viscosity. Viscosity is often measured using an efflux type cup which gives the time required for a given quantity of paint to flow through a hole in the bottom of the metal cup at a given temperature (See Zahn Cup).

Volatile organic compound (VOC): Any organic compound, not specifically exempted by the U.S. EPA, that participates in atmospheric photochemical reactions. VOCs may be emitted during the application and/or drying of coatings. In calculating the VOC content of the coating, exempt compounds and water are excluded. Exempt compounds are acetone, ethane, methane, carbon monoxide, carbon dioxide, carbonic acid, metallic carbides, metallic carbonates, ammonium carbonate, methylene chloride, 1,1,1 trichloroethane (methyl chloroform), 1,1,2 trichlorolotrifluoroethane (CFC-113), trichlorofluoromethane (CFC-11), dichlorodifluoromethane (CFC-12), dichlorotetrafluoroethane (CFC-114), chloropentafluoroethane (CFC-115), trifluoromethane (CFC-23), and chlorodifluoromethane (CFC-22). Although many of these compounds are exempt under the VOC rule, they may contribute to upper atmosphere ozone destruction.

Volatility: The tendency of a liquid to evaporate. Liquids with high boiling points have low volatility and vice versa.

Voltage: measure of the potential difference (force or pressure) in electrical systems.

Waterborne coatings: Coatings in which water is the major solvent or dispersant. Solvents or dispersants include water soluble polymers (water reducible), water soluble colloidal dispersions, and emulsions (including latex).

Water-reducible coatings: see waterborne coatings.

Weir: The (often adjustable) barrier that controls the paint depth in an E-coat tank over which the paint flows to the circulation pump to be filtered.

Wet-on-wet finishing: Applying a new coat over an earlier applied coat which has been allowed to flash-off but not cure.

Wrap around: Electrostatic effect where charged coating particles curve around the part and are deposited onto the rear side of the part.

Wrinkling: Distortion in a paint film appearing as ripples.

Zahn cup: Commonly used efflux cup used for measuring the viscosity of coatings. Other widely used viscosity cups are the Fischer cup and the Ford cup. These instruments measure the time required for a given quantity of paint to flow through a hole in the bottom of a metal cup at a given temperature.