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Paints and Varnishes

Learning Objectives Definition Functions of paints. Composition of oil paints. Preparation of oil paints. Characteristics of good paints. Types of paints, and properties of paints Defects in painting.

Definitions Paint is a coating of fluid material applied over timber and metal surface as protective coating which on drying forms a thin film on surface Paint is a mixture of liquid or medium and a coloring or pigment to impart color and provide protective coating to the surface Oil based paints are polymers or pre-polymer solutions which form a film upon evaporation of the solvent Paint is a dispersion of pigments in a drying oil, with addition of driers and thinners

Definitions Varnish is a transparent or nearly transparent solution of resinous material and oil, alcohol or turpentine to form a clear, tough, glossy film on woodwork Enamel. Bases like zinc oxide ground in varnish. Dry quickly to furnish hard glossy finish.

Definitions Distemper is a comparatively cheap decorative paint for walls and ceilings applied on brickwork, or plastered surfaces Water Wash and Color Wash. Fresh lime slacked with water, mixed thoroughly, screened and added with glue and may be a pigment

Definitions French Polish. Type of spirit varnish prepared by dissolving resin in methylated spirit at room temperature for use on hardwood substances to hide grain defects. Wax Polish. Bees wax dissolved in turpentine used for highlighting the grain over wooden surfaces.

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

Classification of Paints

Classification of Paints

Types of Paints Oil Paint: ordinary paint Bituminous paint: prepared by dissolving asphalt or bitumen in oil or petroleum Cement paint: It consists of cement and hydrated lime mixed along with a coloring pigment Colloidal paint: a paint with no inert material Aluminum paint: aluminum powder suspended in spirit varnish or oil varnish Asbestos paint: peculiar type, applied to places exposed acidic gases and steam. Cellulose paint: prepared from nitro cotton celluloid sheets

Types of Paints Emulsion paint: It consists of polyvinyl acetate and synthetic resin as binding material Enamel paint: It consists of white lead ground in small quantity of oil and mixed with petroleum spirit and resinous matter Graphite paint: black in color, applied on iron surfaces Luminous paint: contains calcium sulphide with varnish Silicate paint: prepared by mixing calcined ground silica with resinous substances Anti-corrosive paint: consists of oil and strong drier Plastic paint: variety of plastic present, applied using brush or spray painting. Synthetic rubber paint: prepared from resin

Composition of Oil Paints Constituents of oil paints Base Vehicle – an oil, generally linseed oil Coloring pigment (s) Solvent or thinner Drier Inert filler By suitable variation of the type and proportion of various constituents the paints are made as Dry Glossy flat

Composition of Oil Paints - Base Base : principal constituent, a solid matter forming the main body of paint and possessing binding properties. White lead, red lead, zinc oxide, iron oxide, metallic powders of aluminum, copper and bronze, etc Makes the paint film harder and more resistant to abrasion Forms an layer to obscure the surface Reduces shrinkage cracks on drying

Composition of Oil Paints - Base White lead Cheapest and most commonly used base Greater covering power than all others Dense so good to obscure surfaces Weathers well Not suitable for delicate works as gets discolored Not suitable for painting of iron work due to rusting

Composition of Oil Paints - Base Red lead Sticks well and protects against rusting. With oil considered best for first coat or prime coat Good drier for linseed oil Lead paints are poisonous. Precautions needed while spraying or scrapping paint

Composition of Oil Paints - Base Zinc oxide or Zinc white Not affected by weather Takes a fine polish hence good for decoration works Not poisonous Less durable and more costly than lead based Iron oxide Used basically in finishing coat for iron work Prevents rust formation Comparatively cheaper Tints vary from yellowish brown to black

Composition of Oil Paints - Vehicle Vehicle: Carrier liquid which carries solid materials of base and helps them to spread evenly on the surface to be painted. Linseed oil, poppy oil, nut oil, soyabean oil, fish oil, latex emulsions Oily liquid in which base and pigment are soluble Facilitates the paint to be conveniently spread evenly over the surface Acts as a binder for the base and causes it to stick to the surface On drying forms a tough and elastic film

Composition of Oil Paints - Vehicle Raw linseed oil Thin, pale and transparent oil Sweet taste, no smell Becomes hard and stiff on exposure to air When spread in thin film, looks like varnish Dries very slowly Used for painting delicate interior work and wood work

Composition of Oil Paints - Vehicle Boiled linseed oil Thicker and darker in color (deep amber to rich brown) Dries quickly. On drying leaves a hard, glossy and durable surface Has more area coverage capacity Used for exterior work Double boiled linseed oil Light in color as raw linseed oil but with different smell Dries quicker and gives better results Requires and thinning agent like turpentine oil

Composition of Oil Paints - Pigment Coloring pigments: finely divided solid coloring matter to provide shade, color to paint Blacks: lamp black, vegetable black, ivory black Blues: indigo, Prussian blue Yellows: chrome yellow, raw Siena, yellow ochre Greens: copper sulphate Browns: raw umber, burnt umber Red: red lead, vermillion, carmine

Composition of Oil Paints - Thinner Solvent or thinner: A volatile liquid added to prepare paint to increase fluidity thus workability and ease of application Thinner helps penetration of paint in porous surfaces Turpentine oil is most common thinner Excessive thinner dulls the colors and gloss Excessive thinner reduces protective value of paint

Composition of Oil Paints - Drier Drier: added to paint to quicken the drying of vehicle Linseed oil dries by absorbing oxygen Drying process expedited by adding oxygen rich substances Common driers are: Litharge, Red lead, Lead acetate, Manganese dioxide, Zinc sulphate Excessive drier destroys the elasticity of paint and causes flaking Drier is added to the paint just before use

Composition of Oil Paints - Filler Inert filler An adulterant mixed to replace the base in part thus reducing the cost of paint Commonly used fillers are silica, charcoal, powdered chalk, aluminum silicate, barium sulphate, etc

Characteristics of Good Paints Should have a good body or spreading power Should work smoothly and freely to be laid in thin coat Should form durable, tough and wear resistant film upon drying Color should not fade or change Painted surface should dry in about 9 hours Should become hard enough in 24 hours to take up another coat Should not crack upon drying Should give a smooth and pleasing finish Should dry quickly Should not damage the painted surface

Manufacture of Oil Paints The base (white lead) is thoroughly ground in oil Thinner (turpentine oil) is mixed to give it necessary workability Pigment and drier are separately ground in linseed oil and mixed with turpentine oil to make it thin Pigment mixture intimately mixed with already prepared base Prepared paint is strained through fine cloth or seive

Defects in Painting 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

Defects in Painting Blistering and peeling: Runs and sage: 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 Runs and sage: Application of too thick or slow drying paint Painting over a glossy surface Use of excessive drier Excessive humidity or rapid thermal changes during drying period

Defects in Painting Chalking: rub off with hands or clothes Use of insufficient oil in priming coat Washing off: deposition of water soluble dissolved matter at lower edges forming streaks Dull appearance: caused by use of excessive drier or on aging

Defects in Painting Slow drying: Yellowing of white paint: use of inferior or old oils Painting over damp surfaces Painting during unfavorable weather Yellowing of white paint: Use white enamel where glossy finish is desired For indoors use linseed oil with yellow tint that does not bleach unless exposed to sunshine

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