Post 552 by Gautam Shah
Coatings are selected in consideration of the substrate (virgin or already coated surface), application technology and atmospheric conditions. Commercially coatings are available for broad categories of surfaces, such as masonry, wood, metals; for specific layering such as primer, undercoating, top or finishing coating; for purposes such as architectural, industrial, marine, clear coatings, food-grade; for technical applications such as water-proofing, insulating, mastic compounds, fire-retardant or proofing, conductive or anti-static etc.
Selection of a Coating is done in view of following factors:
- Appearance (colour -hue, tone) (texture -sheen, gloss)
- Weather properties
- Abrasion resistance
- Adhesion to the substrate
- Impact resistance
- Flexural qualities
- Drying time (touch dry, hard dry or use ready)
- Feasible drying techniques ( Air dry, baking, catalytic)
- Resistance to a given medium (sea water, alkaline or acid, chlorine and sulphur)
- Resistance to sunlight (direct sunlight and UV component)
- Antistatic properties
- Temperature resistance
- Solvent vapour hazards (VOC)
- Ecological value
- Application environment (open, enclosed, controlled, inclement)
General Rules of application
Many of these rules are more pertinent to Alkyd resin-based coatings, rather than Latex or Plastic polymer-emulsion-based coatings. Both often contain anti-settling agents that retard the separation of solids from liquid components. The Plastic paints now contain thixotropic agents that induce false viscosity when the coating material is at rest, but once stirred temporarily gains normal or applicable viscosity.
1 The tins, drums, carboys, Jerry-cans of coatings, primers and undercoats should be delivered on a site, at least 24 hours before the job and allowed to rest, and revert to the site temperature.
2 All packing of coating materials, except solvents, should be placed upside down for at least 24 hours, prior to the job (relevant for An Alkyd resin based pigmented coatings or oil paints, primers etc.)
3 Always draw just sufficient material that can be used immediately (-if enamel, preferably within an hour, and, -if lacquer, much shorter duration).
4 Do not pour back any colour that has been drawn out and exposed to the atmosphere. Before closing a partly used tin, pour a small quantity of miscible solvent or thinner over the surface to prevent skinning and evaporation of volatile substances.
5 Atmospheric conditions should normally remain consistent (ideal temperature range between 10°C to 35°C, humidity 50% and breeze velocity at 20 km per hour) for the entire duration of application, and curing or drying. The surface to be coated must be at the ambient temperature. All direct sun-rays (even for exterior coatings), except of the early morning, should be avoided for the entire period of application and till the coating is `touch dry‘.
6 Tint the undercoat slightly lighter than the final coat to differentiate and ascertain the full coverage by the final coat.
7 Where very light shades (`off whites‘) are to be mixed on a site, first reduce the tinting-paste with a small amount of white paint, and then adjust the final shade with the reduced tinter. To match a very light shade, place a drop of mixed trial colour on white paint wet-surface, and judge the difference. Certain high tinting pigments, such as blue, black and red are required in extremely small portions, and are difficult to mix in base white or lighter shade. Such coatings turn out to be slightly darker and sometimes have streaks, because brushing or rolling enhances the dispersion of pigments. To avoid this it is better to filter out all the site-mixed colours with a Nylon or Polyester fine-mesh cloth (bolting cloths, used for making screens for screen printing).
8 Water based coatings, dry out to a lighter shade then their wet look and Oil or resin-based coatings dry out to a darker shade then their wet look.
9 Never apply a very thin coat, or a thicker coat, than recommended by the manufacturer.
10 Apply the first coat in the direction of fibres or texture grains of the substrate. Apply the next coat in the cross direction, observing that the final coat on vertical surfaces is always in a vertical direction. It is always better to apply the final coat in direction of length then width.
11 Allow sufficient time for each coat to thoroughly cure or dry out, before any new coating or treatments (like sandpapering, rubbing) are done. Learn to differentiate between a `touch dry’ and ‘through dry’ conditions of a coating. Convertible coatings1 should not be re-coated before these are thoroughly dry, but Non-Convertible coatings2 can be re-coated when substantial part of solvent material has evaporated. Non convertible coatings can develop an inter layer adhesion. Both types of coatings, though may not be brush coated before through drying. Non-convertible coatings, however, may be re-coated by spray method.
12 Allow sufficient time before the coated object is used for intensive service (quite often it may take 7/15 days after `through dry’ conditions), because it takes longer for the inner section of coating to become thoroughly dry.
*(1) Convertible Coating > The film forming substance on drying or curing coverts itself into a different material, which cannot be reverted by any process like solution, heating, etc. Dried out Oil paints cannot be dissolved.
*(2) Non-Convertible Coating > The film forming substance on drying can be reconverted into original like material by solution or heat softening. NC Lacquers are such coating materials.