Abrasive Selection and Size
10.15.9 Abrasive Selection and Size
pressed air and cannot be recycled. It removes surface contamination and exist-
Consider some or all of the following when
ing coatings with minimum substrate
selecting a blasting abrasive:
change. Although it produces little or no dust, rinse the substrate with fresh water
• The kind of surface to be cleaned as a final step. The operator requires spe-
• Size and shape of object to be cleaned cial training.
• Type of cleaning facility: outdoor, indoor • Sponge: Synthetic sponge particles
with cabinet, or blast room remove surface contaminants and create a
• Existing surface conditions surface profile suitable for recoating.
Compressed air propels the sponge parti- • Conditions desired after cleaning cles to the surface where, upon contact,
• Desired surface profile and whether or not they expand and abrade the surface. The
the abrasive is to be recycled sponge particles clean the surface by
absorbing contaminants and trapping them • Types of coating to be applied within the sponge particles. The result is a
Each type of abrasive is generally available
very clean surface with suitable abrasion
in more than one size. Abrasives are graded
to remove corrosion and provide an
according to how fine a mesh or sieve screen
anchor profile for industrial coatings. Sponge particles may be discarded after
they can pass through without one particle of
one use or used repeatedly to reduce waste
abrasive remaining on the mesh ( Table 4 ,
and disposal needs. Sponge blasting cre-
Table 5 , and Table 6 ). When in doubt about
ates very little dust, is extremely worker
whether a given abrasive is the correct size,
friendly, and can provide a variety of pro-
perform a mesh or sieve test. Equipment
files, since abrasives may be encased
10-52 Surface Preparation
• An accurate balance • Pour the abrasive into the top of the nested • A set of US National Bureau of Standards screens. The screens are nested so that the (NBS) screens
screen with the largest openings is on top, grading down in screen opening size to the
• A convenient quantity of abrasive to screen with the smallest openings on the weigh (2.2 lbs [1,000 g] is a handy
bottom, for example: from top to bottom, weight)
#8, #15, #16, #40, #50, etc..
Table 4: SAE Shot Specifications
Max.
Max. Screen
Max,
Min.
Screen Max SAE
Screen
Screen
No. and % to Shot No. Retain-
Retain- Aperture
Aperture Pass
ed ed ed ed
S-1320 0 4 (0.187)
7 7 (0.111) 3 S-1110
7 8 (0,0937) 3 S-930
7 10 (0.0787) 3 S-780
12 12 (0.0661) 3 S-660
12 14 (0.0555) 3 S-550
12 16 (0.0469) 3 S-460
11 18 (0.0394) 4 S-390
11 20 (0.0331) 4 S-330
11 25 (0.0280) 4 S-280
11 30 (0.0232) 4 S-230
12 40 (0.0165) 3 S-170
12 50 (0.0117) 3 S-110
10 80 (0.007) 10 S-70
Surface Preparation 10-53
Table 5: SAE Grit Specifications
SAE Max. Limit Screen
Min. Limit Screen Maximum
Nominal Screen
Maximum Grit
Minimum
Grit to Screen No. and Grit No. Retained
Screen No.
Grit
Screen No. and
Pass Aperture %
and Aperture
Table 6: Screen Sizes According to Openings
Opening in Mesh
Opening in
Opening in
Millimeters Size
Inches (in.)
Micrometers (µm)
(mm)
10-54 Surface Preparation
Table 6: Screen Sizes According to Openings
The choice of abrasive is generally determined by the specification and may be the subject of coating manufacturer guidelines provided on application instructions or technical data sheets for a specific product.
turbidity (cloudiness due to sediment) which Test abrasives for cleanliness with the sim-
10.15.10 Abrasive Cleanliness
is usually a sign of excess dirt, dust, or clay ple vial test. Drop some of the abrasive into
in the abrasive.
a small vial of water with known pH (prefer- Use a litmus or pH paper test in the vial of ably distilled or deionized water with a pH
water to tell whether the abrasive is acid or of 7) and shake. Typically, the ratio should
alkaline. pH paper indicates the actual value
be one volume of abrasive to two volumes of of acidity or alkalinity. If the abrasive is water. Inspect the top of the water for a film
dirty, an acid or alkaline, document these of grease or oil. Check the water for visible
Surface Preparation 10-55
results and immediately report them to the owner’s representative.
Litmus and pH papers indicate the presence of chemical salts dissolved in water that form an acidic or basic solution. Litmus and pH papers will not detect the presence of chlorides. If red litmus paper changes to blue, the solution is basic. If blue litmus paper changes to red, the solution is acidic. If, however, the litmus paper does not change, it indicates the solution is neutral.
Even if the solution is neutral, it does not indicate the absence of soluble chemical salts because some chemical salts, such as sodium chloride (common sea salt), form a near-neutral solution. Specific test papers may indicate the presence of soluble chemi- cal salts.