Post 442 -by Gautam Shah


Primer coat is the first surface build up on any substrate. It is designed to receive one or several more coats, immediately or some times later. A primer-coat is a surface preparation layer. It covers the surface irregularities like, chemical reactivity, and physical variations -colour, texture, porosity. A primer coat also offers a next application worthy surface.

Eiffel Tower Painting.jpg

A primer coat serves two distinct purposes: 1 It touches the substrate, often of many different types, and so must adhere to it, well, 2 It receives the next coat so functionally must be compatible with it. A primer coat helps in levelling the surface, and must have ‘body’ to fill up the pores, micro crevices, etc. Primer coat is sand papered for levelling, and to roughen up for application of second coat. Its some superfluous part gets rubbed and removed.

Primer coat is nominally applied on ‘virgin’ or an uncoated surface, and sometimes on very old coated surface. In the later case, the primer coat is slightly coloured white, off-white or some colour closer, but lighter then the final intended colour. When an old coat colour is to be ‘masked’ completely, primer coats or undercoats are coloured by a ‘masking colour’, such as Green by darker Red or Red by Blue. It is desirable to have primer of a slightly different and lighter colour shade than the subsequent coat, to differentiate a freshly coated surface and uncoated surface.

Primer failure

Primer coat is a technical coat, so its colour is not very important. The Primer-colours that we see are mainly due to the filler or body-pigments, called extenders. These are chosen for their protective effects and other technical qualities. Primer coats protect the virgin surface, and so are applied soon after the primary manufacturing is over, such as on pipes, sections, sheets, castings, etc. Good primer must remain adhered to the surface, even after other top coats are removed accidentally, or for renovation.


Clear Sealer Coats: These are priming coats for application of clear coats, mainly wood surfaces. A sealer coat, like a primer prepares the fresh surface (of wood) for subsequent application. Sealer coat must be to be low viscosity, but high solid content material. Nitro cellulose clear has been found to be the most versatile sealer material. Other materials include variety modified pine Rosins. Clear sealer coats are often not categorized as priming coat, where the first application is a coat of ‘extender’ such as china clay, barytes or Calcium carbonate mixed with oxide colour. Such a coat covers, the irregularities of colour and grain patterns, fill up pores and micro crevices on wood surfaces. After sand papering-level rubbing it may be coated with a sealer coat. Sealer coats are preceded by staining compounds or coats, to add a translucent tinge on the surface.

white undercoat

A primer or sealer coat is the first coating applied to the objects’ surface, so it is required to:

● Regulate moisture movement in case of wood or masonry surfaces and provide corrosion resistance on metals

● Regulate the PH value and galvanic activity of the surface.

● Seal the surface so that oils, waxes, gases, vapours, salts and other reactive exudations from the object mass do not leach out in adverse conditions

● Fill up micro pores and crevices to level out the surface

● Provide temporary protection to the substrate from actions like abrasion, oxidation, sparking, ignition, insects attack.


A great variety of primers are available in the market, but of following basic THREE categories:

Wood primers generally function as sealers, so have a high pigment + extender ratio. Wood sealers for clear coats are colourless coatings that help in sealing the grain. Commercial wood primers for pigmented paints are white or pinkish in colour due to the presence of white pigments and extenders, compared to metal primers (red-oxide) which, are dark coloured.

white primer on wooden surface 1566994

primer paint

Metal primers have rust inhibitive pigments or extenders like red lead, zinc oxide, zinc chromate, red oxide, calcium boro-silicate, barium metaborate, zinc molybdate, chromium fluoride, basic lead silico chromate, zinc ferrite, calcium ferrite. Under water (submerged) metal structures are coated with zinc rich primers based on epoxy, polyurethane systems or chlorinated rubber paints are used.


Primer on Metal surface

Masonry primers are applied to alkaline surfaces, so are designed as non acidic mediums. Often such surfaces have high degree of loose particles, so Masonry primers have high proportion of binding materials. Commercially these types of primers are known as cement primers, and are available as water or oil-based formulations. Water-based formulations are mainly used on virgin masonry surfaces.



