Underlying Considerations for Overcoating: Lessons learned for metal building façades

by Katie Daniel | May 24, 2017 10:15 am

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By Gil Giles
For decades, prefinished sheet steel has been used for wall cladding, roofing, and interior panels in architectural building systems, according to the Canadian Sheet Steel Building Institute (CSSBI). (The full fact sheet, “Repainting Factory Prefinished Metal Panels,” was published by the Canadian Sheet Steel Building Institute [CSSBI] in February 2006.) Typically, these components are metallic-coated by the manufacturer using a hot-dip process (CSSBI released “Metallic Coated Sheet Steel for Structural Building Products” in December 2012.) and prefinished with coil coatings in accordance with CSSBI S8-2008, Quality and Performance Specification for Prefinished Sheet Steel Used for Building Products, which includes weathering standards for film integrity, chalking, and colour changes. (CSSBI S8-2008, Quality and Performance Specification for Prefinished Sheet Steel Used for Building Products, was released in August 2008.)

Over time, corrosion of the building envelope will detract from its appearance and could lead to maintenance issues such as a leaking roof. Waking up ‘tired’ metal buildings with a field-applied overcoating system can extend the service life of commercial and industrial structures, protecting them from further corrosion while enhancing the esthetics of the façade.

In its fact sheet, “Repainting Factory Prefinished Metal Panels,” CSSBI states:

Due to the diversity of factory-applied prefinished systems, and the diversity of potential field applied repaint systems, it is impossible to offer one comprehensive repaint procedure for all possible situations. However, it is possible to offer a set of guidelines to be considered in every potential situation.

Successful overcoating of factory-applied coil coatings or galvanized finishes can be achieved when proper evaluation, surface preparation, and application techniques are followed. CSSBI advises consulting with an experienced local coating contractor can be a valuable first step.

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Overcoating risk factors
Among the variables that need to be evaluated to make a decision on overcoating is the condition of the existing coating system—beginning with the amount of corrosion present.

ASTM International has published standard test methods using photographic reference standards to evaluate aged coating systems for rust and blistering. For this purpose, ASTM D610-08, Evaluating Degree of Rusting on Painted Steel Surfaces, uses a numerical scale of one to 10, where a higher rating indicates less rust (Figure 1). Developed in co-operation with the Society for Protective Coatings (SSPC), the visual examples depict the percentage of rusting on existing painted metal surfaces.

Other overcoating risk factors can include film defects (e.g. cracking or blistering), total film thickness, number of existing coatings, and those coatings’ adhesion to the substrate and between coats. Defects such as blistering, cracking, and delamination of an existing coating system can disqualify a metal building as a candidate for overcoating, and extensive blistering can do the same unless spot repairs can be made. The size and density of blistering on painted surfaces can be assessed through visual examination of coatings in accordance with ASTM D714-02, Standard Test Method for Evaluating Degree of Blistering of Paints.

Aged coating systems with multiple layers of paint, or with high film thicknesses and marginal adhesion, are also poor candidates for overcoating due to cohesive stress occurring when new coatings are applied. Typically, the higher the film thickness is, the greater the cohesive stress will be on the existing coating system.

The number of existing coatings and their dry film thickness (DFT) can be measured with a Tooke gauge, magnetic pull-off, magnetic-flux film thickness gauge, or by visual examination of a paint chip cross-section. Prior to determining if an aged paint system is a candidate for overcoating, it is important to check adhesion of the existing paint system.

Using ASTM D3359, Standard Test Methods for Measuring Adhesion by Tape Test, a delamination risk factor can be assigned on a scale of one to five, with one representing the lowest delamination risk after overcoating and five the highest (Figure 2) (For more on this method, see ASTM D3359-09e2, Standard Test Methods for Measuring Adhesion by Tape Test.). A worst-case scenario would be a multi-coat, high-film-thickness, existing coating system with poor adhesion between coats or to the substrate in a freeze/thaw environment. Aged paint systems with high delamination risk factors (4.5 to five) are generally not candidates for overcoating.

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Surface preparation
The importance of surface preparation is emphasized by SSPC in its Protective Coatings Inspector (PCI) Training Program, which is intended for those involved with coating projects. (For more on the Protective Coatings Inspector [PCI] Training Program, visit www.scribd.com/document/240673149/SSPC-Student-Intl-1[5].)

“Preparing the surface for subsequent application of the coating system is the most critical (and typically the most expensive) step in an industrial coatings project,” according to the SSPC PCI Training Program’s section on surface preparation. “In general terms, the better the surface preparation, the longer the life of the coating system.”

The program also advises ambient conditions such as surface and dewpoint temperature should be measured before initiating final surface preparation, recommending professionals verify “the temperature of the surfaces is at least 3 C (5 F) higher than the temperature of the dewpoint, to preclude airborne moisture from condensing on the surfaces.”

Additional information on surface temperatures, curing times, compatibility of coatings, and other technical data is contained in product data sheets (PDS), which are usually maintained on the coating manufacturer’s website or available from company representatives. In some situations, laboratory analysis from a coatings manufacturer may be required to identify the generic type of an existing coating system and assess the suitability of surface preparation methods for a specific project. The analysis can establish whether the old paint system is compatible with self-priming tie-coats and other coating candidates.

Prior to overcoating, existing paint systems must be dry and clean. This can be achieved by solvent cleaning in accordance with SSPC standards for solvent cleaning, hand-tool cleaning, and power-tool cleaning. (These standards include SSPC-SP1, Solvent Cleaning, SSPC-SP2 Hand-tool Cleaning, and SSPC-SP3, Power-tool Cleaning.)

