Painting and defects of painting
Prepared by Architect Perunal Nagaushnam
Paints contain three components: 1.)pigment (colour),2.) a binder (holds the paint together),3.) carrier (disperses the binder).
Why is it necessary to paint a building? The objective of painting is a dual purpose, that is:
- General Protection
to protect the painted parts of the building, making it more durable.For a maximum protection, the painting coating schemes may differ pending on the a length of time of protection to harshest weather conditions.
- Special protections
such as protection against the harsh weather, air pollutions, salt spray environments such as the sea-shores. Steel frames, portal frames or shipping ports structures may require protection against the tendency to rust
the final objective is aesthetics, which is to make the object to be painted to look attractive and presentable.
- Green and sustainable effects
Green paints are materials that do not use toxic chemicals in the paint.The harmful toxic chemicals found in modern paints are harmful to the environment and humans, containing metals such as Cadmium, lead and chromium are found in pigments.
Petrochemicals, solvents, benzene, formaldehyde and VOCs-volatile organic compounds are also used in binders and carriers. Toxic, environmentally harmful, chemicals are used as preservatives, stabilisers and thickener.
- Special functions
Special purposed paints such antifungal, fire retardant, odourless, quick drying
There many types of paint:
a.)Emulsion water based paint
Gloss paints are oil-based which consist of resins to giving them a hard wearing quality. Gloss paints are oil (solvent) based paints and also water based gloss paints . traditionally gloss paint was only oil solvent paints, which is to say that after painting the brushes are washed away using solvent( such as turpentine or oil) or the paint can be diluted only by using a solvent such as turpentine. Water base paints can be diluted with water or washed away with water when it is still wet.
- Liquid gloss needs an undercoat but gives the more traditional high gloss finish and is extremely hard wearing and resistant to dirt.
- Satinwood is a durable gloss paint that gives a more subtle sheen than the conventional shiny gloss effect, however, it is not usually as hard wearing.
- Eggshell is a paint is with flatter matte finish, used for smaller pieces of decoration in timber architraves and skirting.
- Polyurethane glosses an oil-based paint with a polyurethane resin, causing it be tough making it with a hard wearing surface and a greater abrasion than the usual paint.
- Silane is a mix of silicone and polyurethane paints which make the paint with a stronger surface than polyurethane as the silicone, hence giving an extra protection.
As a rule, all painting schemes should be painted with a primer coat, an undercoat, and a finish coat. There is some exemption to this rule, such as weather shield or pu paint polyurethane paints which do not need a primer or sealer coat. The following are the coating system for the different base materials:
Cement base surfaces
Emulsion paints coating system, for the internal wall, are the acrylic acid resisting coat, undercoat, and a finish coat. The Emulsion paints coating system, for external walls, are an acrylic acid resisting coat, undercoat, and a finish coat or 2 coats of weather-resisting painting coat system.
All surfaces external or internal is primer coat(aluminum oxide ) with an undercoat of gloss paint and a gloss finish coat.
Metal surfaces (steel)-gloss paint
All surfaces external or internal is red oxide primer coat with an undercoat of gloss paint and a gloss finish coat.The surface preparation should be mechanical wire brushing, or sand grit blasting to SA standards of 1 to 2.5, as this process would eliminate all rust which will reappear at a later stage. In some standards after sand or grit blasting the surfaces are washed clean of salt and within a minute of preparation the painting coat is applied.
Metal surfaces (steel)-PU paint(polyurethane)
The surface preparation stated above is repeated with 2 coats of Pu paint, which referred to as 2 pack PU paint, which is mixed prior to application as they harden within minutes of application and thus giving excellent protection.
Metal surfaces (GI, aluminum, stainless steel, brass)-gloss paint
The surface preparation as stated above for steel, for the above metals are not necessary as these metals do not rust, but there a small amount of oxidation in aluminum. Nevertheless, the surface needs some treatment such as acid etching to cleans the surfaces of the metal.
1.Zinc chromate primer is necessary on surfaces such as galvanized iron as red oxide as primer will chemically react with the galvanized surface, whereas zinc chromate is a neutral material that stops the reactions.
2.under coat and finish coat of gloss paint
The importance of surface preparation can not be underestimated as the quality of painting durability and aesthetic depends primarily on the quality of surface preparation, as you could see from the above materials the surface preparation vary from material to material.
Defects of Painting
- Painting schemes
The first defect of paint is the lack of provision of painting schemes such as undercoat or the primer coat, sometimes as a way to cut cost the painter applies the finish coat without the undercoat and the primer.
- see images of Blistering
this is a defect where water ingress from the rear background of the material causing the paint to explode with moister inside in blister forms. This may also due to the lack of waiting for the base to dry sufficiently prior to painting, the moisture in the base may result in water escaping eventually which causes the blistering formation.
- see images of crazing
- defective cycle- the waiting time for drying of the surface is not sufficient and this causes the paint to be defective as the moisture attempt to surface.
- see images of Run/sagging
- see images of wrinkling
- Bloom– cloudy and patches on gloss surfaces
- The bleeding-background material is dissolving and causes discoloration.
- Cissing-smooth surfaces repelling the coating of paints
- see images of chalking
- see images of Flashing
- Fading as a result of color pigments fading due to the UV rays
- Flaking-as a result of poor adhesion of painting coats
- Grinning-as a result of background paint showing due to lack of opacity
- Saponification-results of chemical reaction such as alkali surfaces
Reasons for poor quality painting
Some of the reasons for poor quality may be:
a.)poor choice of painting coating schemes,
b.)application on a damp background,
c.)poor preparation works,
d.)poor quality paints types,
f) poor selection of the type of paint for the wrong purpose
Prepared by System Design Architect, Architect Eco Green Perumal Nagapushnam
System Design Architect