because it is inconvenient or the necessary equipment
is not available. To help ensure a quality job, be sure
you use the required equipment and prepare the surface
carefully and thoroughly.
Surface preparation includes three distinct opera-
tions: (1) cleaning, (2) undercutting, and (3) surface
roughening. We will discuss each of them in the next
paragraphs.
Cleaning
You must ensure a good bond between the sprayed
coating and the base material to which it is applied. To
do that, be sure the areas to be coated and the adjacent
areas are free from oil, grease, water, paint, and other
foreign matter that may contaminate the coating.
SOLVENT CLEANING.--Before blasting or
spraying, use solvent to clean all surfaces that have
come in contact with any oil or grease. (Vapor
degreasing is preferred, but you may use solvent
washing.) When using solvent, be very careful that it is
not strong enough to attack the base material; do NOT
leave any residue on the surfaces. Trichloroethane and
28.465
toluene are suitable solvents. Most solvents are
flammable and toxic, so you must follow proper pre-
PREPARING THE SURFACES
cautions when you use them. You also must be very
careful to protect any parts that may be attacked by the
Surface preparation is often the most critical part of
solvents.
the job, yet it often gets the least attention. We cannot
ABRASIVE CLEANING.--You can use a sand
overemphasize its importance because an improperly
prepared surface will cause the part to fail under
heavy or insoluble deposits. Be sure to follow your
operating conditions. For this reason, even the best and
operator's manual. Do not use the same abrasive
most elaborate preparation is still the cheapest part of
blasting equipment to roughen surfaces that you use for
general cleaning.
the job. Quite often, surfaces are not prepared properly
Figure 12-6.--Major steps in restoration of dimensions with thermal spray.
12-4