Galvanized steel ductwork runs through finished basements and open-ceiling lofts with a zinc passivator coating applied at the factory. That passivator exists to prevent the zinc from corroding during shipping and storage. It also prevents paint from bonding. Apply primer directly to new galvanized without removing the passivator and the coating will peel off in sheets within weeks, not because of a bad primer choice but because the underlying surface was never ready to receive it. The passivator is the starting point of every paint adhesion problem with exposed ductwork, and every other approach follows from understanding that single fact.
This guide covers how to clean and prime metal pipes and ducts for reliable adhesion, which paint products handle hot and cold water pipes, how to choose between spray and brush application, and how color strategy can make exposed mechanical systems either disappear into the background or become a deliberate feature of the interior.
Cleaning and Priming Metal Pipes and Ducts for Paint Adhesion
New galvanized ductwork needs the passivator removed before priming. The simplest approach is a white vinegar wipe-down. Soak a clean rag in white vinegar and wipe all surfaces of the duct that will be painted. The mild acetic acid etches through the passivator and creates a surface that primer can grip. Alternatively, phosphoric acid etch products accomplish the same result faster with more aggressive chemistry. After etching, rinse with clean water, allow the duct to dry completely, and prime within 24 hours before any surface oxidation reestablishes the adhesion barrier.
Alkyd and oil-based paints applied directly over zinc cause saponification, a chemical reaction between the alkyd resin and the zinc that produces a sticky, soap-like compound. That compound prevents the coating from ever bonding properly, and the topcoat releases from the metal over time regardless of how well the initial application appeared to go. The rule for galvanized metal is acrylic primer only, with no exceptions.
Zinsser Bulls Eye 123 is the first-choice primer for galvanized ductwork in brush-on form. It is water-based, bonds without saponification risk, and seals the etched galvanized surface before the DTM topcoat goes on. For aerosol application on round ducts where brush coverage is slow, the Rust-Oleum Stops Rust Universal Bonding Primer is the correct aerosol choice specifically noted for galvanized surfaces. One aerosol alternative to avoid is Rust-Oleum High Heat Primer (ATO-56), which is incompatible with galvanized metal and should not be used regardless of other uses you may have for it.
For black iron pipes that carry gas lines or cold water supply, the surface has an oil-based anti-rust coating from the foundry that blocks adhesion in the same way. Clean black iron and cast iron with mineral spirits or acetone before priming to remove the factory coating. After solvent wipe-down, use a primer with a rust inhibitor for ferrous metals. On any pipes in a pre-1978 home, test for lead paint before sanding. Lead test swabs rated for metal surfaces take under a minute and remove any uncertainty before generating dust.
Choosing the Right Paint for Pipes That Carry Hot or Cold Water
Standard acrylic latex paint softens and becomes tacky on pipes carrying hot water or steam. The surface temperature of a steam pipe under pressure can exceed 180 degrees Fahrenheit, and even standard hot water supply pipes commonly reach 120 to 140 degrees Fahrenheit at the pipe surface. Acrylic latex softens significantly above 120 degrees Fahrenheit, which causes it to wrinkle, release adhesion, and peel. The softened film also picks up dust and becomes tacky to the touch.
Rust-Oleum High Heat Spray Paint is rated to 1,200 degrees Fahrenheit and is the standard product for cast iron radiators and steam risers. It is available in flat black, aged copper, and silver, and professionals have used it on cast iron radiators for more than 20 years without peeling reports. The application requires a surface that is clean, primed if porous, and at room temperature. Apply in thin coats rather than a single heavy coat, which causes wrinkling.
Pittsburgh Paints aluminum paint, rated at 450 degrees Fahrenheit in service, is the brush-on alternative for radiators and steam risers where a metallic appearance is acceptable or desired. The aluminum pigment also reflects heat, which makes it a practical choice on supply pipes that run through a room where radiated heat from the pipe is not desirable.
For cold water supply pipes and drain lines, heat tolerance is not the constraint. The constraint is condensation. Cold water pipes in humid months develop surface condensation that runs down the pipe and pools at joints. A paint that lacks flexibility or moisture resistance fails at these joints first. Use a fully cured acrylic DTM topcoat after the correct primer and allow a full cure of at least seven days before the first condensation contact. Waterborne alkyd DTM finishes handle condensation cycling better than standard latex once fully cured.
