BLASTING IN PRACTICE
Copyright 1993/2001 Mario S Pennisi
WHY IS SURFACE PREPARATION IMPORTANT?
Surface preparation is the most important part of a coating
system, because it affects the performance of the coating
more than any other variable. Given that the correct coating
system is selected, if the surface preparation is poor, coating
performance is usually going to be poor. If surface preparation
is good, then the coating applied over it is likely to perform
It is useful to know the reasons why surface
preparation is so important, because knowing why can help
the applicator do a better job.
SURFACE PREPARATION AS A FOUNDATION
Surface preparation is to a coating system what a foundation
is to a building. If a building has a poor foundation, it
can list or lean, as the famous Leaning Tower of Pisa does,
or it can collapse altogether. If a coating system has a poor
foundation (surface preparation) it will fail sooner than
expected (say, after five years rather than 10 years); or
it can fail catastrophically within the first year of application.
In both instances great financial losses can occur to a facility
owner. Surface preparation creates a foundation in two important
* a mechanical way, by providing an anchor for the coating;
* a chemical way, by allowing intimate contact of coating
material molecules with the steel (or other material) surface.
WHY AN ANCHOR PATTERN?
When a surface is very smooth, coatings have a difficult
time adhering strongly. A scraper or even a fingernail, for
instance, easily removes a coating on glass. On the other
hand it is difficult to remove a coating on a rough surface
like sandpaper. Steel, when it is abrasive blasted, has a
surface that is rough like sandpaper, with a series of tiny
peaks and valleys called surface profile. Coatings anchor
themselves to the valleys of the profile, and the peaks are
like teeth. This is why surface profile created by blasting
is sometimes called an "anchor pattern" or "mechanical tooth."
- Visible contaminants
Soils on a metal surface are made up of many different materials.
* Corrosion products
* Oxides/Mill Scale
* Marking pen ink
* Rubber from boots
* Brazing flux
* Weld flux
* Weld scale
* Weld anti-spatter compound
* Adhesive from tape
* Metal chips
* Drawing compounds
* Polishing/buffing compounds
* Mould release agents
* Previous organic coatings
* Previous metallic coatings
* Finger prints
When contaminants such as these are painted,
they interfere with the mechanical and chemical adhesion of
the coating to the substrate so that the coating is likely
to fail. On the other hand, when all soils are removed, the
coating can achieve complete and continuous contact with the
substrate, thus assuring the best possible adhesion. When
a coating adheres well, it is likely to be an effective barrier.
The coating can minimise or prevent moisture (the electrolyte
in the corrosion process) from reaching the substrate.
- Non-visible contaminants
Other forms of soils, not always visible to the naked eye,
are chemical contaminants. The most dangerous forms of chemical
contaminants are soluble salts:
* chlorides and
When such contaminants are painted over, they have the power
to draw moisture through the coating (osmosis) to cause blistering,
detachment, and accelerated corrosion of the underlying metal.
When structural steel is repainted, rough or pitted areas
visible after dry abrasive blast cleaning may contain soluble
salt contamination, especially in the base of the pits. Dry
abrasive blasting does not remove these salts. It is wise
to check for the presence of soluble salts with specially
designed field test kits before painting and if they are present
in detrimental amounts, to take additional cleaning steps
to remove the salts.
DEGREES OF SURFACE PREPARATION
In any job specification, the degree of cleaning required
for a given substrate before painting depends on a number
* The service environment of the coating system. This is perhaps
the most important factor, and normally is the first consideration
when determining the degree of surface preparation. Generally,
the more severe the environment, the better the surface preparation
required. Severe service environments include:
- immersion in liquids,
- exposure to aggressive chemicals or environments, and
- high temperatures, or combinations of these conditions.
* Another consideration is the generic kind of coating used.
Some coatings, such as alkyds, because they flow out and wet
the surface well, can tolerate application over minimally
prepared or hand-cleaned surfaces. In addition, some epoxy
mastics and other "surface-tolerant" coatings are formulated
to be applied over hand-and power tool-cleaned surfaces. Coatings
such as vinyls and inorganic zincs, however, are at the other
end of the spectrum. They require a higher degree of cleaning
than many other types.
* Cost is another factor when selecting the degree of surface
preparation. Blast cleaning to Class 3 (White Metal) is about
4-5 times more costly than to Class 1 (Light Blast Cleaning).
