Applying paint to structural steel
April 15, 2002
This article highlights two of the accepted industry specifications, which are provided by the American Institute of Steel Construction (AISC) and the Society for Protective Coatings (SSPC). It covers many of the more salient topics, point by point, and explains them in detail.
Editor's Note: This article is the second part of a two-part series about structural steel corrosion prevention. Part I, which appeared in the March issue, discussed surface preparation. Part II concerns paint application.
"How difficult can it be? It's just paint!" That sums up a common view of corrosion protection systems. However, a corrosion protection system—the process of identifying the service environment, selecting a coating system, preparing the surface, determining the most appropriate application method, and applying the coating according to the governing specification—isn't as simple as it seems. A corrosion protection system can be difficult and, furthermore, costly.
In fact, a recent survey of steel bridge fabricators conducted by the American Iron and Steel Institute (AISI)—National Steel Bridge Alliance (NSBA) committee revealed that 100 percent of the shops surveyed consider paint to be the most costly item for them. The survey also found that many fabrication shops perceive protective coating systems as problematic. The financial success or failure of a project often depends on whether the coating system is applied correctly the first time or whether it must be removed and reapplied multiple times.
Part I of this article discussed the importance of proper surface preparation and highlighted some of the common specification requirements for preparing steel surfaces for painting. Part II, which is presented in a similar format, addresses common specification requirements for painting of steel surfaces.
Two shop painting certifications are widely recognized for structural and architectural applications. One is available through the American Institute of Steel Construction (AISC, www.aisc.org); the other is available through the Society for Protective Coatings (SSPC, www.sspc.org).
The AISC Sophisticated Paint Endorsement (SPE) is the more common of the two certifications. While it has undergone revisions over time, the intent of the endorsement is to demonstrate, through an independent audit, that a fabrication shop is proficient in surface preparation and painting operations and has the necessary quality control programs.
There is no standard specification for coating work. Requirements vary depending on customer demands, the level of surface preparation required or desired, the coating system to be applied, and the service environment of the finished product.
Following are some of the most common coating application specification requirements. However, this article does not review every aspect of the specifications. Reading and comprehending the governing specification for a particular project, and identifying any requirements that are unfamiliar or potentially problematic, are necessary before beginning a project.
Coatings shall be applied according to the manufacturer's instructions for air and surface temperature, and relative humidity. Coatings shall not be applied unless the surface temperature is at least 5 degrees F above the dew point temperature, and rising.
Coating manufacturers often publish minimum and maximum air and surface temperatures on the product data sheets for their coatings. If the air and surface temperatures are too cool, the coating may not cure properly. Conversely, if the air and surface temperatures are too warm, the solvents may evaporate too quickly and result in dry spray and poor film-forming characteristics.
Further, if the surface temperature is at or below the dew point temperature, droplets of moisture form on the surface before the coating is applied. Very few coatings tolerate this. The industry has adopted a rule requiring the surface temperature to be at least 5 degrees F higher than the dew point temperature to compensate for temperature fluctuations and instrument inaccuracies.
The relative humidity also can affect application and curing properties. For example, a too-high relative humidity can cause epoxy and urethane coatings to develop a haze or blush during curing and can retard the cure of waterborne coatings. Specifications often stipulate a maximum relative humidity of 85 percent.
Coatings that require moisture to cure (for example, ethyl silicate-type inorganic zinc-rich primers and moisture-cured urethane systems) should have a minimum humidity threshold specified rather than a maximum. Otherwise, the coating may not cure. As a general rule, prevailing ambient conditions are assessed every four hours or more often if required by the specification.
All coating materials shall be mixed and thinned according to the manufacturer's written instructions.
Mixing and thinning the coating materials are perhaps two of the most critical steps in the application process, yet they are often overlooked. In fact, many premature coating failures result from improper mixing of multicomponent paints. Without the proper ratio of components, the coating does not cure properly and will not perform in the environment. If zinc-rich primers are to be applied, they should be mixed carefully by power agitating the liquid component while the zinc powder is sifted slowly into the liquid, not vice versa.
After all components are combined (some coatings have as many as three components), the coating may have to induct (also referred to as sweat-in, cook, or digest) for a specified amount of time. This time is based on the coating material temperature, not the air temperature. If an induction period is required and not observed, the coating material may never achieve its full cure, and application difficulties may arise.
The type and amount of thinner added to the coating must be measured and documented. Use of thinners other than those recommended by the coating manufacturer may adversely affect coating performance and void the coating warranty. Further, the addition of thinner in the shop effectively raises the volatile organic compound (VOC) content of the coating. Most shops must record the quantity of VOC emissions generated, which obviously includes the thinner used in the coating. Finally, the addition of thinner affects the target wet film thickness. This can be accounted for, provided the amount of thinner added to the coating is known.
All coating materials shall be applied according to the manufacturer's written instructions.
Coating manufacturers often provide recommendations for spray guns, tips, and atomization and fluid pressures to atomize their coating materials properly. If the specification invokes the manufacturer's recommendations, the specific equipment must be used by contract. If not, the fabricator can choose the application method or methods by considering the shape and size of the parts to be coated, the coating type, transfer efficiency requirements, and skill level of the applicator.
