The Swedish Standard Institution has published a very useful handbook entitled “Degrees of rust on steel surfaces and degrees for pretreatment of steel surfaces prior to application of rust preventing primers” (Swedish Standard 055900-1967). Standardized preparation grades in case of blast cleaning are Sa 0, Sa 1, Sa 2, Sa 21 /2 and Sa 3, for manual scraping and wirebrushing: St 2 and St 3.
- Blast-cleaning
Sa 0: No preparation of surface.
Sa 1: Light blast cleaning. The jet passes rapidly over the surface so that loose millscale, rust and foreign matter are removed.
Sa 2: Thorough blast cleaning. The jet is passed over the surface long enough to remove all mill scale and rust and practically all foreign matter. Finally, the surface is cleaned with a vacuum cleaner, clean and dry compressed air or a clean brush. It should be grayish in colour.
Sa 2,5: Very thorough blast-cleaning. The mill scale, rust and foreign matter shall be removed to such an extent that only remnants appear as shades on the surface. Finally, the surface is cleaned with a vacuum cleaner, clean and dry compressed air or a clean brush.
Sa 3: Blast cleaning to white metal. The jet passes over the surface, long enough to remove all mill scale, rust and foreign matter. Finally, the surface is cleaned with a vacuum cleaner, clean and dry compressed air or a clean brush. It should then have a uniform metallic colour.
- Manual scraping and wirebrushing
St 2: Thorough scraping (with hard-metal scraper) and wirebrushing – disc-sanding – etc. During the operation all the loose scale, rust and foreign matter should be removed. Finally, the surface is cleaned with a vacuum cleaner, clean and dry compressed air or a clean brush. It should then have a faint metallic sheen.
St 3: Extremely thorough scraping and wirebrushing, disc sanding, power brushing etc. Surface preparation as for St 2, but considerably more accurate. After removing the dust, the surface should have a pronounced metallic sheen.
Further reading: ABS Guidance Notes on “The Application and Maintenance of Marine Coating Systems”.