Post 437 – by Gautam Shah 



Coatings are applied as Single or Multi coat systems. The choice depends on several factors, such as:

  • Economics
  • Opportunity offered by technology
  • Emergency or routine application
  • Site accessibility in terms of location, weather, and tools available
  • First or virgin application or a re-coating job
  • Thickness of film required


One-coat system has inherent efficiency, as it requires lesser amounts of material, solvents, curing energy, and takes less time for application. However, in one-coat systems, the coating is required to provide adhesion with the substrate, necessary film build or thickness, and also fully cover up the surface. Single coat systems have to be fail-proof, as there are no remedial opportunities. Performances of single-coat system, depends very closely on the type of (readily available) substrate. If the substrates are large then one has to select different coating system for different substrates or go for ‘All purpose system’.

Ship re painting involves huge effort of surface cleaning before multi-coat application

Multi-coat systems consist of two, three or more layers of applications, such as, Priming-coat (filler or sealing coats), Undercoat (sub coat or intermediate coats) and Finish-coat (top coat or surface coat) These multiple layers are designed to perform specific functions and also take care of any shortcomings of the preceding coats. In multi-coat systems, different coats can be applied at different times and locations, as required. Primer-coat can be a factory application, Intermediate-coat could be part of the installation procedure, while, the Final coat could be applied with other finish systems, just prior to usage. Multi-coat systems have an advantage over many high-build or one coat systems, in that the final coat can be of the same type for different substrates or undercoats. In multi-coat systems, the sub layers could be of economic materials, as it is the final layer that is exposed to the atmosphere, and provides the tangible surface finish.

Rubbing - Sand Papering the primer

Single coat systems are applied to save time and cost of application. Very large walls, giant structures (Eiffel tower, pylons, or Bridges), Road side barricades, Road marking signs, Ships, Marine structures, are some of the entities that must be re-coated in one effort. These are re-application systems, so the substrates have some form of existing coating. The condition could be, severely weathered surface, fatigued or peeled original coating, presence of salts and other reaction products, deposits of dust, grime, etc. Some of these extraneous or by-product matters cannot be removed easily or completely from where they exist (height, nooks, corners, etc.). Such structures are exposed to vagaries of weather, such as very high or low surface temperature, condensation, rains, humidity, high winds and environmental pollution. Where the technology of stripping of aged coatings is available, it may not work for such an extensive surface, including undersides.


Multi-coat systems are as varied as their modes of applications are. Multi-coat systems have a prime or primer coat, followed by one or more finishing coats. Multi-coat systems need an interim ‘rest’ or through drying period, before next coat can be applied. Finishing coats are often designed as ‘removable or stripping system’ by mechanical grinding or chemical scrubbing, or both. Primer coats are specifically designed for the surface to be applied, such as wood, metal or masonry. Multi-coat systems are applied by exclusively by brushing, roller, spraying, dipping, or in combinations of it. With brush application final coating is by vertical strokes.







Post 423 ⇒   by Gautam Shah 


A Paint Thinner nominally changes a material’s solid phase into a liquid phase and heavier liquid into low viscosity liquid. Thinners act by dissolving, suspending the solids and by intervening into viscous liquids.

Paints, Thinners

A thinner is a mix of different solvents that act as a thinning compound, suspending agent and a diluting material. A thinner is also conceived as accelerator or retarder of film formation of a coating. Thinners are used for cleaning of lubricants, machine cutting fluids, coolant materials, greases, waxes, etc. Thinners are used as stripping agent of dry or wet coatings (paint remover). A thinner sometimes may not strip a film of coating, but softens by affecting some of the constituents (through wetting, etching, etc.) of the film. Rest of the stripping act is accomplished with other chemicals, heat softening, singeing, mechanical stripping, scrapping or grinding.


Viscosity of a coating medium can be adjusted by including a low viscosity medium into a high viscosity material or by solvents and diluents. Solvents dissolve by entering the inter-molecular space and changing the intermolecular forces. Diluents by themselves do not enter the inter-molecular spaces but extend the action of a solvent as a liquid to liquid-phase. Often in a multi medium formulation ‘one material that acts as a solvent, to a particular medium, may act as a diluent for the other medium’.

