Generally all new structural steel is specified for

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Generally, all new structural steel is specified for Near-White Metal Blast Clean, SSPC-SP10 or Commercial Blast Cleaning, SSPC-SP6. Requirements for Preparation of Bare Metal Surface preparation is the most critical procedure for successful performance of a coating system. Surface prepa- ration consists of cleaning the bare steel or previously coated surface. It includes establishing an appropriate pro-
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PAGE 43 SYSTEMS file of bare steel and/or an acceptable surface condition of the previously coated surface. Cleaning and surface profile are both critical to the performance of the paint system. Cleaning of the surface includes removal of all soluble salts, oils, grease, dirt, dust and any other contaminants, by whatever means necessary, that will adversely affect the adhesion of the paint coat to the surface. Ensuring that recontamination does not occur, such as from airborne dusts, is also critical to a successful recoating project. When blast cleaning is used to prepare the surface, the compressed air used to propel the abrasive shall be test- ed periodically to ensure it is free from oil and moisture and sufficient volume and pressure to clean the surface in a productive manner to the required profile. For inorganic zinc prime coatings, surfaces shall be cleaned to a level as obtained by SSPC-SP10 for new con- struction. For other primer coats, the surface preferably should be cleaned to SSPC-SP6 or SSPC-SP3 may be acceptable. Preparation Methods and Specifications Power washing. Consists of blasting the steel with water at a pressure of 800 psi to 5,000 psi with the nozzle not more than 12 in. from the surface. If residue containing hazardous substances is removed during the washing process, the water will have to be strained to remove the contaminants or disposed of as hazardous waste. Abrasives. Any abrasives used shall be free of oil, moisture, hazardous substances (i.e., lead, chromium, mercu- ry, etc.) and corrosive constituents (i.e., chlorides, sulfates, salts, etc.). Non-steel abrasives shall be in accordance with SSPC-AB1, Specifications for Mineral and Slag Abrasives . Abrasives with "free" silica contents in excess of one percent should not be used. As surface profile is critical to paint system performance, it must be controlled at the time it is produced, i.e., when the blasting work is conducted. This can be accomplished by controlling the range of particle size and shape in the abrasive used for blasting. The SSPC has published a reference guide, Visual Standard for Abrasive Blast Cleaned Steel SSPC-Vis1-89 . When using automated recycling blasting equipment with steel shot or grit, it is important to consider that a work- ing mix is developed through use, then maintained by addition of suitable quantities of steel abrasive of the cor- rect size range. This mixture of sizes is commonly called the work mix or operating mix. It is important to empha- size that this is indeed a mixture of a range of particle sizes, shape and hardness that is necessary to produce the correct profile. Larger particle sizes are suitable for removing heavy build-ups of mill scale or rust. Smaller size
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  • Fall '19
  • Structural steel

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