Knowledge Centre

Glossary

The key concepts of paint technology: 48 terms with short, clear definitions.

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1K (Single-Component)(Single-Pack): Refers to single-component products that need no hardener and are used as supplied or simply reduced with thinner; the “K” comes from the German word “Komponente”, meaning component. The film dries physically, largely through solvent evaporation; there is no pot-life limit, and leftover material can be stored with the lid tightly closed. Because their chemical and solvent resistance is lower than that of 2K products, they are mostly used for intermediate coats such as fast repair primers and plastic primers.; Related:2K (Two-Component)Solvent Curing
2K (Two-Component)(Two-Pack): Refers to two-component products in which the base component is mixed with hardener in a set ratio immediately before application. The chemical reaction (cross-linking) that starts on mixing produces a hard, glossy, chemical-resistant film. The mixing ratio must be followed exactly and the mixture used only within its pot life; acrylic primers, clear coats and polyester fillers are typical 2K products.; Related:1K (Single-Component)Mixing Ratio Pot Life

A

Adhesion: The strength with which a paint film holds on to the substrate or to the previous coat. Good adhesion requires a surface free of grease, silicone, rust and dust, sanding with the correct grit to create a surface profile that gives a mechanical key, and a primer suited to the substrate (steel, galvanised steel, aluminium, plastic). Poor adhesion shows up over time as blistering, flaking and delamination; the level of adhesion can be checked with a cross-cut test.; Related:Cross-Cut Test Degreasing Primer

B

Baking(Force Drying): Accelerated drying and curing of paint by raising the temperature in a spray booth or oven. A common schedule for 2K clear coats is 30–40 min at an object temperature of 60 °C, after the flash-off time stated in the product information; timing starts once the part reaches this temperature. Baking without the prior flash-off can cause solvent popping, while excessive temperatures can deform plastic parts.; Related:Curing Infrared Drying Solvent Popping
Basecoat: In two-layer (basecoat + clear coat) paint systems, the layer that provides the colour and effects such as metallic and pearl. It usually dries matt and, since it does not on its own provide sufficient gloss or durability, it must always be covered with clear coat. It is applied in thin, even coats until full coverage is achieved; the clear coat should be applied once the waiting time given in the basecoat's product information has elapsed and before the maximum permitted time is exceeded.; Related:Clear Coat Blending (Fade-Out)Flash-Off Time
Blending (Fade-Out): A spot-repair technique in which the basecoat is applied in progressively thinner coats from the repair area out into the surrounding original paint, making the colour transition invisible to the eye. The blend area is first matted down with a fine abrasive or scuff pad and cleaned; the basecoat first covers the repair area and is then gradually extended outwards and faded in. The clear coat is either applied to the whole panel or confined to a limited area, with its edge melted in using blending (fade-out) thinner, and the transition zone is finished by polishing after curing.; Related:Basecoat Clear Coat Polishing
Body Filler(Polyester Filler): A 2K filling material, mostly based on unsaturated polyester and cured with BPO, used to fill the dents, ripples and surface irregularities left after panel beating; it is applied with a spreader in thin, firmly pressed layers and, once cured, is usually shaped with P80–P120 and refined with P180–P240 ready for primer. Types have been developed for different jobs, such as universal, glass-fibre-reinforced, lightweight, finishing (soft), aluminium-filled and galvanised-compatible fillers. Polyester filler must not be applied over 1K primers or acid-etch (wash) primers.; Related:BPO (Benzoyl Peroxide)Pinholes Guide Coat
BPO (Benzoyl Peroxide): A peroxide-based hardener paste for curing polyester fillers, usually supplied in a tube with the filler; the typical addition rate is 2–3% by weight, and the coloured paste should be mixed in until the filler has a uniform colour. Too much BPO causes discolouration in the topcoat (peroxide staining), pinholes and adhesion problems, while too little leaves a filler that stays soft, clogs the abrasive paper and absorbs solvent from the coats above. As an organic peroxide, it must be kept away from heat, and skin contact must be avoided.; Related:Body Filler Hardener Pot Life