Air brush spray nozzle

Solvents and diluents both increase the fluidity of a coating medium. The fluidity of a coating medium is important for applications (spray, brush, etc.), flow or levelling properties, nature of drying and ‘curing’ of the film. Very high fluidity may not mean very low viscosity. Very thinned down coating material leads to separation of solids (like pigments) from liquids (resins, solvents, etc.) Excessively thinned down coating, on drying produces porous film (a film with marked solvent escape pores). Too much thinning often creates non-opaque or patchy (in terms of colourants) film. Solvents also affect the inter-molecular structure of resins affect their interlinking or polymerization, and thus the quality of the film.


Solvents and diluents are added during the manufacture and also prior to application. For the later purpose, a well proportioned an economic blend of solvents and diluents, suitable for specific categories of coatings are marketed as Thinner or Reducer.

paint-brushes-500x500-250x250Thinning solvents included in a coating material, encourage separation and towards the gravity settlement of solids on storage. Nowadays thixotropic compounds are added to water based coatings (such as Plastic or Latex paints). These compounds create a false setting (thickening of liquid mass into a viscous paste), and with little stirring, the coating material gains the original consistency.

Majority of Plastic (or Latex paint as in USA) are water based system, though very small quantity of solvent do exist in them

Constituents of thinners are nominally low boiling point temperature solvents. These evaporate at a faster rate but are affected by the temperature, moisture, movements of air, and the application process. In nominal weather, 25 ° C and 40% humidity, thinner evaporates at an average rate, but in hot weather thinner leaves the coating film before it has time to flow and level out. In case of moist weather (raining periods) the moisture in the air gets trapped in the film and cause blushes or whitish spots. This can be corrected by using retarder additive or slow drying thinner.

Spray Gun formulations require special thinners

Coating applications require special thinner formulation. French polishes and lacquer coatings applied by rubbing pads need very thinned down and slowly evaporating thinner. Brush application of coatings needs film levelling time, high fluidity without reducing the viscosity. Spraying with compressed air creates cooling and moisture condensation and so prescribed quality of thinners must be used. Spraying by airbrushing (thin-narrow and a fine nozzle) for delicate work, need fast evaporating solvents.



Post 359 – by Gautam Shah 



Coatings (Paint, Varnish, etc.) consist of organic as well as inorganic substances. Both the categories of materials can be hazardous at several levels such as: production, application, curing or drying, functional use, and disposal level.

Claude Monet Waterloo Bridge London

At production level the handling and use of certain raw materials can be hazardous. Typically handling of extenders and pigments can pose air-bourne particle spread. Some of these silica containing materials are toxic and carcinogenic and not eco-friendly. Resins (amino) with styrene and formaldehyde are carcinogenic. Solvents are known fire prone materials. At application level, the primary hazard occurs, due to high amounts of volatile organic content going into air. Its vapours or odours are noxious, allergic and unpleasant. The odour suppressants added to coatings are not necessarily benign. At application level, old coated surfaces are rubbed or scrapped, the particulate matter of which causes air pollution and the constituents are often unfamiliar.

Aerosols cause high level of air pollution



Post application, the wearing surfaces and peel-off cause, dust pollution. Some of the plastic materials continue to deteriorate, evolving pollutants. Coatings used on kitchen shelves, utensils, equipments cause contamination. Accidental high heating and fire scorch a coating. Planned removal of coating by sanding and chemical etchings (paint scrubbers or removers) can also leave unwanted residual products. Disposal of coated items by sawing, forced removal, hammering, chiselling, sanding, scrapping, etc. can desecrate the environment.

PIGMENTSClay colours

Prolonged or high intensity exposure to paint and paint fumes can affect three main organs, lungs, kidney and lever. These manifest as headache, allergies and asthmatic reactions, irritation of skin and eyes, burning in nose, airways, urinary tracts.


