Dry Abrasive Blasting Dry abrasive blasting involves propelling abrasive particles against the metallic surface by either high-pressure air or spinning paddle wheel. The striking of these particles against the metal abrades away deteriorated paint and scale. Many abrasive blast machines (portable dry-honing machines) reclaim used grit by cleaning and sifting out dirt, scale, and damaged grit. (See figure 4-28.) Grit that has been recycled after use on steel, brass, bronze, or copper-nickel should not be used on aluminum. Do not blast aluminum with steel or copper slag or chilled iron grit. Table 4-3 lists some common abrasive materials and grit sizes. CLEANING SURFACES WITH POWER TOOLS Power tool cleaning includes devices that impact the metallic surface with an abrasive substance or mechanical object. Impact tools, powered wire brushes, and disk sanders are common power tool cleaners for SE. Usually, electric or pneumatic disk sanders abrade the metal surface of SE with coarse to fine grit. When used with the needle gun, the disk sander can produce a uniform anchor pattern of very closely spaced scratches. WARNING When using abrasive power hand tools, you must wear eye protection to prevent serious injury. Q72. What is the preferred surface preparation method for many   of the components of support equipment? Q73. What should you use during the abrasive process to prevent flash rusting? CORROSION DAMAGE LIMITS LEARNING OBJECTIVE: Recognize the limits in removing corrosion damage. Impact tools, such as the needle gun (pneumatic descaler), provide a rapid means for removing rust and old paint from metal surfaces of SE. These tools must NEVER be used on aluminum. A wire brush powered pneumatically or by electric motor is a method for removing small amounts of paint and rust from SE. Often, the overextended use of a wire brush results in a metal surface that is polished to a glossy appearance. A polished surface produces a poor anchor pattern for paint bonding. Corrosion damage limits refer to the amount of metal that may be removed from a corroded part without impairing the strength and function of the part. When removing corrosion, maintenance personnel must be very careful not to remove more of the metal than is necessary to ensure complete removal of corrosion. Figure 4-29 shows the maximum corrosion depths allowed on the various components of the nose landing gear. When damage exceeds the limits specified in the SRM or the corrosion control section of the MIM, the affected part must be replaced if structural repair of the damage is not possible. Table 4-3.—Recommended Grit for Steel and Aluminum TYPE OF RECYCLING NON-RECYCLING METAL BLASTED GRIT SAE PRES GRIT SAE PRES MESH PSI MESH PSI Steel Angular silica 20/40 75 Silica sand 20/40 75 Sand Crushed garnet 20/40 75 Crushed garnet 20/40 75 Aluminum oxide 20/50 50 Aluminum Silica sand 20/40 75 Silica sand 20/40 75 Aluminum oxide 20/50 50 Crushed garnet 20/40 75 Crushed garnet 20/40 75 4-43