Paints and Coatings

In this section you will learn about:

• Distinguish between the alternatives available in the range of surface coatings,
• Select a suitable finish, taking into account the background, location and durability requirements
• Describe suitable preparation and application techniques

Paint

For hundreds of years people have been finishing the internal and external walls of their buildings with various mixtures or fabrics to decorate, preserve or waterproof them. Very early on, kalsomine (made from powdered limestone) was used to paint interior walls and varnishes and shellac were developed to preserve and decorate timber.

Lacquers, made from resins, came from China originally and became very popular in the late seventeenth and eighteenth century Europe for furniture and wall panels. In sixteenth century France painted hessian was popular as an interior wall finish, later superseded by exotic materials such as brocades. Wallpaper, as we know it, did not become really popular until the middle of the nineteenth century when printing processes made available brightly coloured and patterned wallpapers at prices many people could afford. Many coatings and coverings are now made either entirely or partially from plastics.

Today we expect a surface coating or covering to contribute to or provide any or all of the following:

• decoration
• preservation
• waterproofing
• hygiene
• improved lighting
• safety.

Surface finishes may only represent up to 5% of the initial building cost but contribute greatly to the maintenance costs of the building. Selection of the correct system and adequate preparation of the surface is, therefore, important.

Paints

Broadly speaking, paint is a mixture of:

• the binder
• pigments
• additives and extenders
• the medium.

The binder

The binder, as the name suggests, binds the other ingredients together, forming a solid, elastic film which must adhere to the surface, sometimes penetrating and sealing it as well. A paint is classified according to the type of binder.

Oil-based paints

These are based on oils which react with the oxygen in the atmosphere to solidify. Straight oil paints based on naturally drying oils, such as linseed oil, are rarely used today and have been largely replaced by paints modified with synthetic binders called alkyds. These paints are often called enamels or alkyd enamels.

 

 

Water-based paints

These binders comprise small globules of resin which are suspended or dispersed as an emulsion in water. As the water evaporates, the globules coalesce to form a solid film. Paints based on this type of binder are commonly known as plastic or latex paints and the resins used include PVA, acrylic, polyurethane or combinations of these. They are often referred to as emulsion paints.

Solvent-based paints

These binders are dissolved in a solvent which evaporates leaving a solid film, such as lacquer and chlorinated rubber.

Chemically cured paints

These are usually two-pack paints and the binder forms as the two compounds are mixed together and react chemically. Once mixed, the paint must be applied within a few hours. Epoxy (epoxide) resin paints are examples.

Pigments

Pigments are used to make the paint opaque, to hide the background, and to provide the required colour. For instance, titanium dioxide is used for opacity and another compound such as iron oxide might be used to impart the colour.

Additives and extenders

Additives and extenders are included in varying quantities and have a great influence on the properties of the paint. The roles of additives and extenders tend to merge but basically they are as follows.

Additives might include fungicides and driers in oil and alkyd paints or dispersing and emulsifying agents in latex or plastic paints.
Extenders are used to achieve the required viscosity, body and surface appearance.

Medium

The medium can either be a solvent in which the binder is dissolved or a dispersing medium in which it is suspended. Examples of solvents include mineral turpentine or benzine derivatives. The dispersing medium most commonly used for plastic and latex paints is water.

Thinning and cleaning up depends on the nature of the dispersing medium. Oil-based paints require turpentine or white spirit whereas water-based paints can be thinned and cleaned up with water. Special solvents are required for other types of paints.

Paint systems

Most paint systems include the following:

• primer or sealer
• undercoat(s)
• finishing coat(s).

The choice of system depends on the nature of the surface to be painted and the finish required (refer to Figure 1).

Figure 1: Break-up of a paint system

 

Each component of the system performs a particular function but in some cases, as with plastic paints, a paint can perform more than one function. The type of coat selected must be compatible with the substrate (background) and with adjacent coats.

Primer

The primer can fulfil a number of functions including:

• providing a key to improve the adhesion of the next coat
• sealing porous surfaces which would otherwise absorb part of the next coat and spoil the finish
• minimising ‘bleeding’ of surfaces such as bitumen and timber.

Primers which etch the surface and inhibit corrosion are available for use on metals.

Undercoats

Undercoats must cover the original colour of the surface and fill in any small depressions.

Finishing coats

Finishing coats provide the final colour and texture and offer the final protection against weather, chemical and mechanical damage. Finishing coats are available in gloss, semi-gloss or satin, flat or matt and in various textures.

• gloss –  is highly reflective, resistant to moisture and easy to clean but shows up surface irregularities
• semi-gloss – is less reflective and shows fewer surface imperfections
• flat – has low light-reflection, is usually permeable to moisture and tends to collect grime more easily.

