MIL-HDBK-1110
cleaning or paint spray application to ensure that the air supply
is clean and dry. A blotter test as described in ASTM D 4285 can
be used to determine whether the air supply is free of oil and
moisture. In this test, a clean white blotter is held downstream
about 19 inches from the nozzle for 2 minutes. It should remain
clean and dry.
b) Abrasive. Each batch or shipment of abrasive
should be checked for oil contamination and, if required, soluble
salts. Either can contaminate a cleaned surface and reduce the
service life of the coating. A commonly used test to check for
oil contamination is to take a small random sample of the
abrasive, place it together with clean water in a small bottle or
vial, shake the bottle for a minute and examine the surface of
the water. There should be no sheen of oil on the surface of the
water. Soluble ionic contaminants can be detected using the
electrical conductivity test described in ASTM D 4940. In
addition, the abrasive should feel dry to the touch when it is
placed in the abrasive blasting machine. Recycled abrasives
break down after several cycles, and the number of cycles depend
upon the type of the abrasive. The abrasive should be replaced
when it no longer meets the requirements of the specification.
c)
Blast Hoses and Nozzles. Blast hoses should be in
good condition and kept as short as possible. The nozzle
pressure and diameter of the nozzle orifice both affect the
cleaning rate. A nozzle orifice gage is used to determine the
orifice size. Air pressure at the nozzle is measured using a
hypodermic needle air pressure gage and should be from 90 to 100
psi for optimum efficiency. Usually these parameters are
measured at the start of a job and when production rates are
decreasing. An increase of nozzle size of more than 1/8-inch
causes loss of cleaning efficiency because of the increased
pressure drop. Increased nozzle size also causes increased use
of abrasive. Profile should be inspected when major changes in
cleaning efficiency are noted.
d) Safety. Special safety precautions are required
during abrasive blasting. Refer to Section 13 for more
information. These precautions include use of external couplings
on blast hoses and dead man controls, and electrical grounding of
equipment.
9.6.7.2
Water Blasting. Since contaminants, such as salts and
oils, in the blasting water will be left behind on the blast-
cleaned surface and may adversely affect the adhesion of the
coating to be applied, water should be essentially free of
contaminants. If cleaning agents are added to the water used for
blasting and cleaning, the surfaces must be thoroughly rinsed
with clear water. An exception is the use of flash-rusting
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