Substrate categories

New/unpainted

Hot-rolled steel - Grade A/Grades B & C / Grade D

Hot-rolled mild steel bears a layer of millscale. Initially this is tightly adhering but eventually on exposure it loosens and falls away. It may take several months or even years for this to occur depending upon the conditions of exposure and whether or not the steel has been painted

For visual identification of Grades A B C and D Steel see BS 7079 Part 11989 and ISO Standard 8501 Part 11988 which defines Pictorial Surface Preparation Standards for painting steel surfaces. The nomenclature in this Guide aligns to that standard.

Cold-rolled steel - non-structural new or weathered

Cold rolled steel is normally available in thin gauge plate or sheet form. It is free from millscale. When blast cleaning is employed consideration must be given to likely distortion.

Cast iron - new or weathered

The casting scale formed on cast iron is more adherent than the millscale on mild steel and has some protective value. If the scale is removed cast iron corrodes at about the same rate as mild steel. This may not be evident because the corrosion process leaves behind a non-metallic residue which largely retains the appearance of the original metal although its mechanical strength is negligible.

Wrought iron - new or weathered

In respect of painting this is similar to mild steel although its corrosion rate may differ slightly according to its composition.