 

Figure 2 demonstrates how, on a microscopic level, the medium evaporates leaving various amounts of pigment exposed, thus forming the various finishes.

 

Figure 2: Microscopic cross sections showing how light is reflected, giving characteristic shiny or matt appearances

Choosing a paint system

Nature of the substrate

The substrate is the surface which is to be painted.

Alkalinity, porosity and loose particles on the surface to be painted can affect the adhesion and durability of a paint system.

Materials such as concrete, cement render, mortar and solid plaster contain small amounts of alkaline materials (mainly from the lime) and some paints, such as the alkyd enamels, are susceptible to alkali attack, which causes breakdown of the film. The gloss and semi-gloss enamels are more susceptible than the flat enamels and must be separated from the substrate by an alkali sealer.

 

Gloss and semi-gloss alkyd enamels are also adversely affected by porous surfaces which absorb the medium and binder unequally. The use of a suitable undercoat will prevent unequal absorption of the finishing coats. Plastic or latex paints are not affected by porous surfaces because the globules of resin are not absorbed but sit on the surface.

Loose surface material can reduce adhesion. Enamel paints tend to penetrate the loose material and bind it together but plastic or latex paints just tend to sit on the surface. For this reason, loose material should be removed with a brush or scraper before painting with a plastic or latex paint. If the surface is particularly loose, treatment with a 15% solution of phosphoric acid may be required.

Recommended paint system

In addition to consideration of the nature of the substrate, the choice of a paint system ultimately depends upon:

• the performance specification
• whether you require a fully impervious surface or a porous surface finish which can breathe
• whether you require a high wear, abrasion resistant surface
• whether the surface is to be washable
• whether the surface is inside or exposed to weather and pollution.

Table 1, on the next page, is a summary of paint systems for various substrates.

 

 

Special paints

A variety of paints for special purposes are available, including the following:

 

water-resistant paints chemical-resistant paints fire-retardant paints heat-resistant paints fungus-resistant paints anti-condensation paints luminous paints fluorescent paints phosphorescent paints radioactive paints

low-odour paints quick-drying paints stoving paints insecticidal paints permeable paints floor paints multi-colour paints textured paints metallic paints

 

 

As with all paint systems, it is best to seek the advice of the manufacturer for special requirements and, having selected a manufacturer, it is wise to choose your entire system from your chosen brand to avoid incompatibility between coats.

 

 

 

 

 

 

Substrate	Location	Finish required	Special preparation	Recommended paint
Concrete, concrete masonry, mortar joints, fibrous cement	Interior Exterior Interior/ exterior	Matt Matt Gloss or semi-gloss	Clean loose material Clean loose material Allow to dry for 12 months after construction Apply 2 coats of alkali sealer		Interior or exterior quality latex Exterior quality latex 1 coat undercoat 1 coat gloss alkyd enamel
Fibrous plaster, plasterboard, calcium-silicate and clay bricks	Interior Interior Exterior	Matt Gloss Semi-gloss	Smooth joints 1 alkali Sealer over joints		2 coats latex 1 coat undercoat 1 coat gloss or semi-gloss enamel
Hardboard, particle board, plywood	Interior/ exterior Interior/ exterior	Matt Gloss or Semi-gloss			Alkyd undercoat, latex top coats 1 coat alkyd undercoat 1 coat gloss or semi-gloss alkyd enamel
Timber *	Interior Exterior	Gloss recommended Gloss recommended	Smooth, sand, stop and fill Stop and fill		1 coat wood primer 1 coat alkyd undercoat 1 coat gloss alkyd enamel 1 coat wood primer 1 coat alkyd undercoat 1 coat gloss alkyd enamel or 1 coat wood primer 2 coat gloss latex
Unweathered galvanised steel #	Exterior	–	Wipe surface with mineral turpentine		1 coat calcium plumbate 1 coat alkyd undercoat 1 coat alkyd enamel top coat
Small areas of mild steel	Interior Exterior	Gloss recommended Gloss recommended	Clean carefully with emery paper to remove rust and mill scale		1 coat wood anti-corrosive metal primer 1 coat alkyd undercoat 1 coat alkyd enamel finishing coat 2 coats anti-corrosive metal primer 1 coat alkyd undercoat 1 coat alkyd enamel finishing coat

* Latex paints are too soft for heavy wear and tear on timber.
# With galvanised steel, where possible, it is better to use pre-painted or coated roofing products.
Table 1: Recommended paint systems

Applying the paint

On site, paint can be applied by:

• brush  –      this provides the best adhesion, desirable in priming coats, but skill is required to avoid brush marks
• roller  – which is much quicker but provides a slightly stippled surface finish; edges must be finished with a brush
• spray  –         equipment is expensive but can be economical on very large areas

–      can be used to achieve metallic and graded effects
–      the only suitable method for quick-drying paints (the hot spray process reduces the viscosity of a paint without the addition of a solvent)

In the factory, paint can be applied by:

• dipping smooth – this is rapid and economical, producing a very smooth finish
• flow coating – paint is hosed onto the surface
• roller coating (by machine) – used for continuous lengths.

