In normal sandblasting, the anchor pattern should run about 38 to 50 

 micrometers (1.5 to 2.0 mils) deep. Most manufacturers recommend a 16- to 

 30- (16/30) mesh silica sandblasted at the surface at 690 kilopascals (100 

 pounds per square inch) to produce this profile pattern. An 8/30 mesh sand 

 may be needed to remove tightly adhering rust and paint. Seldom are anchor 

 patterns of more than 75- to 100-micrometer (3 to 4 mils) depths recom- 

 mended, even for high build mastic coatings of up to 5-millimeter dry film. 

 This profile can usually be achieved by use of 8/30 mesh silica sand, 

 blasted at the surface with a nozzle pressure of 690 kilopascals. 



Zinc-rich coatings on the other hand require a blast by 30/60 mesh sand 

 to produce a required profile of 25 to 40 micrometers (1 to 1.5 mils). 



Other abrasives include garnet, flint, steel grit, steel shot, and 

 aluminium oxide. Costly abrasives, such as steel shot and steel grit, are 

 primarily used where they can be recovered and reused such as in cabinets 

 and blast rooms. Special equipment such as surface profile comparator 

 furnished by Zorelco and NACE TM-01-70 visual standard should be used to 

 verify the surface roughness (anchor pattern) produced by various abrasives. 



(4) Sandblasting . 



(a) Requirements . No coating system can perform better than 

 the surface to which it is applied. It must be able to reach and adhere to 

 that surface in order to perform its function. In general, as previously 

 stated, sandblasting steel for use in harbor facilities provides the best 

 and often the only suitable foundation for the majority of materials which 

 should be used in these environments. In addition to removing surface 

 contaminants, sandblasting produces surface roughness known as anchor 

 pattern which enhance coating adhesion. Sandblasting, therefore, is one of 

 the most important steps to consider in any protective coating program. 



(b) Compressed Air Supply . This is perhaps the most critical 

 part of a sandblasting operation. Speed of work and results, will be 

 accomplished only in direct proportion to the volume and pressure of air 

 passing through the nozzle. To achieve good economics and a good profile, 

 sandblasting steel requires a high nozzle pressure of 620 to 690 kilopascals 

 (90 to 100 pounds per square inch) and high volumes of air at 2.5 to 10 

 cubic meters per minute (80 to 350 cubic feet per minute) . The larger the 

 nozzle, the faster the work will be completed, assuming proper air pressure. 



It is generally recommended that an additional 35 kilopascals (5 pounds 

 per square inch) compressor pressure should be used for each additional 15- 

 meter (50 foot) length of blast hose. When only low nozzle pressure is 



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