APPLICATION of COATINGS
Post 300 ⇒ by Gautam Shah →
Coatings are of many types and substances. Some of the simplest (though temporary) coatings are, water sprayed on clothes to be ironed, spreading oil on food being cooked, application of body colours for make-up, skin care creams and lotions and spreading butter or jam over bread. All these substances are in a liquid or liquefiable phase. The liquid phase, at ambient temperature is in the form of solution, suspension, gel, emulsion, or a thixotropic compound, or with application heat and pressure turns into liquid. A coating may turn into a heavier phase like the solid one.
Application of a coating is designed in terms of substance to be applied, complexity of the object to be coated, availability of application tools, environmental conditions (weather and pollution controls) and economics of wastage and reuse.
Coatings of clay and colour ancient times were daubing using hands, palms and fingers. Colours were also sprayed from a mouth. These methods had little control on the spread or intensity of the contents. Daubing with a cloth bundle is very efficient method of coating low viscosity liquids. It is being used since prehistoric times. Lacquer and French polish are still applied for layer by layer build-up by this method. A crude brush formed chewing over the end of a fibrous twig or tied and trimmed bunch of hair were used for coating application for several centuries.
Very small objects like beads and textile fibres were dipped into bowls of colours. Dipping process of coating wastes very little and does not require any accessory like a brush. Dip Coating is used in manufacturing of china ware crockery such as cups saucers and dishes. For dip coatings the viscosity of coating material is very important. Dip coating is used for solid or non-hollow objects without sharp corners or edges. Dip coated parts need little vibration to shake off excess material. Car bodies, fan components etc. are coated by dipping process.
Coating processes became sophisticated when artists started using varied techniques of application like dotes, smudges, scrapping, strokes, washes, etc. These application techniques were conceived for the visual-textural effects like directions, dual colour shades, monochromatic shading, etc. But the application technique was dependent on the formulation such as the amount of solids (pigments + extenders), viscosity of coating, its flow properties, and drying process and time. For example to transfer images from paper, fabric or parchment-based cartoons or master copies high intensity pigments with low viscosity carriers were required. Stroking was possible on high viscosity material and applied with a spatula hor heavy build up. Dual colouring or merging of colours was as much dependent on formulation and craft of application, as it was based on tools used.
Artists used brushes of different forms (flat, round, oval, and shaped flat or angled), sizes (massing and width), bristles, etc. Spatulas of different widths, angles, and thickness or metal strikers were used. Skewers, pointers, blunt chisels, etc. were used for textural scarping. Paint works touched using oils like linseed oil and solvents. Water was rubbed in with solvents and oils to form emulsions for cloudy effects.
Rollers made of wool wrapped over wood were used to coat large flat surfaces. A roller was a sophisticated application tool over the cloth bundle. Rollers were flat, wide and had even spread. Rollers could be extended by a long handle. Rollers could have been used advantageously with water-based coatings of the pre industrial age period. The major problem with lime-wash, calcimine or dry distemper or oil bound distemper was the low viscosity. The Roller technology had to wait for post world war developments in polymer technology. Rollers became popular with plastic emulsion paints, but more so when thixotropic compounds (a temporary and artificial viscosity building material) were added to the plastic paints. Rollers are available in different widths, diameters and nap (of the pad).