Methodology to understand how Protective coating work is done in fabrication shop

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Coating Work in the
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Coating Work in the Fabricating Shop
by Daniel Griffin
International Protective Coatings
Houston, TX. USA.
Painting structural steel at the steel fabricator’s shop offers some real advantages over field painting. Since
painting can usually be done indoors, weather conditions are less of a factor, and the coatings application and
inspection process can be more easily controlled. Well-designed steel fabrication shops have used these
advantages to become sources of high quality protective coatings work .
With the advent of high production, centrifugal shot or grit blasting equipment, some shops have combined the
process of fabricating, blast cleaning, and painting all under one roof. The resulting cost savings and continuity
have led to millions of tons of structural steel arriving at their end-use destinations fully painted and ready for
erection with minimal field touch up required.
Successful fabricator shop painting depends upon many factors that must be controlled to ensure quality
coatings work. Comments on some of these factors follow.
Steel Design Considerations
Back-to-back angles should be designed out of a project because they create surfaces inaccessible for coating,
or, if the angles are set flush and tack-welded, crevice corrosion may occur. If back-to-back angles must be
used, the surfaces should be blast cleaned to a minimum commercial blast (SSPC-SP 6), and the fabricator
should apply at least one coat of primer prior to bolting up the angles. Normally, a zinc-rich or rust inhibitive
polyamide epoxy primer is recommended.
Whenever it is cost-efficient, continuous welding of structural members is preferred. Discontinuous, tack, or stitch
welding often creates areas of crevice corrosion that lead to problems in aggressive environments. The owner
and structural engineer must do a cost-benefit analysis to determine the point at which continuous welding
becomes necessary or cost-efficient, since it does add to the cost of the fabrication process.
Surface Preparation
Proper surface preparation is of great importance in assuring a clean, smooth coating system with good
adhesion. The first step in surface preparation is to degrease and solvent-clean the steel prior to abrasive blast
cleaning.
When centrifugal blasting machines are used, the abrasives themselves must be kept totally free of grease and
oil. Once the metallic abrasives become contaminated, they impart a thin film of oil and metal fines to the blast-cleaned
surface. This contamination of the surface may result in adhesion failures of the coating system.

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The proper size and blend of aggregate must be used and maintained to develop the specified blast profile for
the primer being applied. An efficient practice is to measure the blast profile periodically to ensure that proper
size and blend of aggregate is being used. Generally accepted methods include use of replica tape or
comparators.
A description of procedures necessary to maintain quality control in the centrifugal blast cleaning process is
available from the Steel Structures Painting Council in the monograph, Guidelines for Centrifugal Blast Cleaning
(1984), by A. W. Mallory.
Considerations about the selection and use of metallic abrasives, including the critical importance of maintaining
an effective "operating mix" of abrasive, are described in "Controlling the Centrifugal Blast-Cleaning Process" by
Einar A. Borch in this issue on pp 28-33.
Coatings fail first on prominences such as sharp edges, burrs, welds, and weld spatter. Laminations (slivers) in
the steel may not be easily visible until after blasting. These laminations, burrs, and weld spatter should be
removed and any rough welds smoothed by hand or power tool cleaning. Sharp edges should be ground to a
radius where subsequent coatings systems must perform in highly corrosive atmospheres. When grinding is
required, it should be clearly stated in bid documents.
Even the painting "bucks" or supports are crucial to the quality of a coating system. If allowed to become dirty or
coated with overspray, they can contaminate freshly blasted or painted steel during the turning operation. Bucks
should be made of angle stock, laid with single point up for minimum impingement on the structural member, and
kept clean.
After being spread out on the bucks, and just prior to painting, the steel should be blown with clean, dry air to
remove residue, dust, and aggregate. Other contaminants, such as oil from overhead cranes, should also be
removed.
The steel should be spaced to allow easy access to each member so that the painter is not required to reach or
stretch during the painting operation. Sufficient layout space should be available so there is not an excessive
rush to keep up with the flow of steel that passes through a centrifugal blastmachine. Rushing reduces the