The trend world over is to design entities with their own or integrated surfaces. A variety of metal and polymer surface treatments are available that molecularly change the quality of the surface. Such `surface systems‘, rather than `surface finishes‘ often do not require any application of foreign substances. A variety of single or a combination of treatments like temperature, radiation, sonar, mechanical finishing, stress induction and relief, cathodic protection etc. can eliminate organic coating systems. Such surfaces do not have any applied finishes, but rather have a generated finish.



There are many inorganic coating systems that are akin to metallizing systems. In such systems the role of medium is to carry and spread the pigment and other substances. On baking or firing the medium is evaporated or burnt off leaving metallic or alloying compounds on the surface. Such substances are generally heat or flame cured to cross links with the substrate material.




Post 357 – by Gautam Shah 


Large number of items and components are available in ready to use state with a suitable finish. These products require no extra treatment or coating, before or after the installation. The industrially finished items are class apart from finishes applied on a site, after assembly or erection.

An industrial plant offers streamline production system, in a batch or a continuous fashion. Industrial plant-based surface finish is a process integrated with production and highly articulated one. It is invariably conducted in a controlled environment and observance.


Two men on a platform painting Hull of a ship Himalaya >Wikipedia Image by Australian Maritime Museum

Site applied surface finishes, substantially consist of architectural coatings, and are multi surface applications (general purpose or GP systems). Other on site applications, but not forming part of architectural range, include Road marking paints, Marine paints, and re-application systems. On a site coatings are employed in a wide range of weathers and in open environments. Architectural coatings, for ‘drying’, rely mainly on moisture evaporation, induced polymerization, and in few instances on catalyst curing (popularly known as ‘two-pack’) systems. Architectural coatings are applied, chiefly on masonry, and than to lesser extent on wood and metal surfaces.


Prez Obama and wife help paint Habitat for Humanity site, Washington, Official white house Photo-stream Image by White House (Pete Souza) / Maison Blanche (Pete Souza) 

Industrial finishes are very specific systems, formulated for a particular substrate, object shapes, sizes, pre-treatments, method of application and drying or curing cycles and schedules. As a result, complex surface finish technologies can be used. Industrial finishes offer qualitatively a far superior finish compared to any site-based system.

Dipping treatment for Automotive industrial coating

An industrial coating, forms a very small and often a negligible part of the total cost of the product, its failure in any form destroys the entire value of a product, and manufacturing company’s reputation. Defects of industrial coatings, if any are realized after the product has reached the consumer. Remedial measures are impracticable, and it entails a recall of the product from unknown or far off locations. Industrial coatings must be continuously upgraded. Otherwise, a technologically superior product gets rejected from the market due to its aesthetic and sensorial appeal.

White-goods Consumer appliances

Industrial finishes are generally low in pigments or extenders and high to medium ‘build’. Quality of the finish is largely determined by the type of film forming mediums used. Many industrial finishes are formulated for baking-drying to achieve a hard, stable and durable finish. Amino and epoxy resin coatings dry by catalyst action. In powder coatings thermo-setting materials are heat liquefied to set. Industrial coatings also include air drying, partially air drying & low temp baking, stowing, radiation-curing, solvent-evaporating, oxygen polymerizing (oxidizing) heat polymerizing and catalyst curing, thermo plasticizing carbonating, systems. Industrial products receive pre-treatments to make their surfaces suitable for a single system of coating and through single application. The pre-surface treatment include moisture conditioning, rust inhibiting, galvanic control, texturing and levelling of the surface, etc.


Wikipedia image > Image by


Industrial finishes can be classified as:

  • Traditional systems  

These are based on alkyd resin technology

  • Lacquers

High molecular weight resins, such as vinyls, acrylics, chlorinated rubber, etc. that are dissolved in a solvent and do not undergo any chemical change on drying.

  • Dispersion mediums

  These are dispersions of convertible system.

  • Latex systems

Contain resins dispersed in water or other liquid that do not dissolve them. Resins are invariably high molecular weight polymers.

  • Chemically curable systems

These are made of thermo-setting resins of low molecular weight such as epoxies or urethane, that in the presence of a chemical or catalyst complete the bond formation.