Large structural members such as I beams and girders can be coated by airless spray, which yields high productivity and a good-quality finish. Smaller components such as bracing and K frames are best coated using conventional or air spray because the applicator has more control over the spray pattern and the amount of coating that exits the spray gun.
Some coatings don't lend themselves to all application methods. For example, zinc-rich primers should not be applied by brush or roller, except for small touch-up areas. For improved transfer efficiency, the fabricator may consider air-assisted airless spray or high-volume, low-pressure (HVLP) spray.
All spray techniques require a high degree of applicator skill, and not all applicators are proficient in all methods. Spray application training classes are available from many equipment manufacturers, and some coating manufacturers provide one-on-one spray training for their products. Applicator training should be well-documented.
Even the best applicators can experience difficulty with various coating materials. If the specified coating material is new, or the applicator has never sprayed it before, he should be given the opportunity to experiment with the material before applying it to contract steel components.
Experimentation may include tip selection, pressure adjustments, achieving the wet film thickness without sagging, atomization, and flow characteristics.
A common shop application challenge is topcoating of inorganic zinc-rich primers. These coatings are porous by nature. The dry film is filled with air voids that can cause problems when an overcoat is applied. Once an overcoat is applied, the air inside these voids becomes displaced with paint and solvent. The air is released by blowing a hole (tiny pinhole) in the topcoat. These pinholes also can "mirror through" all subsequent coats and generally are considered to be unacceptable.
To prevent this from occurring, many manufacturers recommend the application of a mist coat of the overcoat, followed by the full coat. A mist coat is a thin, semitransparent application of the second coat, ideally thinned to the maximum allowable. This layer effectively penetrates the pores and seals the inorganic zinc layer. Once the mist coat is tacky, the full coat is applied. The displaced air passes through the thin film while it is still fluid enough to flow together and seal the opening. This process effectively eliminates most of the pinholing, although a few may still occur sporadically. It should be noted that this procedure is unnecessary when topcoating organic zinc-rich primers, because these coatings are not porous (the pores are filled with organic binder, such as epoxy or urethane).
The dry coating thickness shall be 2-4 mils for the primer, 5-7 mils for the intermediate coat, and 2-3 mils for the finish coat. The total coating system thickness shall be 9-14 mils. The thickness of each layer shall be measured according to SSPC-PA2.
Achieving the specified thickness for each layer of the coating system is critical for corrosion protection. One cannot make up for insufficient primer thickness by applying a thicker intermediate layer. Each layer has its unique function and optimum thickness, and it performs that function only if it is applied within the specified thickness range.
SSPC-PA2, Measurement of Dry Coating Thickness With Magnetic Gages, is perhaps the most common industry standard for assessing coating thickness. ASTM D1186, Nondestructive Measurement of Dry Film Thickness of Nonmagnetic Coatings Applied to a Ferrous Base, also describes a procedure for measuring coating thickness, but PA2 is much more common.
While the applicator may measure the wet film thickness, it is the dry film that is dictated by the contract documents. SSPC-PA2 defines coating thickness gauges in two categories: Type 1 (magnetic pull-off) and Type 2 (constant-pressure probe). The standard also prescribes gauge calibration procedures, the number and frequency of measurements, and limits on insufficient and excessive thicknesses.
The standard is described in Volume 1 of the SSPC Steel Structures Painting Manual, Chapter 6, "Inspection," and in the document itself, which is contained in Volume 2 of the SSPC Steel Structures Painting Manual.
Following SSPC-PA2 for shop painting (as currently written) can be challenging, because it is based on total square footage painted, not tonnage or laydowns. The standard is currently under revision to address measurement of thickness on steel beams (girders) and miscellaneous pieces based on length and height of beams, as well as laydowns of beams and miscellaneous pieces. In the current draft, measurement areas include both sides of the top and bottom flanges, both sides of the web, and flange edges (minimum 1-inch thickness) at specified intervals along the length of the beam. It is unknown when this revised PA2 will be released to the industry.
Prior to application of each successive coat, the previous coat shall be adequately cured.
Applying multicoat systems can be a bottleneck, in part due to the cure time required for each layer. However, application of subsequent coats before the previous coat is fully cured can cause premature coating failure and costly rework.
How a coating cures and the degree to which the coating must cure depend on the generic type of coating. For example, acrylic latex, epoxy, and urethane coatings require a minimum temperature to cure (and excessive moisture may be detrimental). Ethyl silicate inorganic zinc primers require moisture to cure, and excessive heat (and a correspondingly low airborne moisture content) causes the coating to dry quickly, without achieving cure. In fact, drying often is mistaken for curing, and the inorganic zinc primer is topcoated too soon, resulting in a cohesive break within the zinc primer and widespread disbonding of the coating system.
Some coating manufacturers recommend misting this coating with water one or two hours after application to generate the cure necessary for successful overcoating. For chemically cross-linked coatings such as epoxy and polyurethane, the next coat should be applied when the coating has dried, but before it is fully cured, to ensure an intercoat bond.
