Metal Finishing Guide Book


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Page 147 of 903

um (hexavalent) poses a health hazard, however, its use is discouraged. Phosphate coatings are usually applied from proprietary baths, all of which should be satisfactory as a paint base. Immersion for 2 to 4 minutes in the following solution will leave the cadmium surface stable enough for painting: Phosphoric acid (85% ortho), 10 oz/gal Zinc phosphate, 3 oz/gal Room temperature CHROMIUM A fine sandblasting with one of the hard abrasives ��� aluminum oxide, silicon carbide, etc. ��� to provide a matte but smooth finish, followed by solvent wiping to remove dust and a thin coat of primer, will develop satisfactory adhesion. Keep the time between blasting and priming as short as possible. COBALT Light sand blasting with aluminum oxide or silicon carbide to provide a matte finish, followed by solvent wiping to remove dust, will be adequate for priming. Wash primer may add additional topcoat adhesion if a traditional primer is not used. COPPER Conversion coatings on copper are usually not necessary; however, the black oxide treatments used for adhesive bonding of copper are also paintable. Removal of oily soils, followed by wash primer, will give sufficient topcoat adhesion. LEAD Cleaning to remove all oily soils will be satisfactory for lead. The dull natural surface resulting from atmospheric exposure should not be removed. Wash primer or a very thin coat of traditional primer is recommended but not required, provided the first topcoat is applied thin. MAGNESIUM Magnesium must be conversion coated or anodized before painting. Products used in mild environments and not subject to mechanical abuse can be conversion coated. The following treatment can be used: Immerse parts for 1 1/2���2 minutes with agitation in the following solution: Ammonium phosphate monobasic( NH4H2PO4), 16 oz/gal Ammonium sulfite [(NH4)2SO3.H2O)], 4 oz/gal Ammonia (30%), 2 fl oz/gal Room temperature After rinsing in cold water, immerse for 1 to 2 minutes in the following solution: Sodium hydroxide, 16 oz/gal Temperature, 160���190��F Mild agitation 144

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