C

Clear Coat(Lacquer): The colourless, transparent topcoat applied over basecoat; it gives the surface gloss and depth and protects the colour beneath from UV rays, chemicals, scratching and weathering. In refinishing it is mostly a 2K acrylic (polyurethane) product and is classified as MS or HS according to its solids content. It is usually applied in two coats to reach a dry film thickness of typically 50–60 µm and can be polished after curing if necessary.; Related:Basecoat UV Resistance Polishing
Corrosion: The deterioration of metals through chemical or electrochemical reaction with moisture, oxygen, salt and pollutants; it appears as red-brown rust on steel and as whitish oxide on zinc and aluminium. In refinishing, the corroded area must be cleaned back completely to sound metal and the bare metal then promptly covered with a suitable primer. Filler and paint applied over rust will blister and flake over time as corrosion creeps underneath.; Related:Galvanised Steel Primer
Cratering(Fish-Eyes): A defect in which the wet paint film pulls away from spots contaminated with silicone, oil, wax or fingerprints, leaving round, recessed marks; it is also known as fish-eyes. The most common causes are silicone-containing polishes and sprays, oil and water in the compressed air, and inadequate surface cleaning. To prevent it, degrease with surface cleaning thinner or silicone remover before every application step, fit oil and water filters to the air line and avoid using silicone-containing products in the workshop.; Related:Degreasing Adhesion
Cross-Cut Test(Cross-Hatch Test): A practical test method for assessing the adhesion of a paint film. Using a special multi-blade cutter, the film is cut through to the substrate in a grid of 6 parallel lines in each direction, crossing at right angles, with a line spacing of usually 1–3 mm depending on film thickness. Adhesive tape is applied over the grid and pulled off sharply, the detached area is then assessed, and the result is classified under ISO 2409 from 0 (no detachment) to 5 (severe detachment).; Related:Adhesion Dry Film Thickness
Curing: In 2K products, the process in which the base component and the hardener react chemically to form a cross-linked, hard and durable film; it differs from physical drying, which takes place through solvent evaporation. The reaction continues after the surface is touch-dry, and final hardness and chemical resistance develop over several days, depending on ambient conditions. Because low temperatures slow curing considerably, most 2K products are not recommended for application below 10 °C, whereas baking and infrared drying speed the process up.; Related:2K (Two-Component)Baking Hardener

D

Degreasing: The removal of oil, grease, silicone, polish residues and fingerprints from the surface before sanding and painting, using a suitable surface cleaning thinner or silicone remover. The cleaner is wiped onto the surface with one cloth and wiped off with a second, clean, dry cloth before it evaporates (the two-cloth method). Inadequate degreasing leads to cratering and adhesion problems, so it should be repeated before and after sanding and before every application step.; Related:Cratering Adhesion
Die-Back(Loss of Gloss): The partial loss of gloss that a paint or clear coat surface, glossy at first, shows hours or days after application. It occurs when solvent retained in the film evaporates over time and the film shrinks; the main causes are excessively heavy coats, insufficient flash-off time, an unsuitable thinner, an incorrect mixing ratio, inadequate curing at low temperature and application over primer that has not fully dried. Slight die-back can be removed by polishing after full cure; in pronounced cases the surface should be sanded and clear-coated again.; Related:Gloss Flash-Off Time Curing
DIN 4 Flow Cup(DIN 4 Viscosity Cup): A standard 100 ml measuring cup with a 4 mm flow orifice in its base; the viscosity of paint is determined by measuring, in seconds, how long the cup takes to empty. With the orifice closed with a finger, the cup is filled to the brim; the stopwatch is started as soon as the orifice is opened and stopped when the stream first breaks. Because the temperature of the material has a major effect on the result, the measurement is made with material at around 20 °C and the value is stated together with the temperature, e.g. “20 s DIN 4 / 20 °C”.; Related:Viscosity Thinner
Dry Film Thickness(DFT): The thickness of the paint film once it has fully dried, expressed in micrometres (µm). It is measured non-destructively with film thickness gauges that work on the magnetic induction principle on steel and on the eddy current principle on non-ferrous metals such as aluminium. Target values are given in the product information, for example typically 50–60 µm for 2K clear coats; too little thickness weakens protection and UV resistance, while too much increases the risk of runs, solvent popping and cracking.; Related:Solids Content Clear Coat Sagging / Runs