Today industrial coatings are also seen in terms of environmental compliance, or the pollution hazard they can cause. The total volatile organic component, VOC. criteria is a factor that defines the worthiness of a coating system.

Water borne coatings: 

These are made from emulsion, water reducible and aqueous colloidal dispersions.

High solid coatings:

High solid coatings are frequently based on conventional polymers but with low molecular weight.

Powder coatings: 

These are based on thermoset and thermoplastic resins with pigments, fillers and additives such as hardeners and flow agents.

Advantages of Industrial coatings: Items which are difficult to coat due to their shape, size or location can be efficiently coated in an industrial set up. Items with sharp corners and edges can be coated evenly. Certain types of pre-treatments, which are not possible on a building site can be carried out at plant level. Materials and methods which are difficult to handle, or are normally hazardous on a site, can be employed at plant level. Industrial application of coating is very efficient and controlled so very thin film thickness can be achieved, and lot of wastage on account of drips and over sprays can be eliminated. Many application techniques, tools, equipment can only be used at plant level set up.

Painting booth

Industrial Coatings include White goods (consumer products of chiefly white-coloured, but not always; such as refrigerates, washing machines, ovens, gas stoves, geysers, fans, air conditioners); Consumer electronic products such as computer and server cabinets; Industrially produced furniture such as chairs, cabinets, tables, partition systems; Vehicle products such as cycles, scooters, bikes, cars, trucks, buses; Extruded or formed Long products such as steel, aluminium and composite sections, and pipes; Besides these there are several applications where clear (pigment less) or non toxic coatings are used such food containers, toys, etc.


GP – General Purpose Paints

Post 165  –by Gautam Shah



In Building Industry many different types of finishes are used for covering non-masonry surfaces such as Ferrous Metals, alloys, aluminium, wood, and wood composites. In many cases the surfaces are composed of many different types of materials, textures and forms. Often the extent of each individual type of surface is so small and its composition so complex that it may not be feasible to attend to different surfaces.

HK_Sai_Ying_Pun_Des_Voeux_Road_West_Dulux_Painting_Material_Shop_3Most paint manufacturers offer a General purpose coating systems (GP) for such situations. The coatings are nominally conceived to be multi layer systems (at least of primer, inner and top-final coat). GP coating systems work for the inner and top coat, however, a primer coat is specific for the surface. It is specifically designed for the substrates such as masonry-cement primer, wood primer, mild-steel or iron primer.


GP or General Purpose coatings are used for following situations:

On items that are difficult to coat– Such as deep grooves, undersides, inaccessible areas, narrow stripes, engravings, sharp corners and edges, very smooth surfaces, small parts like lattices.

On locations with difficult access– High level ceilings, external sides of windows in multi storey buildings, roof trusses.

In variable atmospheric conditions– Rain and snow storms, very high to very low humidity environments, dust, sun rays, wind, flying insects, very high to very low temperatures.

In single or multi coat systems– Some initial coatings at plant level as specific coating system and rest on the site as GP system.

For specific purpose– As a fresh system (on a virgin surface), as a re-application system (re-coating with a similar but over an aged surface) or as a renovation system (removal of aged coating layer, repairs and coating).

With peripheral hazards– Fire, chemical vapour and odours, colour dropping or running, and over-sprays.

GP enamel Painted Interior of Bang Pa In Chinese style palace

Such site applicable finishes are designed to dry out at normal atmospheric conditions. Most of the GP finishes allow multiple methods of application, and are applicable in widely variable atmospheric conditions.

Brooklyn Bridge painters at work high above New York City

GP = General Purpose Paints, as a term are nominally synonymous with Enamel Paints (Oil resin based), but now the term is also used for ‘Plastic’ emulsion paints (Latex paints in American terminology). Such plastic paints are masonry paints for walls,  roof-tiles, floors and stage-sets. These are often favoured over oil based enamel paints due to non-glossy (matt) finish and faster drying capacity.





