The damage started to become apparent during a roof inspection in the spring of 2015. After 25 years of service life, the metal roof on Central Middle School in Dobson, N.C., was showing its age. Back-to-back winters with heavier-than-average amounts of snow and ice caused the blue, factory-applied coating on the standing seam roof to flake and peel off, exposing large sections of gray primer and the underlying steel roof panels to the elements.
As snow and ice slid off the roof, they took chunks of paint with them. The coating failure worsened, to the point where the school district’s director of construction, planning, and design was concerned the exposed metal panels could rust and create opportunities for leaks.
Eager to avoid the expense and disruption of replacing the roof, the district director reached out to the plant operations division of the North Carolina Department of Public Instruction for advice. The division’s engineers inspected the roof and recommended recoating it with a fluoropolymer-based roof coating—a strategy which had been used successfully in other school districts in the state.
Coating removal process: eye on the environment
After a visual inspection of the roof, along with millimeter thickness readings and scratch testing of the existing coating, it became apparent all the existing coating would need to be removed—a process involving more than just simple power washing.
Proper removal of existing coatings is crucial for the ability of a new coating to perform as intended. Without it, the new primer and topcoat will fail. The coating contractor used advanced coating removal methods and a proprietary paint stripping material that are effective and cleaner than other methods such as bead blasting. The stripping material worked quickly; during an initial test, it completely removed a patch of existing coating down to bare metal in under 15 minutes.
As part of its standard waste disposal policy, the contractor took care throughout the removal process to keep the stripping chemical and coating residue from contaminating the environment—a special concern because a farm with a pond was located adjacent to the school property. They set up a series of drainage pipes and collection bins across the 38,100-m (125,000-ft) roof to capture the chemical and solid waste so it would not get into the storm drain system and groundwater. Once captured, all the waste was placed in a dumpster and disposed of according to local waste disposal regulations.
Polyvinylidene fluoride (PVDF)-based coating extends roof lifespan
After stripping the old paint from the roof, the coatings team thoroughly pressure-washed the entire surface to ensure no paint remover or residual materials remained. Then, they sprayed a 1.5 to 2 mm (0.05 to 0.08 in.) thick coat of a bonding primer. The primer is designed to adhere to tough surfaces, including metal and pre-coated metal. Its water-based, low-volatile organic compounds (VOC) chemistry provides early water resistance, protects against corrosion, and applies with a smooth, uniform finish for optimum aesthetics.
Next, they spray-applied two 2.5 to 3 mm (0.1 to 0.12 in.) layers of a PVDF-based metal restoration topcoat in a custom-tinted blue color to perfectly match the roof panels’ original factory finish. Engineered for coated or uncoated ferrous and non-ferrous metal surfaces, the topcoat resists the harmful effects of UV degradation such as fading, erosion, and chalking. It safeguards against salt spray and corrosion, protects against a wide range of abrasions, and has exceptional resistance to algae, mold and fungal growth, dirt pickup, and stains. It also is water-based and low in VOCs, meeting South Coast Air Quality Management District (SCAQMD) Rule 1113 for architectural coatings.
The topcoat’s PVDF resin has super-strong carbon-fluorine bonds that do not break down under exposure to the elements, thus resisting the film erosion common with exterior-grade, acrylic-based latex coatings. When exposed to UV energy and environmental stressors, the additives, pigment, and resin in latex coatings break down, creating a chalky residue. Eventually, the chalked coating wears away from the substrate—or is washed away by rain, wind, pressure washing or cleaning—until the coating film is gone and no longer protects the substrate. In addition, the chalky residue changes the coating’s surface energy and serves as a food source for mold and mildew growth, which further degrades a building’s appearance and creates cleaning and maintenance problems.
Quality at every step
Quality assurance was top of mind throughout the four-month project, which took place primarily during the school’s unoccupied summer months. Even before the job started, the project team conducted multiple installation mockups to test coating adhesion. During installation, the team did adhesion testing and x-cut tests after each coating layer was applied. Since the paint removal process caused some sealant degradation, the team also replaced those sealants, re-waterproofing the entire roof, then coated over the sealant.
Ernie Porco is a product application engineer at APV Engineered Coatings. He is the subject matter expert for the NeverFade Facade Restoration Coatings with Kynar Aquatec product line, along with the original equipment manufacturer-(OEM-) applied coatings products.