Surface preparation, paint systems and galvanising

Surface preparation

Paint only lasts as long as its adhesion. ISO 8501-1 defines visual cleanliness grades for blast-cleaned steel: Sa 1 (light), Sa 2 (thorough), Sa 2½ (very thorough – only slight stains remain, the usual grade for structural paint systems) and Sa 3 (visually clean steel). The surface profile (roughness) and the removal of dust, salts and oil are checked as well.

Choosing a paint system: ISO 12944

ISO 12944 classifies the environment and the desired life of the coating.

Corrosivity category	Typical environment
C1 – very low	Heated interiors with clean air
C2 – low	Unheated buildings, rural atmospheres
C3 – medium	Urban and light industrial atmospheres, production halls with some humidity
C4 – high	Industrial areas, coastal areas with moderate salinity
C5 – very high	Industrial areas with high humidity, coastal areas with high salinity
CX – extreme	Offshore, extreme industrial exposure

Durability is expressed as low (L, up to 7 years), medium (M, 7–15 years), high (H, 15–25 years) or very high (VH, more than 25 years) – the time to first major maintenance, not a guarantee.

Dry film thickness (DFT)

Each coat has a nominal DFT in µm; the system total is measured with a gauge after curing. Too thin reduces protection; much too thick can crack. DFT readings are a standard QC record before release for shipment.

Hot-dip galvanising

Galvanising (ISO 1461) dips the finished member in molten zinc, coating inside and out, including edges and hollow sections, with a metallurgically bonded layer. It suits secondary steel, purlins, bracing and structures in harsh environments. Design for galvanising needs vent and drain holes in closed sections and attention to distortion of thin, asymmetric members. “Duplex” systems paint over galvanising for very long life.