Powder Coating Galvanized Steel and Castings Without Outgassing

The short answer

Hot-dip galvanized steel and metal castings can both be powder coated, and both are known for outgassing: gas trapped in the part escapes while the powder is melting in the cure oven and leaves pinholes, craters or blisters in the film. Cleaning the surface does not fix it, because the gas is coming from inside the coating or the metal.

What prevents it is a sequence, and part of it happens before the parts reach a coater:

1. Tell the galvanizer the parts will be powder coated, so they skip the water quench and passivation bath.
2. Prep the zinc surface per ASTM D7803: clean, then profile it with a light sweep blast, zinc phosphate or light grinding.
3. Pre-bake at or above the powder’s cure temperature to drive the gas out before the powder goes on.
4. Keep the film build in range, and on hard cases add an anti-gassing primer.

Why galvanized steel and castings outgas

Castings. Cast iron and cast aluminum have surface porosity that holds air, moisture and oil. Products Finishing puts it plainly: “The root cause is trapped porosity in the casting.” As the part heats in the cure oven, that gas expands and pushes up through the powder while it is still soft.

Hot-dip galvanized (HDG) steel. ASTM D7803 says gases trapped during galvanizing escape through the powder as it cures, and that pinholing or blistering can follow if an outgassing step is not done first. The AGA describes it as water and air molecules trapped in the zinc coating.

Others to watch. PPG also lists hot-rolled steel, and Columbia Coatings notes that the oxide layer on hot-rolled steel can trap moisture and contaminants.

Heavy film build makes all of this worse. PPG notes that “too many mils may make it difficult for the air to release.” A pinhole is more than cosmetic: it is a path through the barrier film to the metal.

Powder coating vs galvanizing: which one do you need?

Buyers often ask this as either/or. In practice the two do different jobs.

The case for a duplex system is that each layer protects the other. The AGA calls the galvanized coating an ideal primer, and says the zinc extends the life of the powder coat by at least 50%. Its headline figure is that the combination “can last from 1.5 to 2.3 times the combined lifetimes of both systems.” That is an industry association’s estimate, not a guarantee for any particular part.

One scope note. ASTM D7803 covers parts galvanized after fabrication (ASTM A123 or A153 product and hardware). It does not cover galvanized sheet or coil-coated material. Pre-galvanized sheet gets a zinc-compatible pretreatment instead, which our guide on what metals can be powder coated covers. Tell us which one you have.

Before galvanizing: what to tell your galvanizer

The AGA’s guidance starts with the fabricator, galvanizer and powder coater talking before the parts are dipped. Three requests matter most.

• No water quench and no passivation bath. Many galvanizers quench parts in water or a passivation (chromate) bath to cool them fast and prevent wet storage stain. The AGA says the passivation “interferes with powder coating adhesion to the galvanized coating and can lead to blisters and peeling.”
• Dross, runs and drips ground flat. Dross particles are sharp zinc-iron bumps that poke through powder. The AGA says they must be ground flat, and if that is not possible, the part is not acceptable for powder coating. Runs and drip spikes need the same treatment.
• Ship promptly and store dry. After about 48 hours the AGA considers the zinc partially weathered, with oxides that must be cleaned off. Tightly stacked or damp parts can grow wet storage stain, a white deposit that must be removed before coating.

Put it in writing. A line on the galvanizing PO such as “Parts to be powder coated per ASTM D7803. Do not water quench or passivate. Remove dross, runs and drips” saves a round of rework.

How galvanized parts are prepped for powder (ASTM D7803)

After talking to the galvanizer, the AGA’s steps are:

1. Determine the surface condition: newly galvanized or partially weathered. If unclear, treat it as partially weathered.
2. Clean. Grind off excess zinc first, then remove oils and dirt with an alkaline cleaner, a mild acid or a solvent wipe, then rinse and dry. Grinding that throws sparks has reached bare steel, and the zinc must be repaired per ASTM A780 before coating.
3. Profile. The AGA gives three ways to profile the zinc. The first is a sweep blast per SSPC-SP 16 (a sweep blast rather than a standard blast, which would strip most of the coating) at 30 to 60 degrees, no more than 40 psi, with soft media around 200 to 500 microns and no iron in the media. The other two are a zinc phosphate treatment and light grinding that removes no more than about 1 mil. The goal is to remove zinc compounds, not the zinc.
4. Bake to drive out trapped gas (next section).
5. Powder coat as soon as possible, before fresh zinc oxide forms.
6. Cure per the powder maker’s data sheet.

