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SSPC-SP 10 March 1, 1985 posited on the steel surface during blast cleaning A.9 WET ABRASIVE BLAST CLEANING — Steel that whenever the abrasive is contaminated. Therefore, rust- is wet abrasive blast cleaned may rust rapidly.Clean water back can be minimized by removing these salts from the should be used for rinsing (studies have shown that water steel surface, preferably before blast cleaning and of at least 15,000 ohm-cm resistivity is preferred). It may be eliminating sources of recontamination during and after necessary that inhibitors be added to the water or applied blast cleaning. Identification of the contaminants along to the surface immediately after blast cleaning to tem- with their concentrations may be obtained from laboratory porarily prevent rust formation.The coating should then be and field tests.A number of tests for soluble salts are now applied before any rusting is visible. One inhibitive treat- under study by the SSPC, ASTM, Maritime Administration, ment for blast cleaned surfaces is water containing 0.32% and ISO, sodium nitrite and 1.28% by weight secondary ammonium A.7 RUST-BACK-- Rust-back(rerusting)occurs when phosphate (dibasic). freshly cleaned steel is exposed to conditions of high CAUTION: Some inhibitive treatments may interfere with humidity, moisture, contamination, or a corrosive at- the performance of certain coating systems. mosphere. The time interval between blast cleaning and A.10 FILM THICKNESS — It is essential that ample rust-back will vary greatly from one environment to coating be applied after blast cleaning to adequately cover another. Under mild ambient conditions it is best to blast the peaks of the surface profile. The dry paint film clean and coat a surface the same day. Severe conditions thickness above the peaks of the profile should equal the may require coating more quickly while for exposure under thickness known to be needed for the desired protection. If controlled conditions the coating time may be extended. the dry film thickness over the peaks is inadequate, Under no circumstances should the steel be permitted to premature rust-through or failure will occur.To assure that rust-back before painting regardless of the time elapsed coating thicknesses are properly measured, refer to SSPC- (see Appendix A.6), PA 2, "Measurement of Dry Paint Thickness with Magnetic Gages." A.8 DEW POINT — Moisture condenses on any sur- A.11 MAINTENANCE AND REPAIR PAINTING — face that is colder than the dew point of the surrounding When this specification is used in maintenance painting, air. It is, therefore, recommended that the temperature of specific instructions should be given on the extent of sur- steel surface be at least 5 degrees F (3 degrees C) above face to be blast cleaned or spot blast cleaned to this the dew point during dry blast cleaning operations.It is ad- degree of cleanliness. SSPC-PA Guide 4, "Guide to Main- visable to visually inspect for moisture and periodically tenance Repainting with Oil Base or Alkyd Painting check the surface temperature and dew point during blast Systems," provides a description of accepted practices for cleaning operations. It is important that the application of retaining old sound paint, removing unsound paint, paint over a damp surface be avoided. feathering, and spot cleaning. i } l I r� ai z• 4� s ..1 -�Y 3 56