Metal Finishing Guide Book

2012 Organic Finishing Guidebook Issue

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Page 54 of 331

be wiped off with a clean dry wiper. This way the soil is removed, not just spread around. Because these solvents find their way into the atmosphere, the wipers should be placed into closed containers until sent to reclamation or to hazardous waste disposal. Sealed plastic bags will work. "Old work," which is defined as having previously been painted or primed, gen- erally requires different handling, in general. If the work is to be stripped of the coating and started over, one should proceed with the stripping and, when com- pleted, remove or arrest the corrosion and start as if it were new work. If it is not necessary to remove the old paint, the surfaces can be prepared by me- chanically removing any loose material. Wire brushing by hand or with a motor or a light brush-off blast are all methods that work well. The wire brushes should be devoted to a substrate. Although stainless steel can work on all substrates, if it has been used on iron or copper, it should not be used on aluminum or magnesium unless thoroughly cleaned including an acid pickle to remove all foreign metal. Iron wire and copper or brass brushes should only be used on like substrates. Finally, hand sanding, using sandpapers or mat abrasives, or "dust blasting" should be used on the surface of good materials to roughen the surface and to remove the surface oxidized layers of materials. This will promote better adhe- sion between the old and the new paint. After any of the above mechanical treatments, the work should be solvent washed or wiped to remove all of the loose materials, and it should then be final wiped as described above just prior to painting. WASH PRIMER This paint preparation goes by a number of names. In addition to wash primer, it is known as pretreatment coat, resin-acid coat, and acid-etch primer, and it has a number of military and commercial specifications that describe the ma- terial. The coating is a two-part mixture. The first part is composed of a (polyvinyl) butyl resin dissolved in ethyl and butyl alcohol with small amounts of zinc chromate and magnesium silicate added. The second part, which makes up 20% of the volume, is composed of phosphoric acid (85% ortho), ethyl al- cohol, and water. The two parts are mixed just prior to application. The pur- pose of the phosphoric acid is to react with the metal surface, forming an in situ metal phosphate. As the water and alcohol evaporate, the very thin resin forms a stabilizing coating over the phosphate and promotes adhesion of subse- quent coats. This system is only good when the metal substrate will react with the phosphoric acid to form the phosphate. If the acid is not essentially neu- tralized by the reaction, the coating may appear satisfactory upon drying; but on aging, as water vapor penetrates through the coating, the acid will be re- formed and will cause failure of the coating in service. The coating, neverthe- less, is useful as an adhesion promoter on some metals and will be recom- mended for use when appropriate. There are some low-solvent wash primers on the market; these should be investigated if the material usually used will not meet the local air quality district's requirements. CADMIUM Cadmium surfaces should be stabilized with a conversion coating prior to paint- ing. A phosphate coating is the usual way of doing this, but chromate coatings, though usually used for corrosion protection, can be painted. Because chromi- 53

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