Paints, the color of Civil Engineering

Basic Definition:

Paint is a coating of fluid material applied over timber and metal surface as protective coating which on drying forms a thin film on the surface.

Uses of paints:

  1. To protect the surface from weathering effects of the atmosphere and actions by other liquids, fumes, and gases.
  2. To provide pleasing, colorful, and decorative appearance to the surfaces.
  3. To prevent decay of wooden members.
  4. To prevent corrosion of metallic surfaces.
  5. To provide a smooth surface for easy cleaning.

Types of paints:

  1. Oil Paint:

It is an ordinary and most common type of paint.

  • Bituminous paint:

It is prepared by dissolving asphalt or bitumen in oil or petroleum.

  1. Cement paint:

It consists of cement and hydrated lime mixed along with a coloring pigment.

  1. Aluminum paint:

Aluminum powder suspended in spirit varnish or oil varnish.

  1. Asbestos paint:

It is a peculiar type of paint, applied to places exposed to acidic gases and steam.

  1. Cellulose paint:

It is prepared from nitro cotton celluloid sheets.

  1. Emulsion paint:

It consists of polyvinyl acetate and synthetic resin as binding material.

  1. Enamel paint:

It consists of white lead ground in small quantity of oil and mixed with petroleum spirit and resinous matter.

  1. Graphite paint:

It is black in color and is applied on iron surfaces.

  1. Luminous paint:

It contains calcium sulfide with varnish.

  1. Silicate paint:

It is prepared by mixing calcined ground silica with resinous substances.

  1. Anti-corrosive paint:

It consists of oil and a strong drier.

  1. Plastic paint:

It has a variety of plastics present, applied using brush or spray painting.

  1. Synthetic rubber paint:

It is prepared from resins.

Composition of oil paint:

  1. Base:

It is the principal constituent of paint.  It is a solid matter forming the main body of paint and possessing binding properties. It is usually a metallic oxide and it makes the paint film opaque and possess binding properties which reduces the shrinkage cracks in the film on drying. Examples: white lead, red lead, zinc oxide, iron oxide, etc.

  • Vehicle (Carrier):

It is an oil to which the base is mixed. It holds the constituents of paint in suspension and helps spread it over the surface to be painted, imparts durability, toughness, and water resistance to paint film. Examples: linseed oil, nut oil, poppy oil, tung oil,

  • Pigment:

Pigments are used to hide the surface imperfections and to impart the desired color. They are finely ground mineral, organic substance, or metal powders (0.1 to 5.0 micron in dia). They protect the paint film by reflecting the destructive ultraviolet light and help improve impermeability of paint film and enhance weather resistance.

  • Thinners:

These are oils used to thin the paints and increase the spread. They make the paint have workable consistency and evaporate during drying of the film.

  • Driers (plasticizers):

These are added to paint to quicken the drying of vehicle

  • Adulterants:

An adulterant is mixed to replace the base in part thus reducing the cost of paint.  They also help to reduce cracking of dry paint and sometimes help to keep the pigments in suspension. Commonly used fillers are silica, charcoal, powdered chalk, aluminum silicate, barium sulphate, etc.

Characteristics of good paints:

  1. Should have a good body or spreading power.
  2. Should work smoothly and freely to be laid in thin coat.
  3. Should form durable, tough, and wear resistant film upon drying.
  4. Color should not fade or change.
  5. Painted surface should dry in about 9 hours.
  6. Should become hard enough in 24 hours to take up another coat.
  7. Should not crack upon drying.
  8. Should give a smooth and pleasing finish.
  9. Should dry quickly.
  10. Should not damage the painted surface.

Defects in painting:

  1. Cracking:

cracks extending throughout the entire thickness of paint, due to

  • Improper seasoning of wood
  • Excessive use of drier
  • Application of too many coats
  • Crazing and crocodiling:

 Hairline cracks in top-coat, due to

  • Use of excessive oil
  • Use of impure oil
  • Insufficient drying of under coat
  • Blistering and peeling:

Caused due to:

  • Exposure of paint to strong sunshine
  • Leaving oil or grease on the surface to be painted
  • Painting a surface with moisture on surface or in pores of wood
  • Chalking:

It is when the paint easily rubs off with hands or clothes. It is caused due to use of insufficient oil in priming coat

  • Washing off:

Deposition of water-soluble dissolved matter at lower edges forming streaks.

  • Dull appearance:

It is caused by use of excessive drier or on aging.

  • Slow drying:

It is caused due to:

  • use of inferior or old oils
  • Painting over damp surfaces
  • Painting during unfavorable weather
  • Yellowing of white paint:

It is caused due to:

  • Use white enamel where gloss is desired
  • For indoors use linseed oil with yellow tint that does not bleach unless exposed to sunshine.