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Surface Preparation
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Why
is surface preparation so important? What happens if the surface isn't
prepared properly? There's a computer expression that says "garbage
in-garbage out". It means that if insufficient or incorrect work
is done "up front" the outcome will be worth less than the effort.
The same holds true for the end result in painting and decorating projects.
The surface must be adequately prepared so the coating or wallcovering adheres
and has the desired final appearance. Think of it as a rule of "6
- P's". That is: "proper preparation prevents poor
product performance".
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There are two things
to think about in surface preparation:
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1) The surface needs
to be clean.
2) The surface "texture" should be appropriate for the
product applied to it.
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What is meant by
cleaning?
The removal of dirt, dust, loose paint, chalk,
grease, oil, moisture, corrosion products, mill scale, weld spatter and
other foreign matter which will directly affect the adhesion of the coating
or wallpaper. Surfaces with dust, casting burrs, etc. can also affect
the smoothness of the finished product.
Is there a simple rule for cleaning?
Yes, follow the manufacturer's instructions.
Some products require more complete cleaning than others. Different
substrates vary in the degree of thoroughness of cleaning required.
What are some of the different cleaning compounds?
Alkali cleaners like tri-sodium phosphate (TSP)
will remove most soluble compounds. Alcohol containing cleaners will
aid in the removal of grease and oil. Solvent cleaning is one of the
older methods for removing oil and grease but is less favorable due to the
inhaling of vapors and the effects on humans. Vapor degreasing, acid
cleaning and phosphate treatments are often used in industrial applications
on plastic and metal substrates.
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The Profile
of a
Substrate
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Why is surface
"texture" so important?
Some surfaces are extremely rough and need to be
made smoother to assure good appearance and durability of the finished product.
Other surfaces may be too smooth and need to be sanded to provide the "mechanical
adhesion" or anchor which often spells the difference between a successful
job and failure.
How does sanding help?
The tiny hills and valleys created on the surface
will hold the coating or adhesive much better than a perfectly smooth surface.
The average distance from the peak to the bottom of the valley is known
as the height of profile or anchor depth. Any surface
roughening should be fairly uniform, with no deep valleys or high hills.
If a high-grade gloss finish is to be applied, the height of profile should
not be more than about 20% of the dry film thickness. For instance,
if an enamel is applied by a China bristle brush, approximately 2 mils (dry
film) of paint will be deposited (about the thickness of the cellophane
wrapper on a pack of cigarettes). If the dry film thickness is to
be 2 mils, then the height of profile should not be more than 0.4 mil.
Can higher profiles be tolerated?
Structural steel or industrial maintenance coatings
may have a profile as high as 30% of the dry film thickness. This
makes sandblasting a viable option for metal preparation in the field.
The rule of thumb is the smoother the film, the closer the profile should
be to 20% of dry film thickness.
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Does the adhesion of the
coating or adhesive play a role in surface preparation?
Some industrial coatings are formulated to
have very good adhesion and require little mechanical anchoring.
Some trade sales coatings may have limited "inherent" or
built-in adhesion and the surface must be carefully prepared to make up
for the deficiencies in the coating. Automotive enamels, for
example, require the best possible inherent adhesion and good mechanical
anchoring as they are subjected to corrosion, impact, flexing, vibration
and a variety of weathering conditions.
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What are the different types of substrates
and how does their surface preparation differ?
Surfaces are either non-porous (metal and
plastic) or porous (wood, concrete, stucco, plaster and wallboard).
Metal and plastic are often used in manufacturing plants and are usually
cleaned by solvent or emulsion processes. Porous surfaces require a
different approach.
Porous surfaces usually need mechanical
cleaning. Dirt, dust, chalk, spatter and other solid soil which
would roughen the coating or adhesive need to be removed. A wire
brush, scraper or other mechanical operation is required. This can
be done either by hand or with power tools. Holes should be filled
before priming, painting, or hanging wallpaper. Sanding may be
required. This should be followed by a thorough "washing"
of the surface. If allowed in your area, TSP is an effective
detergent. It leaves minimum residue and can be easily wiped off or
rinsed clean.
In what case would a primer be needed?
Porous surfaces have many openings that provide
lots of mechanical adhesion. In fact, they frequently provide too
much adhesion. There is usually too much penetration of the coating
or adhesive into the tiny holes or capillaries, rather than too
little. These surfaces usually require a primer.
Sometimes, the use of specially formulated
primer/sealers may be necessary. There are numerous products on the
market which have been designed to seal interior surfaces that have been
stained or contaminated with oil, grease or wax. The primer/sealers
prevent the contaminants from bleeding through and discoloring the coating
or impeding the drying process of the coating or adhesive. The same
is true for exterior products. Knots, resin streaks and different
types of wood siding may require specially formulated primer/sealers to
avoid bleed through.
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