PAINT

A S M Fahad Hossain

Assistant professor

Dept. of CE, AUST

WHAT IS PAINT

Paint is a fluid usually of a solid pigment in a liquid vehicle, that dries to form a

continuous solid film when spread over a surface or substrate-an underlying

substance or layer.

OBJECTIVES

To protect the material from the destructive action of the atmosphere, harmful

gases and vapors.

To delay the action of flames in case of fire.

To give a specific treatment to impart the desired appearance to a structure.

To improve the sanitary conditions.

To impart the property of corrosive resistance to metals

CHARACTERISTICS OF A GOOD PAINT

Good covering power per unit of paint.

Easy and free application on surface.

It should form a thin film of uniform thickness.

Resistant to weathering action.

Should not show any cracks on drying.

It should form a hard and durable surface.

Economical to suit the type of work

COMPOSITION OF PAINTS

Paints essentially consist of:

i. Base

ii. Vehicle

iii. Pigment

iv. Drier

v. Thinner

Base

• The principal constituent forming its body or main bulk.

• It also possess the binding property.

• Examples: Zinc white, white lead, red lead are common materials.

• Base forms an opaque layer to obscure the surface of the material to be painted.

• A lead paint is affected by weathering action and is therefore, not suited for final

coats of paints. But it is quite well and acts as a good preservative.

• Zinc white is unaffected by weather., but is costly.

• Aluminium powder is used as a base for all aluminium paints.

Vehicle

•The function of vehicle is to contain all the materials of paint in liquid suspension, and

then to allow them to be applied in the form of a thin film on the surface to be painted.

•Linseed oil is the most widely used vehicle in the making of paints. It is obtained from

flax-seed.

•Linseed oil contains acid which reacts readily with oxygen and hardens by forming a

thin film.

•Other vehicles are tung oil, cotton seed oil, nut oil and soya bean oil.

Pigment

•Forms the coloring matter used for giving the required tint or shade to the paints.

•They are solids in a very fine state and usually of colloidal dimensions.

•These fine particles have a reinforcing effect on the thin film of paint.

•The dried film of linseed oil (vehicle) has a tendency to crack on hardening, while

pigment lessens these cracks.

•The best pigments are those that do not change their color when exposed to heat,

sunrays or acid laden atmosphere. Most pigments fail to satisfy this requirement.

•The common pigments are: black magnesia, red lead, chrome for green color,

Prussian blue for blue color, zinc chrome for yellow color etc.

•Powdered metals like aluminium, copper, bronze etc. are also used as metallic

pigments.

Drier

• The function of a drier is to absorb oxygen from the air and to supply it to linseed

oil, which reacts with linseed oil and eventually hardens by forming a thin film.

• They are usually compounds of metals like lead, magnesia, cobalt etc. dissolved in

a volatile liquid.

• Drier has a tendency to destroy the elasticity of the paint and therefore, should not

be used in excess, as this would cause the paint to peel off in scales.

Thinner

It acts as a solvent for the purpose of thinning the paints and thus imparts them

better covering power.

The paint could thereby be spread uniformly on is rough and porous.

It also gives proper consistency to paints.

The common thinning agent are terpentine and neptha.

The thinner evaporates and dries the oil consequently.

HOW DOES PAINT DRY

• When paint dries, a thin polymer film is formed that adheres to the surface and suspends the pigments. Paint dries by a number of different mechanisms:

• Solvent evaporation

• Rust proof paints

• Heating (thermosetting)

• Automotive

• Oxidation

• Drying oils, artistic paints

• Linseed oil

VARIOUS TYPES OF PAINTS

Oil Paint

Water Paint

Aluminum Paint

Cement Paint

Emulsion Paint

Enamel Paint

Distemper

Whitewash

Anti-corrosive paint

Plastic paint

Varnishes

Oil Paint

These are not ready-mix paints.

Mostly white lead is used as base in all light-colored oil paints. White lead is ground in

linseed oil to the paste like consistency. The paste is grinded further by adding some

linseed oil and stirring the mixture with wooden stick.

Drier, if to be used, is also ground in little linseed oil at fine state and the third separate

paste is prepared.

Now all the three pastes (i.e. base+vehicle, drier+vehicle and pigment+vehicle) are

mixed together to obtain the required consistency of cream.

If required, the mixture is further thinner by adding terpentine oil (thinner) or linseed oil

(in this case used as thinner) or both just before its actual use

Water Paint

It includes those paints of which the medium (vehicle) is water, they are prepared

with a medium containing both oil and water to which a certain amount of glue is

added to form the emulsion of oil in water.

Sodium or ammonium soaps are used as emulsifying agents (i.e. glue) in case of

water vehicle, while metallic soaps of magnesia and zinc in case oil-vehicle.

These paints are easy to apply and water dries away within 2 hours.

they are alkali-resistant.

They can be applied on cement plastered surfaces, usually used only for interior

works or for the decoration of exterior surfaces.

Aluminium Paint

These consists of very finely ground aluminium suspended in a medium of slow

drying oil varnish (drier+ vehicle) or quick drying spirit varnish.

These are expensive white paints, usually used for decorative works.

It protects iron and steel from corrosion far better than any other paints

It can resist heat to a great extent.

Cement Paint

These paints are prepared by mixing white cement with coloring matter or pigments

hydrated lime and fine sand as inert filler.

Cement paints are available in the form of powder of a particular color. The

dispersion medium may be water or oil, depending upon the purpose of coating.

