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<br />A. Appendix
<br />A.1 FUNCTION — White Metal Blast Cleaning (SSPC-
<br />P 5) provides a greater degree of cleaning than Near -
<br />White Blast Cleaning (SSPC-SP 10). It should be used
<br />where the highest degree of blast cleaning is required. The
<br />primary functions of blast cleaning before painting are: (a)
<br />to remove material from the surface that can cause early
<br />failure of the coating system, and (b) to obtain a suitable
<br />surface roughness.
<br />A.2 ABRASIVE SELECTION — Types of metallic and
<br />non-metallic abrasives are discussed in the Surface
<br />Preparation Commentary (SSPC-SP COM). It is important
<br />to recognize that blasting abrasives may become embed-
<br />ded in or leave residues on the surface of the steel during
<br />preparation. While normally such embedment or residues
<br />are not detrimental, care should be taken (particularly if
<br />the prepared steel is to be used in an immersion environ-
<br />ment) to assure that the abrasive is free from detrimental
<br />amounts of water soluble, solvent soluble, acid soluble, or
<br />other such soluble materials.
<br />A.3 SURFACE PROFILE — Surface profile is the
<br />roughness of the surface which results from abrasive blast
<br />cleaning. The profile depth (or height) is dependent upon
<br />the size, type, and hardness of the abrasive, particle veloci-
<br />ty and angle of impact, hardness of the surface, amount of
<br />recycling, and the proper maintenance of working mixtures
<br />0 grit and/or shot.
<br />The allowable minimum/maximum height of profile is
<br />usually dependent upon the thickness of the paint to be
<br />applied. Large particle sized abrasives (particularly
<br />metallic) can produce a profile which may be too deep to
<br />be adequately covered by a single thin film coat. Accord-
<br />ingly, it is recommended that the use of larger abrasives be
<br />avoided in these cases. However, larger abrasives may be
<br />needed for thick film coatings or to facilitate removal of
<br />heavy mill scale or rust. if control of profile (min-
<br />imum/maximum) is deemed to be significant to coatings
<br />performance, it should be addressed in the procurement
<br />documents (project specification).
<br />Typical maximum profile heights achieved with com-
<br />mercial abrasive media are shown in Table 8 of the Surface
<br />Preparation Commentary (SSPC-SP COM). Methods (i.e.,
<br />comparators, replica tape, depth micrometers) are
<br />available to aid in estimating the profile of surfaces blast
<br />cleaned with sand, steel grit, and steel shot.
<br />A.4 VISUAL STANDARDS — Note that the use of
<br />visual standards in conjunction with this specification is
<br />required only when they are specified in the procurement
<br />documents (project specification) covering the work. It is
<br />recommended, however, that the use of visual standards
<br />be made mandatory in the procurement documents (proj-
<br />ect specification)
<br />is
<br />toSSPC-SP 5
<br />March 1, 1985
<br />SSPC-Vis 1, "Pictorial Surface Preparation Standards
<br />for Painting Steel Surfaces," provides color photographs
<br />for the various grades of surface preparation as a function
<br />of the initial condition of the steel. The following table lists
<br />the pictorial standards for this specification that are ap-
<br />plicable to the rust grades given.
<br />Adherent Mill Rusting Mill Pitted and
<br />Rust Grade Scale Scale Rusted Rusted
<br />Pictorial
<br />Standards A Sa 3 B Sa 3 C Sa 3 D Sa 3
<br />Many other visual standards are available and are
<br />described in Section 7 of the Commentary (SSPC-SP COM).
<br />A.5 SURFACE IMPERFECTIONS — Surface imperfec-
<br />tions can cause premature failure when the service is
<br />severe. Coatings tend to pull away from sharp edges and
<br />projections, leaving little or no coating to protect the
<br />underlying steel. Other features which are difficult to prop-
<br />erly cover and protect include crevices, weld porosity,
<br />laminations, etc. The high cost of the methods to remedy
<br />the surface imperfections requires weighing the benefits
<br />of edge rounding, weld spatter removal, etc., versus a
<br />potential coating failure.
<br />Poorly adhering contaminants, such as weld slag
<br />residues, loose weld spatter, and some minor surface
<br />laminations, may be removed during the blast cleaning
<br />operation. Other surface defects (steel laminations, weld
<br />porosities, or deep corrosion pits) may not be evident until
<br />the surface preparation has been completed. Therefore,
<br />proper planning for such surface repair work is essential
<br />since the timing of the repairs may occur before, during, or
<br />after the blast cleaning operation. Section 4 of the Com-
<br />mentary (SSPC-SP COM) contains additional information
<br />on surface imperfections.
<br />A.6 CHEMICAL CONTAMINATION — Steel contam-
<br />inated with soluble salts (i.e., chlorides and sulfates)
<br />develops rust -back rapidly at intermediate and high
<br />humidities. These soluble salts can be present on the steel
<br />surface prior to blast cleaning as a result of atmospheric
<br />contamination. In addition, contaminants can be de-
<br />posited on the steel surface during blast cleaning
<br />whenever the abrasive is contaminated. Therefore, rust -
<br />back can be minimized by removing these salts from the
<br />steel surface, preferably before blast cleaning, and
<br />eliminating sources of recontamination during and after
<br />blast cleaning. Identification of the contaminants along
<br />with their concentrations may be obtained from laboratory
<br />and field tests. A number of tests for soluble salts are now
<br />under study by the SSPC, ASTM, Maritime Administration,
<br />and ISO.
<br />A.7 RUST -BACK — Rust -back (rerusting) occurs when
<br />freshly cleaned steel is exposed to conditions of high
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