MIL-HDBK-1110
6.7.1.7
Procedures/General Information. Good blasting
procedures result in efficient and proper surface preparation.
Adequate pressure at the nozzle is required for effective
blasting. Other factors, such as flow of abrasive, nozzle wear,
position of the nozzle with respect to the surface, and comfort
of operator are also important. A well trained operator is
essential to obtaining an acceptable job.
a) Handling the Nozzle. The angle between the nozzle
and the surface and the distance between the nozzle and surface
are important factors in determining the degree and rate of
cleaning (Figure 5). The working angle will vary from 45 to 90
degrees depending upon the job. To remove rust and mill scale,
the nozzle is usually held at an angle of between 80 and 90
degrees to the surface. This is also the preferred configuration
for cleaning pitted surfaces. A slight downward angle will
direct the dust away from the operator and ensure better
visibility. A larger angle between nozzle and surface allows the
operator to peel away heavy coats of old paint and layers of rust
by forcing the blast under them. Other surface contaminants may
be better removed with a cleaning angle of from 60 to 70 degrees.
By varying the distance between the nozzle and the surface, the
type and rate of cleaning can also be varied. The closer the
nozzle is to the surface, the smaller the blast pattern and the
more abrasive strikes it. Thus, a greater amount of energy
impacts the surface per unit area than if the nozzle were held
further away. A close distance may be required when removing
tight scale, for example. However, a greater distance may more
effectively remove old paint. Once an effective angle and
distance have been determined, each pass of the nozzle should
occur in a straight line to keep the angle and distance between
the nozzle and the surface the same (Figure 6). Arcing or
varying the distance from the surface will result in a nonuniform
surface.
b) Rates. The rate of cleaning depends on all of the
factors discussed above. Abrasive blasting of steel to a
commercial degree of cleanliness (SSPC SP 6 or better) is much
slower than painting. No more steel surface area should be blast
cleaned at one time than can be primed the same day, since
significant rusting can occur overnight. If rusting does occur,
the surface must be reblasted before painting.
6.7.2
Acid Cleaning. Acid cleaning is used for cleaning
efflorescence and laitance from concrete.
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