BUILDING BLOCKS
Choosing the right paint
PROF. A.R. SANTHAKUMAR
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One needs a thorough understanding of the materials used and the different surfaces over which they are applied One needs a thorough understanding of the materials used and the different surfaces over which they are applied
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Success in house decoration depends on something more than the use of good paint. At least as important is thorough knowledge and understanding not only of the properties and possibilities of the materials employed, but also those of the different surfaces over which they are used. Here, we will see the materials and methods to achieve best results.
Materials
There is a host of materials used. Fillers are used to repair cracks in plaster and fill small holes and blemishes. Primers are used to apply first coat to bare wood and metal. For galvanised metal a special zinc primer must be used; otherwise your subsequent coats will eventually flake off.
Sealers are used for preserving, waterproofing and preventing problems occurring to subsequent coats caused by chemicals and fungi from the underlying surface. Different sealers are used to counteract different problems. Undercoats/basecoats provide the surface for the following finish coat. This is by far the most important stage in getting a first class finish. If necessary a second or even third undercoat should be applied, if needed.
Gloss, semi-gloss, eggshell and matt are the usual types of finishes used; a high-gloss finish by far is the most common. It is often difficult to get an undercoat to match various stronger colours and manufacturers normally recommend two coats of finish.
Emulsions and latex are paints (which are water-based) for use on ceilings and walls. They are easy to use and dry quickly. They can be used on interior wood but lack the hardwearing qualities of undercoat and gloss.
Which paint to choose?
Generally, quality solvent-borne paint should be applied when repainting over an existing oil-based system and latex paint should be applied over an existing water-based system. Oil paints continue to harden with age and the old surface is sensitive to the added stress of shrinkage that occurs as a new coat of paint dries. Over old oil paint, solvent-borne paints generally provide better adhesion than latex paint and shrink less, reducing the likelihood of pulling the old paint loose. Also as exterior oil paint ages, the binder releases pigment causing a chalky surface. While this should be cleaned off before repainting, a new coat of solvent-borne is more able to penetrate the chalky residue and adhere than latex paint.
Lime wash
Lime wash is an economic traditional water- based finish for exterior walls. It can be reproduced today in its traditional form but will not adhere to previously painted walls. Some modern proprietary lime wash is available with an acrylic sealer that can be applied to painted surfaces prior to the application of lime wash. This achieves a lime wash finish similar to the original finish without removing paint layers back to the substrate.
Distemper
Distemper is a traditional water-based interior paint popular for home applications. It provide a flat velvety finish and is used on walls and decorative plaster. Distemper has a thin consistency and be readily removed with hot water. As with lime wash, this finish can be reproduced today in its traditional form but will not adhere to previously painted walls. There are also some modern proprietary distempers available that require an acrylic sealer undercoat.
Oil finishes
Modern oil finishes are available that can be applied over oils, stains and penetrating finishes on joinery items. Oiled finishes rely on the previous finish being permeable; otherwise the finish will require sanding before the oiled finish is applied. Epoxies and urethanes are hard, inflexible finishes that cannot be easily removed and should not be used to replicate a traditional finish.
Surface condition
When planning to repaint, the surface condition of the existing painted finish needs to be identified and an appropriate method of surface preparation chosen.
Paint surface conditions can be grouped into three basic categories:
Minor blemishes or dirt collection that do not require paint removal.
Failure of the top layer or layers of paint which may require limited paint removal.
Substantial or multiple-layer failure requiring total paint removal.
Under conditions in categories 1 and 2 the following surface preparation methods should be used:
Cleaning, hand scraping and hand sanding; mechanical sanding by orbital sander when used as a smoothing/finishing tool. It must not be used for removal of multiple layers of paint. (Its small diameter and circular motion are useful for feathering areas that have been scraped).
Prior to carrying out any surface preparation the presence of lead paint and the associated health dangers of any paint removal to building occupants and neighbours must be determined and addressed.
The author is emeritus Professor, Department of Civil Engineering, IIT, Madras and former Dean, Anna University.
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