Metallic-coated and pre-painted sheet

Why coated sheet matters

The roof and wall cladding of a pre-engineered building is made from thin high-strength steel sheet – typically 0.40–0.60 mm base metal thickness for cladding and up to about 1.0 mm for liners, gutters and some panels. At this thickness, corrosion protection depends entirely on the metallic coating applied at the mill and, for coloured sheet, on the pre-painted (coil-coated) paint system above it. Choosing the right combination for the building's environment determines how long the roof sheeting and wall sheeting will last before maintenance or replacement.

Metallic coating types

Coating mass is expressed as total grams per square metre on both sides: for example Z275 (275 g/m², equivalent to US G90), AZ150 (150 g/m², close to US AZ50) and AZ200 for more severe environments. Higher mass gives longer life but is less formable.

Pre-painted (coil-coated) systems

Pre-painted sheet is cleaned, pretreated, primed and top-coated on a continuous coil coating line, then baked. Typical systems:

• Polyester (PE): about 20 µm topcoat on 5 µm primer – economical, standard for industrial buildings.
• Silicone-modified polyester (SMP) and high-durability polyester (HDP): better gloss and colour retention.
• PVDF (fluoropolymer): highest UV and colour stability, for architectural and coastal projects.
• Backer coat: 5–10 µm on the reverse, heavier where the underside is exposed (canopies, open sheds, humid interiors).

Product standards include ASTM A755 (US), AS/NZS 2728 (Australia / NZ, classifying products by environment) and EN 10169 (Europe, with corrosion categories RC1–RC5 and UV categories RUV1–RUV4).

Selecting coating for the environment

• Avoid run-off from copper, lead or inert materials onto AZ and Z coatings; avoid contact with wet timber, concrete and soil.
• Livestock, fertiliser, swimming pool and food processing buildings need specific coatings.
• Fasteners must be equal or better in durability – see bolts and fasteners.

Paint system build-ups and coating tests

The paint system is defined by resin type, film thickness and the tests it passes. A clear specification states all three for both the exposed face and the reverse side.

Coil coatings are qualified by a family of laboratory tests. North American data sheets usually cite ASTM methods; European suppliers cite the equivalent EN 13523 parts.

Test results are used to compare products and control batches; they do not predict service life in years. Long-term claims should be backed by outdoor exposure data and a written warranty for the specific site distance from the coast and the industrial environment. For very aggressive sites, aluminium sheet (to ASTM B209, EN 485 / EN 1396 or AS/NZS 1734), stainless steel or GRP cladding may outlast any coated steel.

Sources and WSC supply

In Vietnam, metallic-coated and pre-painted coil is produced by several mills, including NS BlueScope Vietnam, Hoa Sen Group, Dong A, Nam Kim and Southern Steel Sheet; imported coil is also used to meet project specifications. World Steel specifies the substrate grade (e.g. G550 or S350GD), coating type and mass, and paint system in each cladding package, and supplies the matching coil-coated trims, gutters and flashings. See also paint and coating systems for structural steel.