Preparation of surfaces

One of the most common causes of breakdown of painted surfaces is inadequate preparation of the substrate. Sometimes brushing is adequate but in other cases dirt must be removed by washing and scraping, using suitable solvents for oils and stains.

Previously painted surfaces might simply require priming, filling and rubbing down.

However, where a perfect surface is required paint can be removed by burning off and scraping, using solvent and chemical removers or by steam stripping. Water-soluble paints, such as tempera, must be removed before painting as they prevent the formation of a key.

When to paint

Generally speaking, it is best not to paint in wet, damp or foggy weather or below 4°C, in direct sunlight or in dusty conditions. Humid conditions delay drying of ordinary paints.

Each coat should be thoroughly dry before the next is applied.

Good ventilation is required to assist drying and sometimes to remove noxious fumes.

Revision

Undertake the following Questions

• What is an enamel paint?  Describe its advantages and disadvantages.

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• What us plastic paint?  Describe its advantages and disadvantages.

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• What constitutes a paint system?

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Clear finishes

Clear finishes are used to enhance the natural appearance of the substrate and in many cases waterproof and protect it as well. They may or may not include some colour pigment and, depending upon the type of compound, may be available in gloss, semi-gloss or matt finishes.

In general, clear finishes lack sufficient pigment to filter out damaging ultraviolet light and are therefore much less durable than paints in exposed conditions. Consequently, the choice is limited for external conditions.

Interior clear finishes have been formulated specially to suit the substrate. We will deal with them according to the nature of the substrate.

Clear finishes for internal timber

The clear finishes currently available include the following:

• Oil seal – a type of varnish, used to achieve a water and grease resistant, non-slip finish for floors.
• Wax polishes – based on natural waxes, such as beeswax, they can be used as complete system or to maintain other finishes. They are relatively soft and more inclined to collect dirt than other finishes; they discolour when wet and will be stained by ink or heat but are less likely to show scratches and easily are repaired.
• Polymer-based emulsions – based on PVA, acrylic and polyethylene resins; they are easy to apply and maintain.
• French polish – based on applications of shellac and linseed oil in successive treatments, requiring great skill for a good finish. They are considered to be the most beautiful finish for internal timber but are extremely expensive and easily marked by water, heat and solvents.
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• Cellulose lacquer – based on nitro-cellulose and a plasticiser and showing a similar appearance to French polish but less expensive and easier to apply. It is more resistant to water but eventually cracks and must be completely removed before renewing. Nitro-cellulose is extremely flammable and appropriate precautions should be taken regarding storage and use.
• Short-oil varnishes – have a low oil and high resin content, producing a high gloss but reduced flexibility. They are easy to apply with a brush but they dry slowly, collect dust and crack.
• Spirit varnishes – made with resins, such as shellac, they dry quickly by the evaporation of the solvent. They are cheap, brittle and inclined to crack.
• Synthetic resin finishes – made from plastics, such as phenol formaldehyde resins, urea formaldehydes, polyurethane and epoxides. They are available in one-pack or two-pack forms. They are relatively expensive but are very popular because of their ease of application by brush or spraying. They are rapid drying and are extremely hard and flexible, water and chemical resistant and heat resistant. Repairs are difficult because they cannot be removed by normal solvents.

When choosing a clear finish for a timber surface it is important to define your requirements carefully, taking into account the nature of the timber. For instance, the clear finish chosen may actually be harder than the timber substrate and breakdown of the finish has often occurred because an impact has caused denting of the timber below, not the finish itself. The result is a loss of bond between the substrate and the finish. Thus, softer timbers should be finished with the more flexible finishes.

 

 

Preparation of internal timber surfaces

As with painted surfaces, a good finish can only be obtained with adequate preparation of the substrate. In general, the surface must be clean, firm and dry but additional preparation might include:

• bleaching or liming – to give a grey effect
• sanding – to smooth the surface
• stopping or filling of pores or indentations – usually with a tinted, oil-based wood filler
• staining – this may be applied before the final finish or may be included in the finish (the manufacturer’s advice should be followed regarding the compatibility of a stain with a finish).

Clear finishes for external timber

Clear finishes which will help to preserve the natural appearance of timber in exposed conditions include the following:

• Preservatives – These help protect the sapwood and heartwood or timber from attack by fungi and discolouration by moulds.
• Water repellents – These are a mixture of linseed oil, paraffin wax and a fungicide, applied by brushing or dipping, especially to end grain. They help preserve the appearance of the timber by reducing surface cracking due to wetting and drying.
• Stains – Water-resistant stains can provide a degree of ultraviolet filtration change the colour of the timber and revive bleached timber.
• Varnishes – The only suitable varnishes for exterior use are long-oil marine and exterior varnishes but these require frequent recoating – less than four coats will be unlikely to last more than a year. While intact, varnishes seal the timber against water but it is desirable to apply a preservative as well.

