Post 553 by Gautam Shah
Paints have colourants or pigments to cover a substrate and provide new colour. Pigmented coatings constitute the largest section of coating production. Clear coatings, however, provide a see-through film on a substrate, which in additionally endow spacial surfaces, textural and other properties. Clear and pigmented coatings have been concurrently in use since prehistoric times. Both types of coatings usually have similar film forming substances, though the techniques of application and purposes these are employed are distinct. Pigmented and clear coatings, both can form the primer coat or a top coat in a multi coat system. As a primer or first coat the clear coat penetrates micro pores and seals the surface. As a top coat a clear finish endows specific surface quality such as gloss, sheen or matt finish and protection. In a multi coat system a pigmented undercoat creates a colour-equalized surface.
Pigmented coatings, besides colourants like pigments and dyes, have many other substances that add special effects through opacity. These are metal powders and flakes, prilled materials, micro sized lints of glass and polymer fibres, glow powders for phosphorescent, luminescent or photo-luminescent effects. Pigmented coatings usually contain extenders for several purposes. Extenders are minerals of low refractive index. Pigmented coatings for rendered or textured finishes have high solid build resins, additives or false viscosity enhancers and minerals and fillers.
Clear coatings are translucent to transparent, which are natural properties of the resin (film forming substance) or through additives like dyes and very low refractive materials. The need for a clear coating could be functional, such as avoidance of pigment for being unnecessary, toxic, or reduce the bulk of the film and thereby better homogeneity and lesser thickness. Clear coatings offer glossy to dull types of surface textures, and provide shades like tinted, crystal clear or water white. Surface texturing like hammer-tone, wrinkling, crackled, streaking, are also achieved. Clear coatings applied to emphasize substrate colours, grains or patterns as in case of timber, leather, paper metals or fabrics. Clear coatings are applied where for any technical reason it is not advisable to colour a coating, as in case of toys, food cans, etc. Large number of clear coatings are technical applications for rust inhibition, water proofing, static resistance, etc.
Some of the crudest forms of applying clear coatings, are still practised, and these are surface wetting with plain water, waxes, oils or tallow surfacings, starch sizing, gum and other plant exhudents surfacing, milk, and casein coatings. These materials are hydrophilic, and many run in moist weather, collect dust and are vulnerable to fungus attack.
A better alternative for creating clear coating, has been the use of distilled and modified pine tree extracts like rosin. Calcium and zinc modified rosins which are soluble in turpentine were used for cheap quality toys, country furniture and electrical wooden boards and battens. Often raw senna, umber, ochre and dyes like metanil yellow are added to these for colour toning and hide the ugly grain and defects of jungle woods. The finish has very little resistance to wear and tear and slightly softens up in moist weather.
Pigmented coatings are used on sites, in industrial production units, by amateurs, artists, and crafts people. These coatings are applied with crude or rudimentary methods and also through high-tech facilities. Pigmented coatings contain specific minerals and other substances as pigments and extenders for rust inhibition (zinc), water resistance, electrical properties and fire resistance (such as walstonite), anti fouling agents.
Pigmented coatings are of following basic classes:
1. Architectural finishes
- Water-based masonry finishes
- Other masonry finishes
- Non masonry finishes applicable on sites
2. Industrial finishes
- Solvent-based systems
- Water-based systems
- High solid coatings and powder coatings
- Air drying systems, Low and High Temperature baking systems, Catalyst systems
3. Other types of pigmented Surface Finishes and Treatments
- Marine coatings
- Road marking systems
- Toys’ and plastic colours
- Leather colours
- Body colours and make-up utilities