F

Flash-Off Time: The time to wait between coats of the same product, or between different layers, to allow part of the solvent to evaporate; at 20 °C it is typically 5–10 min for many primers and clear coats, becoming longer at low temperatures, in high humidity and with heavy coats. In practice, it is judged by checking that material on the masking tape no longer forms strings when touched or, for basecoat, that the surface has turned evenly matt. Cutting this time short increases the risk of runs, solvent popping and die-back.; Related:Solvent Popping Sagging / Runs Die-Back

G

Galvanised Steel(Zinc-Coated Steel): Steel coated with zinc by hot-dip or electrolytic processes to protect it against corrosion; it is widely used in modern vehicle bodies. Because zinc surfaces can carry oil and zinc corrosion products known as white rust, paint and filler adhere to them less readily than to steel. The surface must be cleaned thoroughly and lightly abraded without breaking through the zinc layer, and a filler and primer compatible with galvanised steel must be used.; Related:Corrosion Adhesion Body Filler
Gloss: The degree to which a paint surface reflects light like a mirror, measured with gloss meters in GU (gloss units). Measurements are usually taken at an angle of 60°; for high-gloss surfaces that read above 70 GU at 60°, the more discriminating 20° angle is preferred. Gloss is directly affected by surface smoothness, sanding marks, film thickness, the choice of thinner, flash-off times and curing conditions.; Related:Polishing Die-Back Clear Coat
Grit Size(P Grade): The number that indicates how fine the grains of an abrasive are; the “P” prefix denotes classification to the European FEPA standard, and the higher the number, the finer the abrasive. In refinishing, P80–P120 is typically used for shaping filler, P180–P240 for finishing filler, P400–P600 for sanding primer and P1500–P3000 for removing defects before polishing. Avoid large jumps between grit sizes, and remove all scratches from the previous grit completely before moving on to the next.; Related:Guide Coat Polishing Primer Filler
Guide Coat: A control material, usually black and in dry-powder or aerosol form, applied as a thin layer to filler or primer before sanding. As sanding levels the surface, the guide coat is removed from the high spots but stays in low spots, scratches and pinholes, making defects visible. Renewing it before each change of grit makes it easier to achieve a ripple-free surface; once all traces of the guide coat have disappeared, the surface has been levelled sufficiently with that grit.; Related:Grit Size Body Filler Primer Filler

H

Hardener(Activator): In 2K products, the component that reacts chemically with the base component to cure the film; it is isocyanate-based in acrylic primers and clear coats and BPO-based in polyester fillers. The hardener specified in the system for each product must be used at the stated mixing ratio, and hardeners developed for different product groups must not be used interchangeably. Because isocyanate-based hardeners are sensitive to moisture, the container must be tightly closed after each use, and hardener that has turned cloudy, formed sediment or gelled must not be used.; Related:Isocyanate BPO (Benzoyl Peroxide)Mixing Ratio
HS (High Solids): Describes products with a high solids content and a low solvent content in the ready-to-use mixture. Because they build a thicker dry film with the same number of coats, the target thickness is reached with fewer coats, saving material and time and reducing VOC emissions per unit of dry film achieved. With heavy coats there is a risk of runs, so particular attention must be paid to correct viscosity, gun set-up and flash-off time.; Related:MS (Medium Solids)Solids Content VOC (Volatile Organic Compounds)
HVLP (High Volume Low Pressure): Spray gun technology that atomises paint using a high volume of air at a low pressure of no more than 0.7 bar (10 psi) at the air cap. Transfer efficiency is usually above 65%, which markedly reduces overspray and material consumption compared with conventional guns. For a good result, use the inlet pressure recommended by the gun manufacturer (around 2 bar for most models), a nozzle size suited to the product and a constant gun distance.; Related:Nozzle Size Viscosity Orange Peel

