The lifetime of pre-finished steel roof or wall cladding can be determined by using a suitable coating material and applying it properly. A contractor who chooses the wrong kind of finish may damage the cladding product and increase its costs.
Most steel used in construction is already coated for corrosion resistance. This work is carried out before any finial finish and profiling. Any subsequent coatings may be divided into metal and organic finishes.
Metal finishes
Zinc is the most common coating for construction steel applications. It is suitable for light coatings and protects steel during storage, but it looks dull and is not used for exterior decoration. It will also deteriorate slowly in rainfall.
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| Coating Types for Pre-Finished Steel |
Aluminium-zinc alloys: ZalutiteThis is an alloy of 55 per cent aluminium, 43.5 per cent zinc and 1.5 per cent silicon. It has between two and six times the lifetime of plain zinc coating and can be used externally. It is a useful substrate for organic colour coatings. Its best use is in cladding agricultural building internally, such as barns used for storing dry crops.
Terne: Lead-tin alloy
A lead tin allow with 12 per cent tin, terne can be spot welded, brazed and seamed. It is useful in industrial roofing that could be vulnerable to severe corrosion risks.
Organic coatingsThe success of steel as roofing and cladding material has led to the development of a huge range of choices of coloured organic coatings that may be used internally and externally.
The term 'organic' applies to a sheet of steel that is drawn through a process where a thin film of PVC, plastisol or polyvinylidene fluoride is applied on both sides of the sheet. PVC plastisol is a proprietary product of Colorcoat (
http://www.colorcoat-online.com). The bond between the film and the metal is such that the coating and the bending of the cladding can be carried out in one process without the protective coating being damaged.
PlastisolPlastisol is colourfast, high durable and abrasion resistance. It can be applied in 200-micron thick metallic or non-metallic colour finishes. It is suitable for interior and exterior work in all locations with the exception of tropical climates.
Polyvinylidene fluoridePolyvinylidene fluoride (PVDF) has a spectacular property of retaining its gloss finish and ca last for over 30 years when applied correctly. However, it has a lower abrasion resistance than plastisol.
PolyestersPolyesters are a cheap alternative for organic coatings of steel strips. They come in a wide colour range but will need repainting every seven to 10 years. Polyesters are also preferred more in recent times because they are more environmentally friendly than plastisol. The addition of acrylic beads to the mixture improves its strength and the coating can last up to 40 years. An even cheaper version of this type of coating is silicone polyester.
AgristeelAgristeel is a multiple layer protective coating that has been developed for use in single skin buildings. The combination of layers gives a 35-micron thick coating that can last about 15 years. It can be used both externally and internally, especially where corrosive conditions could be present such as stables and other animal housing.