In some severe environments and with some coating types, rigorous
cleaning is necessary; but in other instances, cost and cost-benefit
in the form of longer coating lifetime will become an important
factor in selecting the degree of surface preparation.
* Finally, regulations may have an impact on the degree and
method of surface preparation. - in residential or congested
urban environments, open blasting may be prohibited and -
in addition, where lead-or chromate-based paints are being
removed, hazardous waste regulations may require containment
and use of special surface preparation methods.
Determining the degree of surface preparation,
as described above, is the job of a specifier or engineer.
The task of doing the work is the contractors. No matter what
degree of surface preparation is required, it must be done
thoroughly. If hand-tool cleaning is required, then all the
surface area specified must be hand-tool cleaned, after it
has been cleaned by water or solvent to remove dirt, oil,
or grease. Similarly, if a Class 3 blast is specified, then
conformance with the written description of this must be achieved
on all surfaces. When preparing metal, it is also important
to follow the proper sequence.
* First, remove dirt and other soils. It is a lot easier to
sweep mounds of dirt and other loose material off a surface
with a broom than to try to remove it with surface preparation
* The next step is removing visible oil and grease by solvent
cleaning. Then conduct the mechanical cleaning operation whether
hand tool, power tool, or blast cleaning. If these steps are
reversed, particularly with blast cleaning, the force of the
blasting abrasive can drive the soils into the roughened steel
surface or profile. Then it is not easy to remove, and it
may interfere with coating adhesion. In addition, it is important
to achieve the surface profile required by the specifications,
- When the profile is too rough, the coating may not cover
the peaks of the profile, and the result will be pinpoint
- When the profile is not rough enough, the coating may not
anchor well to the surface, and the result will be loss of
adhesion. To make sure that a coating system will perform
well as a barrier to prevent/reduce corrosion, all soils must
be removed so that the coating contacts the entire surface
of the metal for chemical adhesion and that the surface is
roughened for mechanical adhesion as well. These two conditions
of cleanliness and profile ensure that a proper foundation
has been created for applying the coating system. This good
foundation should help to provide many years of service life
for the coating.
HOW CLEAN IS CLEAN?
Protective coatings have been used for centuries to protect
substrates subjected to the environment. The most severe environments
are near the ocean. Many industrial plants have severe exposures
that require protection.
For years, most coatings were applied by brush to hand-cleaned
surfaces. The fish oils and long and medium oil alkyd coatings
were pigmented with some very effective inhibiting pigments
such as red lead, and they were literally scrubbed into the
surface by the brushing action. These materials were good
at sticking on the surface, and they could be applied over
mill scale and tight rust to provide some protection.
Of course, they would protect longer if they were put on a
blast-cleaned surface. Many of the new, high performance coatings
do not stick well to mill scale, tight rust, or a smooth surface.
Abrasive blasting is needed to clean and roughen the surface.
Media -Garnet .....................Glass
* Abrasive blasting
There are four (4) levels of abrasive blasting described in
AS1627-Part 4. The following lists them in increasing levels
- Class 1
- Class 2
- Class 2 1/2
- Class 3.
These specifications define the physical cleanliness that
must be achieved on the surface.
- Cleanliness The last three of these specifications requires
all the mill scale, rust, and old paints to be removed. All
that can remain on the surface are stains of these contaminants.
The following amount of stains is allowed (by
· Class 2 - 33 percent of each sqcm,
· Class 2 1/2 - 5 percent of each sqcm
· Class 3 - none.
The most common tools used in the industry to assist in determining
surface cleanliness are
· SSPC - Vis 1-89,
· NACE coupons, and
· Swedish pictorial standards as specified in AS1627.9
- Anchor profile The height of the anchor profile
is specified independently from cleanliness. The manufacturer's
application data sheet will give this information. There is
no standard anchor profile height that is good for all coatings.
The surface must be roughened sufficiently to get the coating
to stick. Coatings that are applied in thin coats, such as
oil-based coatings, require a low anchor profile ie, 50 microns.
Too heavy an anchor profile will result in the peaks of the
profile in the steel sticking out and causing pinpoint rusting.
Thick coatings such as coal tar epoxies require a deep anchor
profile ie, 100-125 microns, to get them to stick properly.