I

Infrared Drying(IR Drying): A method in which radiation from infrared lamps heats the paint film and the substrate directly, speeding up drying. Short-wave IR in particular heats the substrate so that the film dries from the bottom up, reducing the risk of trapped solvent, and it considerably shortens the drying time of filler, primer and clear coat in spot repairs. Lamp distance and time must be set according to the manufacturer's instructions, the flash-off time must be observed beforehand, and overheating of plastic parts and panel edges must be avoided.; Related:Baking Curing Solvent Popping
Isocyanate: The reactive component in the hardeners for 2K acrylic (polyurethane) primers and clear coats; it reacts with the hydroxyl groups in the base component to form a durable, cross-linked film. It is highly sensitive to moisture: reacting with humidity in the air or with water can cause cloudiness, gelling and bubbles in the film, so the container must be tightly closed after each use. As it can cause respiratory and skin sensitisation, suitable respiratory protection, gloves and good ventilation must be used during application; for details, refer to the product's Safety Data Sheet (SDS).; Related:Hardener 2K (Two-Component)Pot Life

M

Mixing Ratio: The ratio between the quantities of base component, hardener and, where applicable, thinner in a 2K product, expressed by volume or by weight. For example, “4:1 + 10–20%” means that 1 part by volume of hardener is added to 4 parts by volume of primer and 10–20% thinner is then added to the mixture; in Türkiye these ratios are also often written as “4+1” or “5+1”. Ratios must be followed exactly using graduated mixing cups, mixing rulers or scales; too little or too much hardener leads to inadequate curing, a soft or brittle film, loss of gloss and durability problems.; Related:2K (Two-Component)Hardener Thinner
MS (Medium Solids): Describes products with a lower solids content and a higher solvent content than HS products; it is a common classification, especially for 2K acrylic clear coats and hardeners. Their more fluid consistency makes them flow out easily and tolerate variations in application better, although reaching the same film thickness usually takes more coats or fuller coats. Thanks to their balance of cost and performance, they are frequently chosen for general repair work.; Related:HS (High Solids)Solids Content Clear Coat

N

Nozzle Size(Fluid Tip Size): The internal diameter, in millimetres, of the nozzle through which material leaves the spray gun; it directly determines how much material is sprayed. HVLP guns mostly use 1.2–1.4 mm nozzles for basecoats and clear coats and 1.6–2.0 mm nozzles for primer fillers. A nozzle that is too small for the product can lead to poor build and dry spray, while one that is too large can cause excess material, runs and poor atomisation; for the best choice, follow the recommendation in the product's Technical Data Sheet (TDS).; Related:HVLP (High Volume Low Pressure)Viscosity

O

OEM (Original Equipment Manufacturer): Stands for original equipment manufacturer; in the paint industry it refers to a vehicle's original paint, applied at the factory on the production line (OEM paint). Factory paint is usually baked at high temperatures of around 130–140 °C, whereas refinish paints are designed to cure at room temperature or at around 60 °C. In refinishing, the aim is to achieve a result as close as possible to the OEM appearance in terms of colour, gloss and orange-peel texture.; Related:Orange Peel Gloss Baking
Orange Peel: A wavy, slightly uneven texture on the paint surface that resembles the skin of an orange. The main causes are high viscosity, poor atomisation (low air pressure), holding the gun too far from the surface, using a thinner that is too fast for the ambient temperature and applying coats that are too thin (dry). Factory finishes also show a certain amount of orange peel; in repairs, the aim is a texture that matches the surrounding panels, and pronounced orange peel is removed by sanding and polishing after curing.; Related:Viscosity HVLP (High Volume Low Pressure)Polishing

