Painting, Varnishing and Distempering – Building Construction Notes

Painting, Varnishing and Distempering

1. General

Painting, varnishing and distempering are finishing processes applied on surfaces such as walls, ceilings, woodwork and metalwork. Depending on the surface and the type of finish required, it may be coated with paint, varnish or distemper. These materials provide protection, durability and good appearance to the surface. Normally, they are reapplied after an interval of about 2 to 5 years.

2. Objectives of Painting

• To protect surfaces from weathering effects of atmosphere, liquids, fumes and gases.
• To prevent decay of wood and corrosion of metals.
• To improve appearance and make surfaces hygienic, clean and attractive.
• To provide a smooth surface for easy cleaning.

3. Characteristics of an Ideal Paint

• Good spreading power — maximum area covered with minimum paint.
• Economical and not too costly.
• Easy and uniform application on the surface.
• Reasonable drying time — neither too quick nor too slow.
• Colour should remain stable for a long time.
• Forms a hard and durable surface.
• Should not affect the health of workers during application.
• Resistant to weathering actions of atmosphere.
• Pleasing appearance and no cracks after drying.
• Uniform thin film formation on the surface.

4. Pigment Volume Concentration Number (P.V.C.N.)

The pigment volume concentration number (P.V.C.N.) is given by the formula:

P.V.C.N. = V1 / (V1 + V2)

Where:
V1 = Volume of pigment in the paint
V2 = Volume of non-volatile vehicle/carrier in the paint

The higher the P.V.C.N., the lower is the durability and gloss of paint.

No.	P.V.C.N.	Type of Paint
1	25–40	Primer coat on metal
2	28–40	Exterior surfaces of house
3	35–40	Primer coat on wood
4	35–45	Semi-gloss paint
5	50–75	Flat paint

5. Ingredients of an Oil-Borne Paint

An oil-based paint generally consists of:

1. Base – the solid substance forming the bulk of paint, gives body and durability.
2. Vehicle/Carrier – the liquid that holds ingredients in suspension and spreads them.
3. Drier – substances that accelerate drying.
4. Colouring Pigment – gives colour and opacity.
5. Solvent – provides proper consistency and ease of application.

6. Common Bases for Paints

Name	Description
White Lead	Carbonate of lead, dense and waterproof; used on wood but not on iron (discolours with sulphur vapours).
Red Lead	Oxide of lead; used for priming iron surfaces and as a drier with linseed oil.
Oxide of Zinc (Zinc White)	Smooth, transparent, non-poisonous; expensive and less durable.
Oxide of Iron	Cheap, durable; used in primers for iron surfaces.
Titanium White	Non-poisonous, highly opaque; used for enamel coats.
Aluminium Powder	Forms aluminium paints; prevents moisture and cracking in wood surfaces.
Lithophone	Mixture of zinc sulphide and barytes; cheap but discolours in sunlight, used for interior work.