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SSPC•SP 10 March 1, 1985 and appropriate section of SSPC-SP COM. merciai abrasive media are shown in Table 8 of the Surface Subject Commentary Section Preparation Commentary (SSPC-SP COM). Methods (i.e., Abrasive Selection . . . . . . . . . . . . . . 5. comparators, replica tape, depth micrometers) are Degree of Cleaning11.10 available to aid in estimating the profile of surfaces blast . . . . . . . . . . . . cleaned with sand, steel grit, and steel shot. Film Thickness . . . . . . . . . . . . . . . . . 10. Wet Abrasive Blast Cleaning. . . . . . 9. A.4 VISUAL STANDARDS — Note that the use of Maintenance Painting . . . . . . . . . . . 3.2 visual standards in conjunction with this specification is Rust Back(Rerusting) S. required only when they are specified in the procurement Surface Profile . . . . . . . . . . . . . . . . . B. documents (project specification) covering the work. It is Visual Standards . . . . . . . . . . . . . . . 7. recommended, however, that the use of visual standards Weld Spatter . . . . . . . . . . . . . . . . . . . 4.1 be made mandatory in the procurement documents (proj- ect specification). A. Appendix SSPC-Vis 1, "Pictorial Surface Preparation Standards A.1 FUNCTION — Near-White Blast Cleaning (SSPC• for Painting Steel Surfaces," provides color photographs 10)provides a greater degree of cleaning than Cummer Sia for the various grades of surface preparation as a function tial Blast Cleaning (SSPC•SP 5) but fess than White Metal of the initial condition of the steel.The following table lists high the pictorial standards for this specification that are ap- Blast Cleaning(SSPC-SP 5)-It should be used where a hplicable to the rust grades listed below. degree of blast cleaning is required.The primary functions of blast cleaning before painting are:(a)to remove material Adherent Mill Rusting Mill Pitted and from the surface that can cause early failure of the coating Rust Grade Scale Scale Rusted Rusted system, and (b) to obtain a suitable surface roughness. Pictorial A.2 ABRASIVE SELECTION — Types of metallic and Standards A Sa 2-'/2. B Sa 2-'/z C Sa 2-1/2 D Sa 2-1/2 non-metallic abrasives are discussed in the Surface Many other visual standards are available and are Preparation`Commentary (SSPC-SP COM), it is important described in Section 7 of the Commentary (SSPC-SP to recognize that blasting abrasives may become embed- COM). ded in or leave residues on the surface of the steel during A.5 SURFACE IMPERFECTIONS -- Surface imperfec- preparation. While normally such embedment or residues tions can cause premature failure when the service is are not detrimental, care should be taken (particularly if severe. Coatings tend to pull away from sharp edges and the prepared steel is to be used in an immersion environ- projections, leaving little or no coating to protect the ment) to assure that the abrasive is free from detrimental underlying steel.Other features which are difficult to prop- amounts of water soluble, solvent soluble, acid soluble, or arty cover and protect include crevices, weld porosity, other such soluble materials. laminations, etc. The high cost of the methods to remedy A.3 SURFACE PROFILE — Surface profile is the the surface imperfections requires weighing the benefits roughness of the surface which results from abrasive blast of edge rounding, weld spatter removal, etc., versus a cleaning. The profile depth (or height) is dependent upon potential coating failure. the size,type,and hardness of the abrasive, particle veloci- Poorly adhering contaminants, such as weld slag ty and angle of impact, hardness of the surface, amount of residues, loose weld spatter, and some minor surface recycling,and the proper maintenance of working mixtures laminations, may be removed during the blast cleaning of grit andlor shot. operation. Other surface defects (steel laminations, weld The allowable minimumlmaximum height of profile is porosities, or deep corrosion pits)may not be evident until usually dependent upon the thickness of the paint to be the surface preparation has been completed. Therefore, applied. Large particle sized abrasives (particularly proper planning for such surface repair work is essential metallic) can produce a profile which may be too deep to since the timing of the repairs may occur before,during,or be adequately covered by a single thin film coat. Accor- after the blast cleaning operation. Section 4 of the Com- dingly, it is recommended that the use of larger abrasives mentary (SSPC-SP COM) contains additional information be avoided in these cases. However, larger abrasives may on surface imperfections. be needed for thick film coatings or to facilitate removal of A.6 CHEMICAL CONTAMINATION — Steel contam- heavy mill scale or rust. If control of profile(minimumlmax. inated with soluble salts (i.e., chlorides and sulfates) imum) is deemed to be significant to coatings perform- develops rust-back rapidly at intermediate and high ante, it should be addressed in the procurement documents (project specification). humidities.These soluble salts can be present on the steel Typical maximum profile heights achieved with tom- surface prior to blast cleaning as a result of atmospheric contamination. In addition, contaminants can be de- 55 -