P

Pinholes: Tiny holes, the size of a pinpoint, in the surface of paint or filler. They are usually caused by air being trapped in the filler during mixing, air or solvent escaping from porous surfaces, applying too thick a film or drying the surface too quickly. Filler should be mixed by folding rather than whipping and applied in thin, firmly pressed layers; any remaining holes should be sealed with finishing filler or primer filler before further coats are applied.; Related:Body Filler Solvent Popping Primer Filler
Polishing(Compounding): The process of removing defects such as dust nibs, slight orange peel, small runs and fine scratches from a cured clear coat and restoring a high gloss to the surface. It is usually carried out after the defective area has been levelled with very fine abrasives such as P1500–P3000, using a polishing machine, pads and compounds working from coarse to fine. The clear coat must be sufficiently cured before polishing; to avoid heat build-up, the machine should not be held on the same spot for long, and care is needed at edges where the clear coat is thin.; Related:Gloss Orange Peel Grit Size
Pot Life: The time for which a 2K product, once mixed with hardener, keeps a workable consistency in the container; the product information usually states it for 20 °C. For polyester fillers it is limited to a few minutes (typically 3–6 min), whereas for 2K acrylic primers and clear coats it is mostly around 1–2 h, and it shortens considerably as the temperature rises. A mixture that has exceeded its pot life must not be used by adding thinner, even if it still looks fluid; otherwise curing, gloss and adhesion will be impaired.; Related:2K (Two-Component)Mixing Ratio Hardener
Primer: The intermediate coat between the substrate and the topcoat that provides adhesion, protects against corrosion and prepares the surface for the topcoat. Primers can be 1K or 2K and are divided into different types according to their intended use, such as filling, plastics, fast repair or wet-on-wet application. The right primer must be chosen according to the substrate (steel, galvanised steel, aluminium, plastic, existing paintwork) and the type of job, and must be compatible with the paint system that goes over it.; Related:Primer Filler Adhesion Wet-on-Wet
Primer Filler(Primer Surfacer): A high-build primer, usually 2K acrylic, used to fill the fine scratches, small pores and unevenness left on the surface after filling and sanding; it is usually applied in 2–3 coats and, once fully dry, sanded with P400–P600 before basecoat to give a smooth, uniform base. By isolating the filler underneath, it also stops solvent from the coats above soaking into the filler and showing through over time. In Türkiye, primer fillers with 4+1 and 5+1 (HS) mixing ratios are widely used.; Related:Primer Body Filler Grit Size

S

Sagging / Runs: The downward flow of a wet paint film under gravity on vertical or sloping surfaces, leaving drips, curtains or wave marks. It is usually caused by applying too heavy a coat in one go, not observing the flash-off time, using too much thinner or a thinner that is too slow, low ambient temperature, or holding the gun too close to the surface and moving it too slowly. Small runs can be removed by fine sanding and polishing once the film has fully cured; heavy runs may require the area to be repainted.; Related:Viscosity Flash-Off Time Dry Film Thickness
Solids Content(Non-Volatile Content): The percentage of a paint or primer that remains on the surface after the solvents have evaporated and forms the dry film (binder, pigments and extenders), expressed by weight or by volume. Volume solids determine how much of the wet film becomes dry film: for example, with a product containing 50% solids by volume when ready to use, a 100 µm wet film gives approximately 50 µm of dry film. Products with a high solids content reach the target thickness with fewer coats and have a lower VOC value.; Related:Dry Film Thickness HS (High Solids)VOC (Volatile Organic Compounds)
Solvent: Volatile, flammable liquids that dissolve the binder in paint, adjust the product's viscosity and flow, and leave the film by evaporation after application; they are the main source of VOC emissions. Their evaporation rate directly affects flow, drying time and the formation of defects: a solvent that is too fast can lead to orange peel and dry spray, while one that is too slow can lead to runs and long drying times. In 2K systems, solvents containing alcohol or water, which can react with isocyanate, must not be used.; Related:Thinner VOC (Volatile Organic Compounds)Solvent Popping
Solvent Popping(Solvent Boil): A defect in the form of small bubbles, craters or pinholes, caused by solvent trapped in the film trying to escape after the surface has closed over too early. The main causes are excessively heavy coats, not observing the flash-off time, using a thinner that is too fast for the ambient temperature, and starting baking or infrared drying without a flash-off period. To prevent it, keep to the recommended film thickness and waiting times; once it has occurred, the defect is removed by sanding and repainting after full cure.; Related:Flash-Off Time Baking Pinholes
Stone Chip Protection(Anti-Gravel Coating): An elastic coating with a granular (textured) finish, applied to protect areas exposed to stone and gravel impacts, such as sills, lower panels, wheel arches and the chassis. It is usually applied over a primed surface with a special stone chip gun; the texture is adjusted via air pressure and gun distance, and the coating can be overpainted once the drying time given in the product information has elapsed. In Turkish workshop jargon, the word “pütür” is also used for roughness on a paint surface caused by dust or dry spray.; Related:Corrosion Primer