Cleaning with wire brushes, scrapers, and other non-power tools can remove loosely adhering old paint and flaking mill scale. Rusty areas require power-tool cleaning with grinders and pneumatic chisels and spot priming before overcoating.

CSSBI recommends special attention and treatment be given to areas that may have already begun to corrode by removing all traces of white, black, or red rust, then applying a corrosion-resistant, zinc-rich primer.

“Low-pressure water cleaning (LPWC) or ‘pressure washing’ is often specified for overcoating projects,” according to the SSPC PCI Training Program. “LPWC can be very effective in removing dirt, chalking, bird droppings, and other contaminants from the surfaces, although mechanical agitation of the surface during LPWC is often required to ensure adequate removal.”

Overcoating requires the edges of existing coatings to be feathered using sandpaper or power-tool attachments to form a smooth transition prior to spot-priming. Failure to feather edges can result in poor adhesion and an undesirable appearance.

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Choosing an appropriate overcoat system
When selecting an overcoating system, it is important to consider the cohesive stress exerted on the existing system will vary with generic type (Figure 3).

Nonflexible, conventional two-component epoxies and aliphatic urethanes will develop more cohesive stress upon curing than more flexible coatings such as acrylic emulsions, medium-to-long alkyds, moisture-cured urethanes, and epoxy mastics.

The higher the film thickness of the overcoat, the greater the cohesive stress on the existing paint system. Applying two-component epoxies at 12 mils DFT to an aged paint system with marginal adhesion could result in the old system pulling apart at its weakest adhesion link—between coats or from the substrate.

Geographic location also plays a crucial role in the selection of an appropriate overcoat system. In areas where frequent freeze/thaw cycles occur, coating systems are subject to more stress, so flexible, low-cohesive-stress overcoat systems are the appropriate choice.

Among the new coating products intended for use as overcoats on minimally prepared, sound-rusted steel and previously coated surfaces is a single-component, mastic waterborne acrylic coating that can be applied in a wide range of environments. This coating offers 200 per cent elasticity, which enables it to expand and contract with the substrate as the temperature varies.

The development of new infrared (IR) heat-reflective pigments in finish coatings can significantly reduce the amount of heat absorbed and retained by a metal building’s exterior wall and roof surfaces. The result is savings in fuel and cooling costs.

While most reflective coatings are white, coatings manufacturers have been able to incorporate IR-reflective pigments into field-applied, air-dried fluoropolymers offering a wide range of colours. This new technology enables the architectural community to gain the benefits of heat-reflective coatings without having to sacrifice colour and esthetics. The use of IR-reflective coatings on a structure can also earn points in four Leadership in Energy and Environmental Design (LEED) categories, described in Heat Island Reduction Option 1 (non-roof and roof). (Details on these categories can be found in LEED v4 for Building Design and Construction, Sustainable Sites (SS) Credit,: Heat Island Reduction, pages 38 to 39.) LEED requires roofing materials used on low-slope roofs with a slope of less than 2:12 to meet an initial solar reflectance index (SRI) of 82 (64 for three-year aged SRI). Steep-sloped roofs with a slope of more than 2:12 must have an initial SRI of 39 (32 for three-year aged SRI).

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Before a final decision is made, the overcoat system should be tested for adhesion to the existing paint system. After ensuring the existing system is dry and clean, it is best to use ASTM D5064, Standard Practice for Conducting a Patch Test to Assess Coating Compatibility, as a guide in conducting adhesion patch tests. ASTM D3359 Method A is best used to evaluate adhesion of the new system to the old. The overcoat system should not wrinkle, lift, or show any other adverse response to the existing system.

Another method for determining if an aged coating system is sound enough to accept overcoating is the ‘recoatability test’ recommended in CSSBI Fact Sheet 4:

1. Clean and otherwise prepare several small test areas representative of the entire surface to be repainted.
2. Apply a coat of the desired repaint per the manufacturer’s instructions. Allow each test area to dry per the manufacturer’s instructions.
3. After drying using about 200 mm [8 in] of gray “duct” tape for each area to be tested, firmly smooth about 75 to 125 mm [3 to 5 in.] of the tape onto the repainted areas. Rapidly pull off the tape, attempting to remove the recently applied air-dried coating.
4. Unsatisfactory adhesion/compatibility is indicated if the new coating is removed with the tape.
5. If an unsatisfactory test occurs, it may be necessary to conduct a different or additional cleaning procedure, apply an intercoat adhesion primer, or select a different type or different manufacturer’s repaint coating.
6. Repeat the “recoatability test” until satisfactory results 
are obtained.

Conclusion
Whether or not overcoating a metal building is a feasible alternative to complete removal and repainting depends on factors such as the condition of the existing paint system, the amount of corrosion present, and the number of old coatings and their adhesion to the substrate/between coats. Surface preparation of existing coatings and geographic exposure conditions are other major considerations in determining whether overcoating is practical or if complete removal is required. Environmental service conditions are especially important in Canada, where freeze/thaw cycling during winter months causes significant stress on overcoat systems. With the availability of new high-performance coatings products for overcoating metal building components, specifiers can achieve enhanced protection against corrosion, extended colour and gloss retention, and energy savings.

[8]Gil Giles has worked in the paint and coatings industry for more than a decade in various functions, such as coatings sales, specification writing, and inspection. Giles currently serves as an independent representative with Tnemec Company Inc., assisting architects, engineers, and applicators with protective coatings and linings in New Brunswick, Nova Scotia, Prince Edward Island, and Newfoundland. He is a NACE III coating inspector, is certified by the Master Painters Institute (MPI), and holds a bachelor’s degree in commerce from Dalhousie University. Giles can be reached via e-mail by contacting ipaint@nb.sympatico.ca[9].

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