On older homes, cast iron drain stacks may have multiple paint layers from previous occupants. Test the existing paint before adding more. Each additional coat of incompatible product pushes the total film thickness higher and increases the chance of adhesion failure at a layer junction. If the existing paint is unknown-type, sand with 120-grit to mechanically degloss and prime with a universal bonding primer rather than assuming compatibility.
Spray vs Brush Application on Exposed Ductwork
Round duct sections, T-connections, elbows, and damper housings are irregular shapes that do not take well to brush application. A brush held at a consistent angle against a flat surface produces a smooth, even film. The same brush applied to a round 10-inch diameter duct creates lap marks at every rotation point, heavy buildup where the brush changes direction, and light spots on the back of round curves that are difficult to reach at all. The more complex the duct assembly, the more pronounced these problems become.
Spray application is the preferred method for exposed ductwork. For aerosol work on individual pipes and smaller ducts, the Rust-Oleum Direct-to-Metal aerosol line covers straightforward sections efficiently. For whole-ceiling ductwork in a finished basement, an airless sprayer or HVLP gun with a DTM latex coating is faster and more consistent than aerosol. Keep the spray tip 10 to 12 inches from the surface and rotate the gun around round duct sections rather than holding it at a fixed angle. Two thin coats with full dry time between produce a more even result than one heavy coat.
Masking is the primary preparation step for spray work on exposed ductwork. Cover the subfloor and any finished wall surfaces adjacent to the ductwork before spraying. Overspray from airless or HVLP equipment at the volumes required to coat an entire duct run lands on every uncovered surface in the space. For basement work with exposed joists above, drape canvas or plastic sheeting over any finished floor areas below the duct work zone.
Brush application is practical for supply and return trunk lines that run in a straight, flat horizontal plane, such as a rectangular duct running along a basement ceiling. A 3-inch or 4-inch brush can cover rectangular duct faces in long, consistent strokes without the rotation problem that round duct creates. On flat duct surfaces, brush marks from Zinsser Bulls Eye 123 primer smooth out under the DTM topcoat. Use Floetrol at 8 ounces per gallon in the topcoat to extend open time and allow the brush marks to level before the paint skins.
For visible mechanical systems in open-plan spaces or design-conscious interiors, the spray approach produces a cleaner result that reads as intentional rather than improvised. A brushed finish on complex ductwork in a finished space always shows the brushwork at some angle of light.
Color Strategies for Making Pipes and Ducts Disappear or Stand Out
The most common residential approach to exposed ductwork is to paint everything, including the duct, the surrounding joists, any fire blocking, and the subfloor above, the same flat black. In open-plan kitchens and finished basements designed to show industrial structure, flat black makes all the mechanical, structural, and electrical elements recede into a visual field that reads as a deliberate dark ceiling rather than a collection of systems. This approach requires two coats of flat black on galvanized with proper galvanized prep, since one coat on galvanized rarely produces a fully uniform finish.
The second common strategy is to paint ducts and pipes the same color as the ceiling they run against. This approach makes exposed systems recede visually without the dramatic commitment of full black, and it suits rooms where the ceiling is a warm white or off-white rather than the industrial palette that flat black implies. The important note for this approach is that galvanized prep requirements remain the same regardless of what color the topcoat is. Ceiling-color ductwork that was painted without etching the passivator and priming with acrylic primer will peel exactly as quickly as any other color applied over improperly prepared galvanized.
A third approach, used in deliberate industrial and loft interiors, is to keep exposed pipes and ducts metallic by applying clear DTM enamel or satin metallic paint over the primed galvanized surface. This preserves the industrial look without requiring color commitment while still providing a controlled, clean finish that resists dust accumulation better than raw galvanized.
For exposed copper supply pipes, the natural copper color is often worth preserving. A clear water-based DTM sealer over lightly cleaned copper maintains the warm copper tone and prevents the greenish patina (verdigris) that forms on uncoated copper in humid environments. If patina has already developed, clean with a commercial copper cleaner, rinse thoroughly, dry fully, and seal before the patina rebuilds.
In rooms where a single pipe or duct run cannot be fully concealed, paint it the closest possible match to the wall or ceiling surface it runs nearest to. A single pipe painted to match a white wall becomes nearly invisible under normal room lighting. The same pipe left in raw galvanized reads as a feature regardless of whether a feature was intended.