 

Preparation of external timber

In general, a lower standard of preparation is required for external timber but any stopping or filling must be water-resistant and galvanised nails should be driven well below the surface and filled to avoid rust stains.

External clear finishes on other materials

Clear finishes designed to reduce soiling and make the surface impervious to water are frequently applied to masonry surfaces, finishes based on silicone being the most effective and the most expensive alternatives.

Finishes based on acrylic resins and polyurethane two-pack systems are available to give some protection to metals such as copper. They must be applied by spraying and preferably in a factory.

Other coatings

Vitreous enamel coatings

Vitreous enamel (often called porcelain enamel) is actually glass which is fused under extreme heat to metal surfaces.  The process is expensive but the resultant coating is extremely hard and durable and adheres firmly to the substrate so that where damage exposes the underlying surface, rust will not creep under the rest of the coating.

The finish is applied after fabrication is complete and the number of coats required depends upon the location of the finished component.

A wide range of colours is available and finishes can be gloss, semi-gloss, matt or textured.  The latter collect grime easily and are not suitable for external use.

Vitreous enamel coatings are used for metal-wall infill panels, mullions, lift panels, steel rainwater components and baths.

 

Plastic coatings

Plastics can be applied in a number of ways to metal, timber and other surfaces and form continuous protective coatings which, in general, are more durable and tough than ordinary painted finishes.

Some are extremely durable (eg polyvinyl fluoride and nylon) but others (eg, polyethylene) deteriorate in exterior conditions, fading and becoming brittle.

Many colours are available though some are not suitable for external use and the finish obtained is usually warm to the touch, and smooth, easily cleaned and provides electrical insulation.

The coatings are applied to the components or sheet materials in the factory and are used for sheet metal, and extruded components, such as handrails, in particular.

Revision

Undertake the following Questions

• Why do many clear finishes break down rapidly when exposed to the external environment?

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• How should the timber substrate be prepared for an internal clear finish?

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• What preparation is required for exposed timber which is to be clear finished?

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• What are the major differences between vitreous enamel coatings and plastic coatings?

Sheet coverings

As briefly mentioned at the beginning of the unit, sheet coverings such as wallpapers and fabrics have been used to decorate wall and ceiling surfaces for hundreds of years.

Wallpapers and textiles are still the easiest way to obtain large areas of highly patterned or textured wall surfaces and in addition can contribute to acoustic modification of the space.

Light-fastness varies and few are suitable in areas receiving strong sunlight.

Type of sheet coverings

Sheet coverings used frequently include the following:

• Lining papers:  These are used to cover imperfect plaster surfaces which are subsequently painted or wallpapered.  They are hung horizontally under wall paper to minimise coincidence of joins
• Expanded polystryrene:  This is a great deal thicker than wall paper and it provides some thermal insulation, often sufficient to prevent surface condensation.
• Wallpapers:  These can be machine-made or hand-made – the latter being more expensive, with dense colours but some imperfections.  Wallpapers are available in the following types:
• pulps:  patterns printed directly onto the paper
• embossed:  with a raised design
• duplex:  two-ply papers
• ingrain:  having fibres incorporated into the surface
• washable: coated with a plastic emulsion, vinyl-faced papers are washable but maximum dirt resistance is provided by PVC coated papers
• shiny:  surfaced with mica
• flock:  raised applied pattern created by blowing fibres onto patterns printed in adhesive

 

Preparing the surface to be papered

The wall surface should be dry and chemically neutral with a slight suction.  This is achieved by removal of efflorescence by brushing and painting with an alkali-resistant primer.

If mouldy, old wallpaper should be removed and the surface treated with a fungicide.  Depressions and cracks should be filled and a lining paper could be applied to improve the substrate.

 

 

Galvanising

Galvanising is the process of coating steel and iron with zinc to form a protective coating.  The steel is lowered into a molten bath of zinc heated to approximately 5000C and emerges with a shiny coating of zinc.  The zinc coating acts as a ‘sacrificial’ anode and corrodes to protect the steel.  Since its rate of corrosion is slow, the steel can remain protected for hundreds of years, depending on the environment.

 

Zincalume

Zincalume is a newer protective coating and is a combination of zinc and aluminium (45% & 55% respectively), which is applied in a factory process to sheet steel used for roofing and cladding in the building industry.

Revision

Undertake the following Questions

• How would you prepare a wall surface for wallpapering?

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• What precautions should be taken when hanging wallpaper in order to achieve a good result?

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