Two common tools are used to determine anchor profile:
· a surface profile comparator and
· replica tape.
ASSURING QUALITY OF ABRASIVE BLASTING OPERATIONS
performance of a coating depends in large part on the quality
of surface preparation. This is because coatings have been
formulated to perform properly under particular conditions,
such as over a specified degree of surface cleanliness and
a specified anchor profile, and under certain environmental
conditions. If these and other conditions are not met, coatings
may not achieve their expected performance. When dry abrasive
blasting is the specified method of surface preparation, many
conditions must be taken into consideration by the blaster.
* Checking conditions before blasting
Most coatings do not adhere well to surfaces contaminated
with oil and grease. Blasting actually drives them further
into the steel rather than removing these contaminants and
thus contributes to premature coating failure. Therefore,
a check for visual surface contaminants before blasting is
essential. If oil and grease are present, they should be removed
with solvent cleaning, in accordance in AS1627-Part 1.
All of the blast cleaning specifications from AS1627-Part
4 require this step with the statement, Before blast cleaning,
remove visible deposits of oil or grease by any of the methods
specified in AS 1627 Part 1.
Ambient conditions should be measured before blasting. If
blasting is not to be followed immediately by coating application,
then it may be all right to proceed first with rough blasting
to remove the existing coating, rust, and mill scale, and
to check ambient conditions before the final blast. If blasting
is to be followed immediately by coating, then ambient conditions
should be checked before blasting begins.
It is essential that the dew point, air temperature,
relative humidity, and surface temperature are suitable for
blasting. This insures that condensation will not be forming
on the metal surface during or after blasting and cause flash
oxidation (rust), which can be detrimental to the overall
quality and coating performance. Dew point is the temperature
at which moisture condenses on a surface. If the dew point
is 10C, condensation will occur if the metal is at or below
this temperature. As a general rule, final blast cleaning
should take place only when the surface is a least 3C above
the dew point. For example, if the dew point is 10C, the steel
temperature should be at least 13C. This rule provides a margin
of error, in case of instrument inaccuracies, quickly changing
weather conditions, or human error. Dew point is calculated
using the relevant psychrometric tables. The psychrometer
is a hand-operated instrument that has 2 glass thermometers.
To measure ambient conditions, the following
equipment is required:
- a surface temperature gauge or surface thermometer.
- a psychrometer for measuring dry bulb (air) and wet bulb
temperature, and psychrometric tables for calculating dew
point and relative humidity.
* Checking blasting abrasives and equipment
Abrasives and equipment should also be checked for cleanliness
before blasting, and the equipment should be checked for efficiency.
There are several parts of the blasting equipment that need
to be checked for contaminants:
- the compressor,
- the moisture separator, and
- the air that comes through the hoses.
* Checking the surface after blasting
After blasting, remove all dust from the blast-cleaned
surface, either by blowing down the surface with compressed
air or by vacuuming. Dust on the surface can interfere with
the coating's ability to bond to the surface. After blowing
or vacuuming the surface, brush a clean white cloth across
the surface (without touching the substrate with the hands
as body oils or salts can be transferred easily to the surface
and contaminate it). If dust appears on the cloth, blow down
or vacuum the surface again. Check for non-visible contaminants,
especially soluble salts, which are detrimental to coating
performance. Once the blasted surface is free of dust (and
other contaminants), check the surface profile and degree
of cleanliness to see that the specifications have been met.
The quality control checks should be documented and kept
as part of the quality control records for the job. This way,
historical information is available for verifying compliance
There's an old cliche that a top quality coating put
on a marginally prepared surface will perform no better than
a cheap coating applied to a squeaky clean surface. This statement
is probably an oversimplification of the problem, but there's
a great deal of truth in the statement as well.
Surface preparation is an important step that affects the
life of a coating. The life of an oil-based paint, for example,
is longer on a blast-cleaned surface than on a hand-cleaned
Many of the high technology coatings such as zinc-rich primers
require a blast-cleaned surface to stick and provide protection
to the steel. Coatings stick better to a rough surface than
to a smooth surface.
Abrasive blasting both cleans a surface and roughens it. This
roughness is called anchor profile. The specification will
specify how rough the surface must be before the paint is
Quality assurance will help ensure that abrasive blasting
operations create a surface suitable for coating application
and should be followed even if customer's inspectors are on
the job conducting similar checks.