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quality of application, and can result in damage or failure from handling or topcoating before sufficient cure is
achieved.
Storage and Mixing Of Coating Material
Proper handling, storage, and mixing of coating products are equally important to a coating system’s success.
Maintaining materials within specified temperature limitations is vital. Manufacturers generally recommend
protected storage in the range of 40° F (4° C) to 100° F (38° C). For optimum application properties, it is also a
general practice to recommend that material temperatures range from 70° F (21° C) to 80° F (27° C) prior to
mixing and application.
Power agitation for mixing a coating, whether it is a single- or multiple-component material, assures even
dispersal and smooth blending of ingredients. The motor and agitator blade should be properly sized to mix the
material uniformly.
Coatings should always be mixed in whole unit sizes as supplied by the manufacturer. Attempting to mix a partial
unit of a multiple-component zinc-rich primer, for example, may lead to premature failure of the system because
of an incorrect ratio of components.
If thinning is required after the coating is mixed, the manufacturer’s recommended thinner should be used.
Thinning should be kept to a minimum because excessive thinning can increase cost and reduce film build. Only
the amount of thinner required to attain the proper spray fan pattern and film formation should be used. The
manufacturer’s recommended clean up solvent should also be used at prescribed intervals (e.g., breaks, lunch)
during the application process and at the end of the work day.
Application Equipment
Many manufacturers test various types of application equipment with their products to recommend the
equipment that gives the most desirable results with their coatings. For this reason, the application equipment
recommended on the coating manufacturer’s written application instructions should be used.

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All coatings can cause wear of spray tips, but when abrasive material such as a zinc-rich primer is applied by
airless spray, the abrasive ingredient will wear and enlarge the spray tip orifice quite rapidly. Good application
practice, then, includes periodic inspection and replacement of tips that become excessively worn. Tip wear can
be recognised by changes in the spray pattern or by film build problems such as sagging.
Conventional air spray equipment, particularly the gages, must be maintained in proper functioning condition.
However, when gages break, as they frequently do, it is possible to adjust the spray pattern as follows: turn off
the regulators to the fluid and atomisation pressures. Hold the gun at your hip. With the atomisation pressure off,
gradually increase the fluid pressure until triggering the gun yields a material flow stream that falls approximately
4 ft in front of you. Then adjust the atomisation air to break up the material flow, spreading it into a satisfactory
fan pattern.
Effective Work Procedures
Prior to spray application of the primer, the steel should be inspected for areas that will be difficult or tight to
spray paint (for instance, end clips, gussets near a flange, or nuts and bolt heads). These tight areas should
receive a brush coat prior to the spray application.
Both primer and topcoat should be specified with minimum and maximum thicknesses, and those targets should
be consistently reached. Too little coating usually causes premature failure in service, while excessive
thicknesses may lead to "mudcracking," associated with zinc-rich primers, or to coating delamination due to
curing stresses. Fabricators who are painting structural steel must be careful to avoid too much film thickness on
flat and web areas of structural pieces and too little thickness on the vertical and flange areas. The minimum and
maximum limits for the specific coating being applied should be determined by the coating manufacturer. This
information should be clearly stated in the specifications.
Sometimes, the turning and spraying technique used when T-braces are coated leaves uncoated areas on the
edges. This occasionally occurs on the outside edges of flanged steel. The painter should be made aware of this
potential problem so that the edges will be properly coated. Other practices to avoid include dragging material
lines over freshly coated steel, walking on freshly coated steel, or handling steel coated with uncured paint (even
with gloved hands).
An interesting phenomenon is that the first and last few feet of I-beams or other structural members tend to have
lighter or lower coating thickness than the midsection. This is a problem created by the method of application.
Generally, a painter will coat the first 5 ft to 6 ft, then move down the beam. At the beginning of the next
sequence of passes, he will over lap some of the previously coated area. Because the first and last few feet do
not receive an over lap, they obviously will have a lower coating thickness. An extra pass over these areas can
eliminate this problem.
Topcoating
Where topcoats are applied in the fabricating shop, special considerations must be taken into account. They
include assuring that the appropriate cure of the primer has been achieved before topcoating, setting up the
physical layout and work flow of the paint shop to allow for holding steel between priming and topcoating, and
improving the care in handling and storage of topcoated steel to minimise damage that must be repaired
subsequent to erection.
Assuring cure of the primer before topcoating can be accomplished by following the coating manufacturer’s
recommendations on recoat intervals and on the ambient conditions necessary for recoating.

