You also may use masking compound to protect the
Clean, dry air is essential for proper blasting. Traces
spray booths and other equipment that is subject to
of oil in the air that cannot be readily detected can
overspray, such as rotating spindles, chucks, lathes, and
seriously affect the bond.
the like. When you use masking compound for this
Before reuse, screen the aluminum oxide grit using
purpose, be sure you clean the surfaces on a regular
a 30-mesh screen, and visually inspect it for debris and
schedule and reapply the compound. If you don't, it will
oil contamination according to MIL-STD-1687A(SH).
eventually dry out and the sprayed material will then
Cover and mask all areas of a component that will
stick to the substrate.
not be grit blasted to prevent damage or contamination
In situations where you cannot protect holes, slots,
by the abrasive blasting medium and debris. Grit
keyways, or other types of recesses with tapes or
rebounding from the walls of the blast room or blast
shields, use inserts of carbon, metal, or rubber. Install
cabinet may scratch and damage those uncovered areas.
them before you begin abrasive blasting and spraying,
Masking may be an expensive part of the operation and
and leave them in place throughout the thermal spray
you should consider cost when you select the masking
operation. Remove them after you complete the surface
method. When you finish blasting, remove any masking
finishing but before you apply the final sealer.
material that is unsuitable as masking for the thermal
spray process and replace it with suitable material. We
Spraying the Coating
will discuss this in later paragraphs.
Spray the component using the specifications you
Rubber or aluminum masking tape is satisfactory
will find in the approved procedure for the material
for all operations where you can do hand masking
being sprayed. These specifications cover gun-to-work
economically. Since rubber is not cut by the blasting
distance, rotational or linear speed of the gun to the
operation, you can use it almost indefinitely. You can
workpiece, air, fuel, gas, primary and secondary
use thin rubber tape for heavy blasting protection.
pressures, and power output. Figure 12-9 shows a shaft
MACROROUGHENING.--Macroroughening is
being thermal sprayed.
a lathe operation performed on bearing areas of shafts
or similar surfaces. It consists of cutting a narrow,
Applying the Sealant
low-pitch, shallow groove or thread in the surface.
Sprayed coatings must be treated with a sealant to
APPLYING THE COATING
prevent corrosive attack or fluid leakage. Select the
sealant on the basis of the maximum use temperature of
You will apply the coating by using three distinct
the component and the purpose of sealing the coatings.
procedures: (1) mask the component, (2) spray the
Apply the sealant after spraying and before finish
coating, and (3) apply a sealant to the coating. We will
machining.
discuss each in the next paragraphs.
SURFACE FINISHING
Masking for Spraying
Use surface finishing to reduce the component to
its specified dimensions and to provide a finer surface
You can use tape, liquid masking compounds,
finish than the sprayed finish. Remove all masking
silicon rubber, or metal shielding as thermal-spray
materials before you finish a surface. The acquisition
masking materials. Tape must be designed for high-
specification or drawing will dictate the finished
temperature use. Masking materials must not cause
dimensions.
corrosion or contamination of the sprayed coatings.
Generally, you will mask materials to be sprayed with
Coatings may be used in the as-sprayed condition
masking tape and masking compound. Use a
whenever permitted. The type, hardness, and thickness
pressure-sensitive masking tape designed to withstand
of the coating determines the finishing method. You
the usual spray temperatures.
will often finish softer coating materials by machining.
You can get a good machine finish in most applications
Use masking compound where a liquid masking
by using high speed and carbide tools. Generally, you
material is more convenient. It is a water-soluble
will grind harder coating materials to get the best finish.
material that can be brushed onto any surface to prevent
the adhesion of sprayed material. Approved masking
Take care in any machining or grinding operation
compound will not run or bleed at the edges.
to avoid damage to the coating. Improper machining
12-6