T

Thinner(Reducer): A blend of solvents used to bring paints, primers and clear coats to application viscosity. The thinner suited to the product's system should be added at the ratio stated in the Technical Data Sheet, typically 5–10% for clear coats and 10–20% for primer fillers; too much thinner lowers film thickness and increases the risk of runs. Using cellulose thinners intended for cleaning to reduce 2K acrylic products can cause loss of gloss and curing problems.; Related:Viscosity Solvent DIN 4 Flow Cup
Thixotropy: The property of a material that becomes thinner and more fluid under a force such as stirring, spreading or spraying, and thickens again when left to stand. This property lets fillers and primers spread easily while staying in place on vertical surfaces without sagging. Thixotropic products should be stirred thoroughly before use, and viscosity should be measured immediately after stirring.; Related:Viscosity Sagging / Runs Body Filler

U

UV Resistance: The ability of a paint film to withstand the ultraviolet rays in sunlight without deteriorating through loss of gloss, yellowing, chalking or cracking. The UV absorbers and light stabilisers used in automotive clear coats protect both the clear coat film and the basecoat beneath it; for this protection to be effective, the recommended dry film thickness must be reached. Primers and fillers are not designed for UV resistance, so they should not be left exposed to sunlight for long periods without being overcoated.; Related:Clear Coat Dry Film Thickness Gloss

V

Viscosity: A liquid's resistance to flow – in other words, its consistency – which directly determines how paint atomises and flows out, how much film it builds per coat and how prone it is to sagging. In the workshop it is usually measured in seconds with a DIN 4 flow cup and adjusted with thinner to the value given in the product information. Temperature has a strong effect on viscosity (cold material thickens, warm material thins), so it should be measured and adjusted with the material at around 20 °C.; Related:DIN 4 Flow Cup Thinner Thixotropy
VOC (Volatile Organic Compounds): Organic compounds that evaporate readily at room temperature; in paint products they come mainly from solvents. Because they harm air quality and pose health risks, many countries apply legal VOC limits to vehicle refinish paints, usually stated in g/L for the ready-to-use mixture. HS products, the correct thinner ratio and spray guns with high transfer efficiency all help to reduce VOC emissions.; Related:Solvent HS (High Solids)Solids Content

W

Wet-on-Wet: A system in which a coat is applied over the previous one without sanding, after only a short waiting time and before the coat underneath has fully dried. In refinishing it mostly means overcoating a wet-on-wet primer with basecoat, without sanding, after the waiting time given in the product information; applying clear coat over basecoat works on the same principle. It saves process time and labour, but because the primer is not sanded, the surface must be prepared perfectly beforehand and the specified minimum and maximum waiting times must be observed.; Related:Primer Basecoat Flash-Off Time

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