Finishing Grades for Ductwork Fabrication

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When planning heavy ductwork fabrication, finishing is a crucial consideration because atmospheric conditions and temperature extremes can compromise heavy ductwork integrity.

Stainless-steel ductwork does not rust and does not need a protective finish, but for most applications this material is very cost prohibitive. Carbon steel is a more affordable alternative, but requires special finishing such as hot-dip galvanizing, where the fabricated ductwork is immersed into a vat of molten zinc to form a protective coating that is bonded to the steel.

Although not as expensive as using stainless steel, hot-dip galvanizing an intricate carbon-steel duct system is still cost-prohibitive for most budgets, and basically overkill.

When contemplating ductwork fabrication for exhaust and HVAC applications, the most cost-effective solution is to manufacture the ductwork using carbon steel and simply add a layer of protective paint, especially when there’s minimal risk of chipping or gouging the protective coating and creating exposed areas vulnerable to corrosion. It’s fast, comparatively easy, and delivers a favorable quality-to-cost ratio.

We’ve walked through the process of how heavy ductwork is custom fabricated in a previous blog post, but wanted to take a closer look at what goes into the painting process because many of our customers see this as the best solution for their needs.

Prepping Carbon Steel Ductwork for Painting

The first step in finishing carbon-steel ductwork is preparing the surface to be painted, to ensure a proper degree of paint adhesion. The standards for this come from the Society of Protective Coatings (SSPC) and a joint standard with the National Association of Corrosion Engineers International (NACE):

SSPC-SP1: Solvent cleaning to remove all visible contaminants such as oil, grease, and salts; solvent cleaning does not remove rust or mill scale, the flaky surface of hot rolled steel

SSPC-SP2: Hand-tool cleaning (e.g., scraping or wire brushing) removes loose contaminants such as rust and mill scale

SSPC-SP3: Power-tool cleaning (e.g., power sanding or power grinding) also removes loose contaminants

SSPC-SP5/NACE 1: White metal blast cleaning to eliminate all contaminants visible without magnification; this process is often performed through sandblasting or metal-bead blasting to create a gray-white color with a slightly roughened surface to help anchor paints

SSPC-SP6/NACE 3: Commercial blast cleaning to eliminate contaminants of at least 66 2/3-percent of a unit area (defined as a 3″ x 3″ square) when viewed without magnification; slight streaks and minor discolorations caused by rust or stains in less than 33 1/3 percent of a unit area is allowed

SSPC-SP7/NACE 4: Brush-off blast cleaning to eliminate contaminants visible without magnification; mill scale, coatings, and rust may remain if they cannot be removed with a dull putty knife

SSPC-SP10/NACE 2: Near-white blast cleaning to eliminate unmagnified visible contaminants of at least 95 percent of each unit area

For ductwork fabrication, meeting the necessary grade of surface preparation is as important as the painting itself; improperly prepared surfaces can result in coating failures and decreased service life.

Applying Paint During Heavy Ductwork Fabrication

Once the surface has been properly prepared, it’s time to apply the paint itself.

The thickness of the primer and protective paint is directly related to its temperature tolerances and lifespan. If your heavy ductwork has strict guidelines that need to be met, it’s important to take measurements with each coat to ensure precise conformity. Protecting your ductwork investment from its environment will properly amortize its cost over its lifecycle and prevent costly repairs.

At Southern Metal Fabricators, we have a seamless process for finishing heavy ductwork. Our on-site finishing shop enables us to blast clean the carbon steel to white metal using steel grit. This blast room has a recirculation system that recaptures the blast media for the next application to maximize efficiency.

During the painting stage of ductwork fabrication, we use a state-of-the-art paint booth to provide uniform coatings and prevent debris or dust settling on the freshly painted surface as it dries. Each hand-sprayed layer is applied with a meticulous eye thanks to a sophisticated lighting system that enables us to see clearly every nook and cranny and deliver a thorough coating. The paint booth itself measures 1,300-square feet to accommodate large components and can deliver thermal curing at temperatures up to 150° F to ensure quick turnarounds and complete hardening.

Our certified in-house staff carefully measures and documents each layer, and we often use a third-party inspector to corroborate the fidelity of our final work, as well as invite representatives from our customers into our shop to conduct their own reviews.

We also assemble all our ductwork before shipping to ensure everything fits properly and has no assembly challenges, documenting every step before disassembling and preparing for shipment. Our goal is to eliminate any potential problems before shipment so our clients can execute on-site assembly worry free.

If you need heavy ductwork fabricated to exact standards, let us help you meet those requirements in the most affordable and efficient way possible. Please contact us or call us at 1.800.989.1330 and we can help you find the best solution for your situation.