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Paint Specification Preparation Standards for Metal
Paint specifications produced for larger painting projects are often heavily abbreviated, below we have focused on some of the most common preparation standards for metal surfaces that are often abbreviated – blast cleaning to pure metal being Sa 3, for example. Against each preparation standard we have given a short description of how the metal surface should be prepared and how it should appear after preparation is complete. In addition to the processes below it is assumed that prior to treatment the surface has been cleaned of any surface contaminants (dirt, grease, oil, etc.) and that any heavier layers of rust if present have been removed by chipping.
Sa 3 – Blast cleaning to pure metal. Mill scale, rust and foreign matter shall be removed completely. Finally, the surface is cleaned with a vacuum cleaner, clean dry compressed air or a clean brush. The surface of the steel is metallic-white, there are no traces of corrosion or shadow after blast cleaning.
Sa 2.5 – Very thorough blast cleaning. Mill Scale, rust and foreign matter shall be removed to the extent that the only traces remaining are slight stains in the forms of spots or stripes. Finally the surface is cleaned with a vacuum cleaner, clean dry compressed air or clean brush. The surface is metallic-white, there are no traces of corrosion but some lightly coloured greyish shadows might remain.
Sa 2 – Thorough blast cleaning. Almost all mill scale, rust and foreign matter shall be removed. Some points of corrosion might remain and some very adherent mill scale particles after blast cleaning. Finally the surface is cleaned with a vacuum cleaner, clean dry compressed air or a clean brush.
Sa 1 – Light blast cleaning (Sweep blasting). Loose mill scale, rust and foreign matter shall be removed. Some oxidised parts and scale remain. The metal maintains the colour of rust.
St 3 – Very thorough scraping and wire brushing (machine brushing, abrasive discs, grinding, etc.) Surface preparation as for St 2, but much more thoroughly. Operation usually applied to pre-primed steel or de-scaled raw steel. After removal of dust, the surface shall have a pronounced metallic sheen. The surface must have a very clean metallic-white appearance.
St 2 – Thorough scraping and wire brushing (machine brushing, abrasive discs, grinding, etc.) The treatment shall remove loose mill scale, rust and foreign matter. Finally, the surface is cleaned with a vacuum cleaner, clean dry compressed air or a clean brush. It should have a faint metallic sheen. The aim is to eliminate powdery, non-adherent and scaly corrosion. The appearance is a light metallic gloss.
Note: After blast cleaning iron or steel, the surface will have a rough profile which will affect the coverage of paint. The surface area is thus larger and hence will require more paint.