Paints

It is a coloured substance which is spread over a surface and dries to leave a thin decorative or protective coating.

Composition

The actual composition of any paint can be complex but the basic components are:

  • Binder – this is the liquid vehicle or medium which dries to form the surface film and can be composed of linseed oil, drying oils, synthetic resins and water. The first function of a paint medium is to provide a means of spreading the paint over the surface and at the same time acting as a binder to the pigment.
  • Pigment – this provides the body, colour, durability and corrosion protection properties of the paint. White lead pigments are very durable and moisture resistant but are poisonous and their use is generally restricted to priming and undercoating paints. If a paint contains a lead pigment the fact must be stated on the container. The general pigment used in paint is titanium dioxide which is not poisonous and gives good obliteration of the undercoats.
  • Solvents and Thinners – these are materials which can be added to a paint to alter its viscosity.

Paint Types

There is a wide range available but for most general uses the following can be considered:

  • Oil Based paints these are available in priming, undercoat and finishing grades. The latter can be obtained in a wide range of colours and finishes such as matt, semimatt, eggshell, satin, gloss and enamel. Polyurethane paints have a good hardness and resistance to water and cleaning. Oil based paints are suitable for most applications if used in conjunction with correct primer and undercoat.
  • Water Based Paints most of these are called emulsion paints the various finishes available being obtained by adding to the water medium additives such as alkyd resin & polyvinyl acetate (PVA). Finishes include matt, eggshell, semi-gloss and gloss. Emulsion paints are easily applied, quick drying and can be obtained with a washable finish and are suitable for most applications.

Application

Paint can be applied to almost any surface providing the surface preparation and sequence of paint coats are suitable. The manufacturers specification and/or the recommendations of BS 6150 (painting of buildings) should be followed. Preparation of the surface to receive the paint is of the utmost importance since poor preparation is one of the chief causes of paint failure. The preperation consists basically of removing all dirt, grease, dust and ensuring that the surface will provide an adequate key for the paint which is to be applied. In new work the basic build-up of paint coats consists of

  • Priming Coats – these are used on unpainted surfaces to obtain the necessary adhesion and to inhibit corrosion of ferrous metals. New timber should have the knots treated with a solution of shellac or other alcohol based resin called knotting prior to the application of the primer.
  • Undercoats – these are used on top of the primer after any defects have been made good with a suitable stopper or filler. The primary function of an undercoat is to give the opacity and buildup necessary for the application of the finishing coat(s).
  • Finish – applied directly over the undercoating in one or more coats to impart the required colour and finish.

Paint can be applied by

  • Brush – the correct type, size and quality of brush such as those recommended in BS 2992 (painters and decorators brushes) needs to be selected and used. To achieve a first class finish by means of brush application requires a high degree of skill.
  • Spray – as with brush application a high degree of skill is required to achieve a good finish. Generally compressed air sprays or airless sprays are used for building works.
  • Roller – simple and inexpensive method of quickly and cleanly applying a wide range of paints to flat and textured surfaces. Roller heads vary in size from 50 to 450 mm wide with various covers such as sheepskin, synthetic pile fibres, mohair and foamed polystyrene. All paint applicators must be thoroughly cleaned after use.

Basic Surface Preparation Techniques

  • Timber – to ensure a good adhesion of the paint film all timber should have a moisture content of less than 18%. The timber surface should be prepared using an abrasive paper to produce a smooth surface brushed and wiped free of dust and any grease removed with a suitable spirit. Careful treatment of knots is essential either by sealing with two coats of knotting or in extreme cases cutting out the knot and replacing with sound timber. The stopping and filling of cracks and fixing holes with
  • putty or an appropriate filler should be carried out after the application of the priming coat. Each coat of paint must be allowed to dry hard and be rubbed down with a fine abrasive paper before applying the next coat. On previously painted surfaces if the paint is in a reasonable condition the surface will only require cleaning and rubbing down before repainting, when the paint is in a poor condition it will be necessary to remove completely the layers of paint and then prepare the surface as described above for new timber.
  • Building Boards – most of these boards require no special preparation except for the application of a sealer as specified by the manufacturer.
  • Iron and Steel – good preparation is the key to painting iron and steel successfully and this will include removing all rust, mill scale, oil, grease and wax. This can be achieved by wire brushing, using mechanical means such as shot blasting, flame cleaning and chemical processes and any of these processes are often carried out in the steel fabrication works prior to shop applied priming. Plaster the essential requirement of the preparation is to ensure that the plaster surface is perfectly dry, smooth and free of defects before applying any coats of paint especially when using gloss paints. Plaster which contains lime can be alkaline and such surfaces should be treated with an alkali resistant primer when the surface is dry before applying the final coats of paint

Paint Defects

These may be due to poor or incorrect preparation of the surface, poor application of the paint and/or chemical reactions. The general remedy is to remove all the affected paint and carry out the correct preparation of the surface before applying in the correct manner new coats of paint. Most paint defects are visual and therefore an accurate diagnosis of the cause must be established before any remedial treatment is undertaken.

Typical Paint Defects

  • Bleeding – staining and disruption of the paint surface by chemical action, usually caused by applying an incorrect paint over another. Remedy is to remove affected paint surface and repaint with correct type of overcoat paint.
  • Blistering – usually caused by poor presentation allowing resin or moisture to be entrapped, the subsequent expansion causing the defect. Remedy is to remove all the coats of paint and ensure that the surface is dry before repainting.
  • Blooming – mistiness usually on high gloss or varnished surfaces due to the presence of moisture during application. It can be avoided by not painting under these conditions. Remedy is to remove affected paint and repaint.
  • Chalking – powdering of the paint surface due to natural ageing or the use of poor quality paint. Remedy is to remove paint if necessary, prepare surface and repaint.
  • Cracking and Crazing – usually due to unequal elasticity of successive coats of paint. Remedy is to remove affected paint and repaint with compatible coats of paint.
  • Flaking and Peeling – can be due to poor adhesion, presence of moisture, painting over unclean areas or poor preparation. Remedy is to remove defective paint, prepare surface and repaint.
  • Grinning – due to poor opacity of paint film allowing paint coat below or background to show through, could be the result of poor application; incorrect thinning or the use of the wrong colour. Remedy is to apply further coats of paint to obtain a satisfactory surface.
  • Saponification – formation of soap from alkali present in or on surface painted. The paint is ultimately destroyed and a brown liquid appears on the surface. Remedy is to remove the paint films and seal the alkaline surface before repainting.

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