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Advice on the care necessary in handling, shipping, and storing topcoated steel is available in Tinklenberg and
Culp, "Michigan’s Answer: Total Shop Painting," JPCL, June 1984, and in Bayliss, "The Handling, Transport, and
Storage of Coated Items," JPCL, August 1984.
After the steel is coated, it must be allowed to sit undisturbed for the specified time prior to handling. The
minimum time depends upon shop and surface temperatures when the finish coat is sprayed, but is usually in
the range of eight to 24 hours. When the steel is handled, care must be exercised to avoid excessive damage to
the coating, since the coatings have not reached their ultimate toughness during this period. Nylon slings work
well, though they may cause wider abrasions than chains do, if the steel is handled too rapidly after coating.
Chains can be used effectively if only one piece of steel is moved at a time, and the hooks are placed in the end
clips, not wrapped around the steel.
Stacked steel should be protected with dunnage to minimise metal-to-metal contact from damaging the coating
system. In transportation, coated steel needs the same careful stacking with dunnage and secure anchoring to
truck or railcar bed to prevent damage from load shifting. Steel temporarily stored outside the fabrication shop
also requires careful handling and stacking so that ponding of water and dirt entrapment is kept to a minimum.
Recent Trends In Fabricator Shops
As the trend to in-shop application grows, some fabricator shops are beginning to insulate and heat their painting
areas to facilitate movement and handling of painted steel. A few have also installed misting or fogging devices
to elevate the humidity in the painting shop and induce faster curing of some inorganic zinc primers.
Many fabricators are upgrading their equipment to enable them to apply high performance coatings. The more
experienced applicators are also using larger equipment versatile enough to handle a wide range of products.
The Coating Manufacturer
Coating manufacturers are responding to fabricators’ needs by developing new products that dry and cure in
much less time and at lower temperatures than standard coatings. They are also developing products to meet
another trend affecting not only the steel but all industry – increasing government regulation of pollution caused
by solvent emission. To fulfil needs brought about by this trend, manufacturers are developing water-based and
higher solids coatings to eliminate or reduce the emission of volatile organic compounds (VOC’s).
Conclusion
Many fabricating shops are beginning to recognise and accept that painting is an integral and essential part of
the steel fabricating industry. It can no longer be dismissed as a nuisance or hindrance to the demands of high
throughput of steel. These fabricators have accepted the challenge of applying high quality coating systems and
of protecting the coatings to the final field-site destination.

6. Disclaimer
The information given in this sheet is not intended to be exhaustive and any person using the product for any purpose other than that specifically recommended in this sheet without first
obtaining written confirmation from us as to the suitability of the product for the intended purpose does so at his own risk. Any warranty, if given, or specific Terms & Conditions of Sale are
contained in International’s Terms & Conditions of Sale, a copy of which can be obtained on request. Whilst we endeavor to ensure that all advice we give about the product (whether in
this sheet or otherwise) is correct we have no control over either the quality or condition of the substrate or the many factors affecting the use and application of the product. Therefore,
unless we specifically agree in writing to do so, we do not accept any liability whatsoever or howsoever arising for the performance of the product or for any loss or damage (other than
death or personal injury resulting from our negligence) arising out of the use of the product. The information contained in this sheet is liable to modification from time to time in the light of
experience and our policy of continuous product development.
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