Several methods can be used to verify a coating's cure. Ultimately, the coating manufacturer should be consulted for the recommended procedure. ASTM prescribes solvent rub tests for thermoset organic coatings in D5402, Assessing the Solvent Resistance of Organic Coatings Using Solvent Rubs, and for inorganic zinc primers in D4752, Measuring MEK Resistance of Ethyl Silicate (Inorganic) Zinc-Rich Primers by Solvent Rub. Pencil hardness also can be used (ASTM D3363, Film Hardness by Pencil Test), and ASTM D1640, Drying, Curing, or Film Formation of Organic Coatings at Room Temperature, prescribes several procedures for assessing the drying phases of organic coatings.
The adhesion of the coating layers to each other and of the coating system to the underlying substrate shall be measured in accordance with ASTM D4541-95. The adhesion shall be no less than 750 PSI.
In some cases, specifications require adhesion testing of the coating system. This should be performed only on the fully cured system, and only if required by the contract. Adhesion testing is destructive to the coating film, and the tested area must be repaired. ASTM D4541, Pull-off Strength of Coatings Using Portable Adhesion Testers, prescribes a procedure that entails attaching a pull stub to the coating film using an adhesive, then detaching the pull stub using a pull-off tester, which applies perpendicular force to the pull stub. The pull-off strength (in PSI) and location of break are recorded.
When evaluating this type of adhesion, it is important for the contract to specify the type of adhesion tester to be used, because different testers yield different adhesion values.1
The adhesion of the coating layers to each other and of the coating system to the underlying substrate shall be measured in accordance with ASTM D3359. The adhesion shall be no less than 4A.
Alternatively, adhesion testing can be performed using a simple knife and tape test. Similarly, this should be performed only on the fully cured system, and only if required by the contract, as it too is destructive. ASTM D3359, Measuring Adhesion by Tape Test, describes the method, which entails making an X-shaped cut through coating systems that are greater than 5 mils thick or a series of cross-cuts through coating systems that are less than 5 mils thick. Cuts are made with a utility knife blade. Tape is applied and pulled from the cut area, and the adhesion is rated on a scale from 5 (little to no detachment) to 0 (significant detachment).
It is important to use a new knife blade when performing this test, because a dull blade can generate false low values. The type of tape used also can influence the results. Currently, Permacel 99 is the tape that is referenced in the standard. Using tapes with stronger or weaker adhesives can influence the results. Finally, multiple tests should be performed to characterize the adhesion of the coating system, rather than relying on a single test.
The coating system shall be free of all visible defects. Mudcracking of the zinc primer shall be repaired by removal down to bare metal, followed by re-application of the primer.
Runs, sags, and drips in the coating film are not only aesthetically unpleasing, but they represent areas of excessive thickness that may crack and disbond while in service. These defects should be brushed out during application or sanded out before application of the next coat. Mud cracking of inorganic zinc primers occurs when they are applied too thick. While some specifications only require screening down to "sound zinc," most require removal down to the substrate.
If abrasive blast cleaning is used for the removal process, it must be done very carefully to prevent overblast damage. Power tools or vacuum blasting likely are better alternatives. If grinding wheels are used and the surface becomes polished, the profile, or anchor pattern, may require regeneration using a needle gun, rotopeen, or other impact tool.
What's in the future for shop painting? Although difficult to predict, here are a few thoughts.
Despite the complexity of applying three-coat systems in the fabrication shop, total shop painting likely will become commonplace in the future, as the logistics and cost of field painting continue to escalate. Further, control over temperature and humidity and accessibility to steel make total shop painting attractive to facility owners and state agencies.
Fabrication shops that embrace the technology, invest in equipment and training for their personnel, and can successfully complete paint jobs with minimal rework will find three-coat jobs lucrative.
VOC regulations have led to the use of more and more waterborne coatings in the shop. Some state agencies require complete waterborne systems, including waterborne zinc primers. As these coatings evolve, they will continue to improve in both application characteristics and field performance.
Many agencies also are considering electric arc metallizing as an alternative to zinc-rich primers. A Federal Highway Administration (FHWA) study showed metallizing provides the highest degree of corrosion protection of steel bridges. It can provide 30 to 50 years of service if sealed with a liquid-applied coating. With the right equipment and proper training, shops can diversify their coating application services with the addition of metallizing. It should be noted, however, that metallizing is not a surface-tolerant coating material, and quality control throughout the surface preparation and coating application processes is paramount to the success of this type of coating system.
Finally, more private and public agencies probably will require shop certification through AISC's Sophisticated Paint Endorsement or SSPC's QP3 program. Shops already certified in one of these programs should strive to maintain their certification through continuous employee training and quality control records. Shops not yet certified may want to take a hard look at becoming certified to be eligible to bid on contracts that contain a painting component.
William D. Corbett is technical services administrator for KTA-Tator Inc., 115 Technology Drive, Pittsburgh, PA 15275, phone 412-788-1300, fax 412-788-1306, e-mail firstname.lastname@example.org, Web site www.kta.com. KTA-Tator provides services and products for the coatings industry, including consulting; inspection; laboratory testing; environmental, health, and safety; and measurement instruments.