How we handle the blast step. We don’t blast in-house. When a galvanized or cast job needs a sweep blast, we send the parts to our blasting partner, get them back, and run them through our own prep, pre-bake and coating. It is quoted as part of the coating job, on one quote and one PO; we don’t offer blasting on its own. The transit step adds some lead time. Our prep and masking page lists what we do in-house.

The outgas pre-bake, and why the sources disagree

Heat the part at least as hot as it will get in the cure oven, so the gas comes out while there is no powder on it. Then cool, coat and cure. Less gas is left to escape through the film.

How much hotter, and for how long, is where the published guidance splits:

They agree on the principle, at or above cure temperature, and differ on the margin. The AGA’s number is the most conservative. PPG’s advice to set the cycle by trial reflects the real variables: a thin bracket and a thick casting do not need the same cycle.

In every case the part itself has to reach temperature, and Columbia Coatings notes thicker parts need longer holds, so tell us wall thickness when you quote. After the bake, the AGA says to cool the part only enough to apply powder and coat as soon as possible.

If your drawing or spec calls out a specific outgas bake, we follow it. If it doesn’t, we set the cycle by part mass and substrate. On a new casting or galvanized part number, we suggest coating a few first-article parts before releasing the full batch. The AGA recommends the same thing: coat a sample piece first.

Anti-gassing primers and other fixes

When a pre-bake alone doesn’t stop the pinholes:

An anti-gassing primer. Cardinal makes H304-GR312 Anti-Gassing Primer, a gray epoxy-polyester hybrid that cures 10 minutes at 400°F metal temperature. In a 2018 recommendation for galvanized structural steel prepared to Minnesota DOT specifications, Cardinal called for:

• H304 primer at 3 to 5 mils dry film, cured at least 80% before topcoat
• A superdurable topcoat at 3 to 5 mils, fully cured to the temperature on its data sheet

The letter points to Cardinal’s data sheets for the cure temperatures. In that system the hybrid primer sits under a superdurable topcoat, which is the layer that faces the weather.

A degassing powder or a lower-cure powder. Talbert notes that powders formulated with an outgassing additive are “sometimes the best solution, but not always effective,” and that lower cure temperatures generally help, since temperature affects how much air is released.

Film build control. Keeping the topcoat inside its specified range gives gas less film to fight through.

The trade-off is cost: Talbert notes a primer’s second application and oven cycle add significant cost. A reasonable order is pre-bake first, then add a primer if first articles still pinhole. For outdoor or corrosive service, a two-coat system may be worth specifying from the start.

Castings: what’s different

Castings are harder to predict, because the porosity is set at the foundry. Talbert notes that better casting quality or resin impregnation can help, but often “the casting is what it is.”

• Oil in the pores. A part that smokes during the pre-bake still has contamination in it, and Columbia Coatings recommends running it again until it stops.
• Cast aluminum needs aluminum pretreatment. Iron phosphate is ineffective on aluminum, so a cast aluminum housing needs a conversion coating made for aluminum.
• Machined surfaces and threads. Bores, sealing faces and tapped holes usually get masked. Our masking guide covers what to call out.

What to put on the drawing or PO

For galvanized and cast parts, add these to the usual coating callout:

• Substrate: hot-dip galvanized (and to which spec, A123 or A153), galvanized sheet, cast iron or cast aluminum
• Galvanizer instructions: no quench, no passivation, dross removed
• Prep standard: “Prepare per ASTM D7803” for HDG
• System: single coat, or primer plus topcoat, with the powder or color and a dry film range for each coat
• Masking: threads, bores and machined faces
• Acceptance: your criteria for pinholes or surface defects, if you have them, and whether you want a Certificate of Conformance

Our coating callout checklist covers the rest of a complete callout.

Cost and lead time

A pre-bake cycle, a second coat and a sweep blast each add handling or oven time, which is why “galvanized” or “cast” on an RFQ changes the quote. Our standard lead time is about a week; jobs with a blast step or a two-coat system take longer. See powder coating cost factors for what else moves the price.

To get a quote, send drawings, quantities and the substrate. If the parts haven’t gone to the galvanizer yet, send the RFQ first so the galvanizing instructions are settled before the parts are dipped.