Introduction

Paint manufacturers base their recommendation for the protection of steel, on a careful assessment of the structure to be painted, its environment, how long the paint film has to last and ease of maintenance.  Good surface preparation, effective application and proper consideration of the paint system as a whole are essential to achieve maximum protection and optimum performance.

Various standards for cleaning steel prior to painting exist, among them the SWEDISH STANDARDS INSTITUTION SIS 055900 - 1967 : Surface Preparations Standards for Painting Steel Surface is widely used and accepted world-wide. Its cleaning degrees such as Sa 2, Sa 2½, etc. are universally recognized.

The Swedish Standard SIS 055900 was the first to use pictorial representations to specify degrees of cleaning. This standard has now been superseded by the International Standard ISO 8501-1:1988 which uses the same photos as the SIS standard plus four additional photos from the German standard DIN 55928, Part 4, Supplement 4 (flame cleaning).

Other prominent standards are:

Steel Structures Painting Council (USA): Surface Preparation Specifications (SSPC-SP 2, 3, 5, 6, 7, and 10) 

British Standards Institution: Surface Finish of Blast-cleaned steel for Painting (BS 4232)

DIN 55928: Protection of steel structures from corrosion by organic and metallic coatings. Preparation and testing of surfaces (Germany)

The British Standard uses drawings to indicate degrees of cleaning whereas the American and German standards use the same photos as ISO 8501 -1:1988. In addition, DIN 55928 includes photos of secondary surface preparation.

With the exception of BS 4232 the above standards take into account the state of the raw steel surface before cleaning. Four rust grades are specified:

A: Steel surface largely covered with adherent mill scale but little, if any, rust.

B: Steel surface which has begun to rust and from which the mill scale has begun to flake.

C: Steel surface on which the mill scale has rusted away or from which it can be scraped, but with slight pitting visible under normal vision.

D: Steel surface on which the mill scale has rusted away and on which general pitting is visible under normal vision.

Throughout this procedure, reference is made to the various degrees of cleaning of ISO 8501-1:1988. These are listed in the following pages and the texts are quoted literally.

Note: The ‘notes” and ‘photographs’ mentioned in the texts are not included in this manual. The information is however available at our Technical Department, Mauvilac Industries Ltd.

Blast Cleaning, Sa

Surface preparation by blast-cleaning is designated by the letters “Sa”.

Prior to blast-cleaning, any heavy layers of rust shall be removed by chipping. Visible oil, grease and dirt shall also be removed.

After blast-cleaning the surface shall be cleaned from loose dust and debris.

Sa 1, Light Blast-Cleaning

When viewed without magnification, the surface shall be free from visible oil, grease and dirt, and from poorly adhering mill scale, rust, paint coatings and foreign matter. See photographs B Sa 1, C Sa 1, and D Sa 1.

Sa 2, Thorough Blast-Cleaning

When viewed without magnification, the surface shall be free from visible oil, grease, and dirt, and from most of the mill scale, rust, paint coatings and foreign matter. Any residual contamination shall be firmly adhering. See photographs B Sa 2, C Sa 2 and D Sa 2.

Sa 2½, Very Thorough Blast-Cleaning

When viewed without magnification, the surface shall be free from visible oil, grease and dirt, and from mill scale, rust, paint coatings and foreign matter. Any remaining traces of contamination shall show only as slight stains in the form of spots or stripes. See photographs A Sa 2½, B Sa 2½, C Sa 2½ and D Sa 2½.

Sa 3, Blast-Cleaning to Visually Clean Steel

Hand & Power Tool Cleaning, St

Surface preparation by hand and power tool cleaning, such as scraping, wire brushing, machine-brushing and grinding, is designated by the letters “St”. 

Prior to hand and power tool cleaning, any heavy layers of rust shall be removed by chipping. Visible oil, grease and dirt shall also be removed.

After hand and power tool cleaning, the surface shall be cleaned from loose dust and debris. 

St 2, Thorough Hand & Power Tool Cleaning

When viewed without magnification, the surface shall be free from visible oil, grease and dirt, and from poorly adhering mill scale, rust, paint coatings and foreign matter.

St 3, Very Thorough Hand & Power Tool Cleaning

As for St 2, but the surface shall be treated much more thoroughly to give a metallic sheen arising from the metallic substrate.

Flame Cleaning, Fl

Surface preparation by flame cleaning is designated by the letters “Fl”. Prior to flame cleaning, any heavy layers of rust shall be removed by chipping. After flame cleaning, the surface shall be cleaned by power tool wire-brushing.

When viewed without magnification, the surface shall be free from mill scale, rust, paint coatings and foreign matter. Any remaining residues shall show only as a discoloration of the surface (shades of different colours). See photographs A Fl, B Fl, C Fl and D Fl. 

Abrasive blast cleaning methods and hand and power-tool cleaning methods, including treatment prior to,  and after,  the cleaning procedures,  are  fully  described  in  ISO   8504-2: 1992  and   ISO 8504-3:1992 respectively.

Surface Profile

Blast cleaning produces a surface profile the depth of which depends on the size, shape and hardness of the abrasive used and the volume and pressure of air.  A rough guide to the depth of profile obtained using different media is:

• Fine sand or slag: 30 microns
• Coarse sand or slag: 80 microns
• BSS 30 Steel Shot: 45 microns
• BSS 18 Steel Shot: 90 microns
• 14/18 Chopped piano wire: 95 microns

A useful guide to follow is that the peak to valley profile should not exceed 1/3 of the dry film thickness above the peaks e.g. for a total dry film thickness of 240 microns the peak to valley profile should not exceed 80 microns.