Post by – Gautam Shah



Oil bound distemper is one of the most common and oldest oil and water dispersion coating system for masonry surfaces. These emulsion-distempers, are better then Calcimine. This is now outdated technology in many parts of the world. Such  paint, however, are produced and used in many developing countries of the world. It is rather easy to procure the raw materials and produce it with very little use of power, tools or equipments. It is comparatively an eco-friendly system, as it has very small amounts toxic materials or petroleum solvents.

The Oil Bound Distempers are made by emulsifying Bodied drying oils or their alkyds with water in the presence of casein, glue dextrine (which also provide the film).

Traditionally distemper or glue based interior paints were made with either animal hide glue or rabbit skin glue. Glue is heated and mixed with a paste of slaked (water soaked) whiting or calcium carbonate and pigment paste. Glue-based distempers were preferred as its pH was compatible with alkaline masonry and plaster surfaces. Oil paints were difficult and costly to produce, and peeled of from the alkaline surfaces. Distemper are water based systems and so dry out very fast, so preferred for not only interior surfaces but wood, paper and canvas  surfaces in art works. These are used for restoration of architectural heritage of 19th C later periods.

Distemper on palm leaf Buddhist art 12th C

Water is mixed with casein, glue or dextrine and an alkali, such as lime or soap, is combined with the oil component such as the Bodied drying oils or their alkyds. The mixture is heavily agitated and emulsion is formed. Stabilizers (to preserve the state of suspension) and antibacterial agents are then incorporated for prolonged storage life.

The film is first formed due to evaporation of water, The Glue component, on removal of water provides the binding. The oil component then forms a latticed (porous) film on the top part of the surface. Oil oxidation of the oils may take more then 24 hours. The hardened film becomes moderately wipe-able or the commercial signature ‘washable distemper’ (compared with the Calcimine) .

After two or three coats over a period of 5/10 years, repainting becomes difficult. Fresh coat of OBD over old OBD softens it to some extent, and causes it to swell. The strength of coating is less than that of oil paint. During drying, the OBD film shrinks substantially and exerts a considerable pull over the underlying film.

Raphael The Miraculous Draught of Fishes 1515

Distempers are essentially masonry coatings so contain substantial amounts of, extenders as bodying agent. Extenders are low refractivity white mineral powders like Calcium Carbonate, Barytes, talc, etc. Extenders also reduce the need for White pigments (high refractivity powders) such Zinc Oxide, Titanium Dioxide. etc.


Oil Bound Distempers do not have rich hue colours. Majority of commercial shades are of pastel variety(white added tones). Due to a low amount of Oil or related constituents in the emulsion the finish is low gloss or dull (low sheen or matt).

Oil bound-distempers or OBD, are sold in viscous paste form, reducible with plain water or special thinning liquids, to brush-able viscosity. Proprietary thinning compounds are very light <oil in water> emulsions, often with additives such as silicone oil, stabilizers, plasticizer, and antibacterial agents and occasionally organic solvents.

MiG-3 aircraft in winter Washed out distemper as a camouflage World_War II

Water-based latex paints

Oil bound distempers are difficult for re-coating purposes. Their water resistance is also not very good. So some time in 1950-1960s a range of Vinyl-based emulsion polymers began to replace the caseins and natural gums -the most moisture susceptible components. Later even the oils were eliminated. Resultant new product was known as the water-based latex paint. Latex is a term more used in USA, where all plastic emulsion products were also known as Latex systems. In most other countries Plastic emulsion paint is popular term.

Dieric Bouts The Entombment Artron

Vinyl-based emulsion coating systems had good binding and slightly better water resistance, but their UV (solar light) resistance was poor. It made the film tacky -attracted moisture (on the breakdown of polymer chain). The product soon saw a replacement with Acrylic Polymer-based emulsions. Both the products were marketed as Latex or Plastic Distempers, fully washable distempers or synthetic distempers.

Pieter_Janssens_Elinga Room_in_a_Dutch_House

Water bound systems are favoured by films, drama and TV scenographers for set making and and scenery backdrops, for three reasons, Fast drying quality, matt (non reflective) surface and high level of opacity (presence of whiting offers better hiding power).