Standard Guide for Painting Inspectors (Metal Substrates) – ASTM D3276
Surface Preparation Methods and Requirements
The inspector should determine whether the specifications are being followed with reference to the painting or prohibition of painting of contact surfaces in bolted or riveted surfaces of construction. He should ensure that surfaces not in contact but that will be made inaccessible by assembly or erection, have received the full number of specified coats before they become inaccessible.
Surface Preparation Commentary for Metal Substrates – SSPC-SP COM
Selection of Abrasives, Blast Cleaning Parameters, and Equipment
Centrifugal Blast Cleaning
A blast cleaning process (usually enclosed) that uses rotating, motor-driven, bladed wheels to hurl abrasive (usually steel shot, steel grit, or a shot/grit mixture) at the surface being cleaned. The abrasive material is fed to the center of the wheel and then onto blades that radiate from the hub. Centrifugal force produced by the turning wheel accelerates the abrasive to the ends of the blades from which it is thrown against the surface.
Metallic Abrasives
Steel shot consists of nearly spherical particles of steel obtained by granulating a molten stream of metal with water, air, etc. Steel shot will generally conform to SSPC-AB 3 “Ferrous Metallic Abrasive” in terms of hardness, chemical composition, size, and microstructure. Cast steel grit consists of angular particles produced by crushing steel shot (SAE J827). Steel grit is available in a wide range of hardness, from 30 to 66 on the Rockwell C scale (Rc), produced by varying the tempering time cycles to which the grit is subjected. Generally, three hardness ranges are most commonly produced: 40 to 50 Rc, 55 to 60 Rc, and 60 to 66 Rc. The first two hardness ranges are used for structural steel, and the latter is used primarily for selective application where deep, consistent, sharp etched finishes are required, or where moderate etches on extremely hard surfaces are needed. In addition to steel shot and grit, iron grit may also conform to SSPC-AB 3. Iron abrasive is characterized by high carbon content and hardness typically over 55 Rc. The particle shape requirements for iron grit are less stringent than those for steel grit.
Iron abrasive cannot be recycled as many times as steel abrasive, but it can be recycled many more times than non-metallic abrasives. Iron abrasive is also less costly to purchase than steel abrasive. In situations where full recovery is not possible, such as a bridge containment, iron abrasive has an economic advantage over steel abrasive. Steel abrasive is more cost effective in centrifugal wheel machines in a shop.
Metallic shot will produce a peened surface texture, whereas grit produces more of an etched surface texture. The etch becomes more pronounced with increasing abrasive hardness.
Typical applications of various steel abrasives, referring to rust condition classifications described in Section 4.1, are:
• Shot: Commonly used on new steel (rust conditions A and B) to remove mill scale with centrifugal wheel machines
• Grit (40-50 Rc): Most effective on rust conditions C and D, but also commonly used for rust conditions A and B
• Shot/Grit Mixture (Shot 40-50 Rc/Grit 55-60 Rc): Used on new steel to remove both mill scale and rust. Shot/grit mixes demand careful attention and close control of abrasive additions to maintain the shot/grit ratio.
AISC American Institute of Steel Construction, https://www.aisc.org/
Coatings & Corrosion Protection, https://www.aisc.org/why-steel/resources/coatings-and-corrosion-protection/
Faying Surfaces, https://www.aisc.org/steel-solutions-center/engineering-faqs/6.7.-faying-surfaces/
In snug-tight and fully tensioned bearing connections, paint is unconditionally permitted on the faying surfaces. In slipcritical connections, however, if paint is present, it must be a qualified paint. A qualified paint is one that has been tested in accordance with the RCSC Specification Appendix A and offers a defined slip-coefficient. Other paints that do not offer a defined slip-coefficient are not permitted in areas closer than one bolt steelwise MAY 2015 steelwise diameter but not less than 1 in. from the edge of any hole and in all areas within the bolt pattern of slip-critical connections, even when due to inadvertent over-spray.
In a slip-critical connection, the faying surfaces are those that resist relative movement (or slip) of the plies. This occurs on the contact surfaces between the plies, not those surfaces under the bolt head or nut.
What is the difference between the surface preparation requirements for class A and B surfaces in slip-critical connections?
With uncoated faying surfaces, clean mill scale provides a Class A slip resistance, μ=0.30, whereas blast cleaning is required to obtain the higher Class B slip coefficient, μ=0.50. With painted faying surfaces, the slip resistance is determined by the tested performance of the paint system as meeting Class A, B, or some other intermediate slip coefficient and the steel to be painted must be blast-cleaned in all cases. Roughened (see 6.7.4) hot-dip galvanized surfaces also provide a Class A slip coefficient, μ=0.30.
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