Cement paints have marked water proofing capacity. This paint proves to be useful

for damp surfaces.

Cement paint is preferable on rough plastered surface. For painting surface of

corrugated iron sheets, cement paint may be mixed with boiled linseed oil. It should

be constantly stirred during use.

Emulsion Paint

A paint composed of small beads of resin binder which are dispersed, along with

pigments, in water. On evaporation of the water, the resin particles coalesce to form a

film which adheres to the surface and binds the pigment particles.

The surface of the paint is tough and it can be cleaned by washing with water. It has

a strong resistance against action of alkali. It dries very quickly in about 1-2 hours.

Enamel Paint

This is a superior quality paint and contains a high grade bases like zinc-white or

white lead ground in varnish (vehicle+drier).

An enamel paint is characterized by an ability to form an especially smooth film.

The amount of pigment is usually much less because a much higher degree of

dispersion of the pigment in the vehicle is required (than is generally necessary with

paints).

The enamel paint film presents a glossy surface. They are not affected by

atmosphere and stand well the adverse effects of weather.

Distemper

It is a form of water paints which contains water and oil as its vehicle and thinning

agent.

Glue, starch or resin are used as base together with a pigment to suit the desired

color.

They are two types mainly: a) oil bound distemper are generally available in the form

of paste (i.e. in oil medium) and they are thinned by water before use b) Ordinary

distempers , contains no oil, but are available in the form of powder, before use, they

are to be mixed with water

Used in all interior surfaces of buildings for decorative process.

Whitewash

A mixture of lime and water, a type of inexpensive paint made from slaked lime

(calcium hydroxide) and chalk (and glue and other additives), that is used to whiten

walls, fences, or other structures.

Anti-corrosive paint

These paints are used mainly to protect the surface of metallic steel work against the

negative effects of acids, corrosive chemicals fumes etc.

This paint consist of linseed oil as vehicle and red lead, zinc oxide, iron oxide, zinc

chromate etc. as their base. Drier and inert fillers are sometimes added while required.

These paints are cheap, durable and black in color.

Plastic paint

The paint contains a variety of plastics in suspension and is available in the market

under different trade names.

This paint is available in very attractive and pleasing shades. It can be applied by

spray or by brush.

Varnishes

Do not contain any pigments.

They are liquid compositions which may be converted to transparent or solid film after

application in a thin layer.

Usually consist of melted resin (as base) dissolved in hot linseed oil, terpentine or alcohol (as

vehicle), a drier is also used.

Varnishes give brilliance to which they are applied and protected the coated surface from the

atmospheric action.

They are the most commonly used for brightening the plain surfaces of timber in

furniture and other building works.

Two types: oil varnish and spirit varnish.

Oil varnish contains resin (base), oil (vehicle, say, linseed oil), drier (cobalt or manganese powder)

and thinner (say, terpentine oil, naptha etc.)

Spirit varnish contains one or more resins (as base) in volatile liquid (vehicle), usually

methylated spirit is used); this varnish is used commonly in Bangladesh and is prepared

by dissolving gums and resins in methylated spirit; French polish or shellac varnish is

also classified under this category.

SOME IMPORTANT ACTIVITY

RELATED TO PAINTING

Knotting

It is the process by which knots on the surface of wood-work are killed by

applying certain substances over the knots.

The substances applied over the knots prevent exudation of wood resin from the

knots and thus preventing the wood-knot from destroying the film of paint by way

of cracking, pealing or discoloring

This process contains of filling up all the nail holes, cracks, open joints, dents and

other depression on the surface by putty.

Putty is prepared by kneading white chalk powder into linseed oil; it should be given

the same color that will match the shade of the finishing coat by adding suitable

coloring agents.

After putty has dried up, the surface is rubbed by means of sand paper or glass paper

so as to make it smooth with having no irregularities.

Then the surface ready to apply primer.

Stopping

Primer Coat

This is actually the first coat of the paint, whose function is to fill the pores of

wooden surface by penetrating into the surface.

It also acts as a foundation for subsequent coats of the paint.

In general, the constituents of the priming coat should be kept same as those of

subsequent coats, but their proportion may vary.

Second or Subsequent Coat/ Under-coat

After the primary coat is dry, second or under-coatings are applied. The color of

the undercoat should approximately be of the same shade as that of the desired

finish. Two or more under-coatings are sometimes necessary to achieve the object.

Sufficient time must be allowed for each coat to dry before applying the

succeeding

Finishing Coat

This is the last coat applied on the surface after the under-coat is

properly dried

This coat should be applied very carefully so that the finished surface

can be free from thin patches and brush marks.

Spray Painting

For rapid execution, efficiency, perfection and durability, spray painting is

recommended on metal and woodwork.

The paint is put into a container and forced by air pressure through a hose to a metal

gun. The gun is provided with a spray nozzle and a hose conveying compressed air.

When the trigger of the gun is pulled, both the air and the fluid valves get opened. The

compressed air strikes the paint and forced out in the form of a spray from the nozzle.

Selection of tools for painting

Roller, brush and spray gun are generally used for painting. The bristle or brushes,

which are elastic and flat shapes, should be firmly fixed and possesses a good paint

holding capacity. After use, the brushes should be cleaned very thoroughly with

kerosene oil.

Other Topics

To be covered from Dr. Latifee sir’s book: “Construction Materials”

P-401: Functions of paint system components

P-411: The application of paints

P-412: Methods of painting

P-413: Common Paint problems.