The quality of steel surface preparation is the single most important function that can influence paint performance and has a direct relation with the lifetime of a system.
For optimum service life, the steel surface preparation must remove all contaminants that might impair performance and offer the best adhesion of the paint system.
One of the most commonly used methods of steel surface preparation, blast cleaning involves an abrasive jet of particles in a compressed air stream to remove rust, millscale, old paint and other impurities. Remember that prior to any blast work, steel should be degreased and all weld spatters removed.
Different degrees of surface cleanliness can be achieved depending on the original surface condition, the abrasives used and the length of time the surface is exposed to the abrasive jet. The surface appearance resulting from blast cleaning has been defined by several different bodies with the most widely used being the Swedish Standard SIS 05 59 00.
In this post, we will identify the different surface finish standards identified by the Swedish Standard SIS 05 59 00
See the full range of Airblast Australia blasting equipment.
Steel Surface Preparation Standard SIS 05 59 00
Steel Surface Preparation Brush Off Blast Cleaning – SA 1
Brush Off Blast Cleaning refers to the removal of loose mill scale, loose rust, and loose paint by the impact of abrasives propelled through nozzles or by centrifugal wheels. It is not intended that the surface shall be free of all mill scale, rust, and paint. The remaining mill scale, rust, and paint should be tight and the surface should be sufficiently abraded to provide good adhesion and bonding of paint. A brush-off blast cleaned surface finish is defined as one from which all oil, grease, dirt, rust scale, loose mill scale, loose rust and loose paint or coatings are removed completely but tight mill scale and tightly adhered rust, paint and coatings are permitted to remain provided that all mill scale and rust have been exposed to the abrasive blast pattern sufficiently to expose numerous flecks of the underlying metal fairly uniformly distributed over the entire surface.
Commercial Blast Cleaning – SA 2
Removal of partial mill scale, rust, rust scale, paint or foreign matter by the use of abrasives propelled through nozzles or by centrifugal wheels, to the degree specified. A commercial blast cleaned surface finish is defined as one from which all oil, grease, dirt, rust scale and foreign matter have been completely removed from the surface and all rust, mill scale and old paint have been completely removed except for slight shadows, streaks, or discoloration caused by rust stain, mill scale oxides or slight, tight residues of paint or coating that may remain if the surface is pitted, slight residues of rust or paint may be found in the bottom of pits at least two-thirds of each square inch of surface area shall be free of all visible residues and the remainder shall be limited to the light discoloration, slight staining or tight residues mentioned above.
Near White Blast Cleaning – SA 2-1/2
Removal of nearly all mill scale, rust, rust scale, paint, or foreign matter by the use of abrasives propelled through nozzles or by centrifugal wheels, to the degree hereafter specified. A near-white blast cleaned surface finish is defined as one from which all oil, grease, dirt, mill scale, rust, corrosion products, oxides, paint or other foreign matter have been completely removed from the surface except for very light shadows, very slight streaks or slight discolorations caused by rust stain, mill scale oxides, or light, tight residues of paint or coating that may remain. At least 95 percent of each square inch of surface area shall be free of all visible residues, and the remainder shall be limited to the light discoloration mentioned above
White Metal Blast Cleaning – SA 3
Removal of all mill scale, rust, rust scale, paint or foreign matter by the use of abrasives propelled through nozzles or by centrifugal wheels. A white metal blast cleaned surface finish is defined as a surface with a gray-white, uniform metallic colour, slightly roughened to form a suitable anchor pattern for coatings. The surface, when viewed without magnification, shall be free of all oil, grease, dirt, visible mill scale, rust, corrosion products, oxides, paint, or any other foreign matter. White Metal Blast Cleaning should be used where maximum performance of the protective coating is required due to severe environmental conditions such as immersion in water or chemicals.
Besides the cleanliness of the steel, consideration needs to be given to the profile roughness created by the impact of abrasive on the steel surface.
The substrate profile is generated by:
1. Shape, type and grading of abrasive.
2. Blasting method and velocity of abrasive impaction.
3. Steel condition prior to abrasive blasting.
The etched profile of the surface enables adhesion of the protective paint coatings. Of great importance is the level at which this is achieved as too severe a profile will cause a waste of paint and time in application whilst a profile which is too shallow could result in a lack of adhesion.
The profile of the surface can be measured by various items of test equipment. The best method of obtaining the correct profile is by ensuring that the correct blasting equipment is used coupled with the correct type and size of abrasive which we will cover in future posts.
Airblast has become the industry standard in manufacturing and supplying surface finishing equipment worldwide. With showrooms in Perth, Brisbane and Melbourne we can offer product demonstrations, sales and service for all of our abrasives, spray equipment and ppe products. Contact us to find out more.[insert page=’23’ display=’excerpt’]