Outgassing in Powder Coating: Causes, Fixes, Process Control
Why pinholes and bubbles appear in powder coating on castings and galvanized parts, and the preheat, cure and prep steps engineers use to stop them.
Outgassing in Powder Coating: Causes, Fixes, Process Control
Outgassing is what happens when air, moisture or other volatiles trapped in a part escape during the cure oven cycle and break through the film while it is still soft. The result is bubbles, pinholes or craters that spoil the appearance and open a path for moisture to reach the metal. It shows up most on castings, zinc-plated or galvanized parts and anything with rust or residue in the surface. You fix it by removing what is trapped before the powder goes on, by controlling how the part heats up, and by choosing a powder and film build that give gas time to leave. This guide covers each of those levers and how to specify them for production parts.
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Control Outgassing on Complex Parts
Professional Powder Coating provides automated coating, in-house quality testing, and documented quality assurance for demanding industrial components.
Outgassing is trapped air or volatiles expanding during cure. It leaves pinholes, blisters and craters, and the pinholes are also corrosion entry points.
The usual sources are porous castings, galvanized or zinc-plated surfaces, rust, oils and absorbed moisture.
The core fix is a degas (preheat) cycle at least 15°C (30°F) above the cure temperature, then coating the part while it is still warm.
Lower-temperature, longer cures, low-cure powders and film thickness held to the data sheet all give gas more time to escape.
If it stays uncontrollable, look at casting density. Switching or qualifying the casting supplier can matter more than any oven tweak.
Powder coating melts, flows and then cross-links in an oven. During the gel phase the film is a viscous liquid that gas can still push through. If a bubble of air or vapor rises after the film has gelled and hardened, it cannot heal over. It leaves a crater or a pinhole.
That is also why the defect is more than cosmetic. A pinhole is a small break in the barrier layer. On a part that lives outdoors, on a telecom enclosure, a power component or marine hardware, moisture and salts get in through exactly those breaks.
Where the gas comes from
Different substrates outgas for different reasons, and the right fix depends on which one you have.
Source
What is trapped
Typical sign
First move
Die or sand castings (aluminum, cast iron, zinc)
Air in surface pores
Scattered pinholes, often clustered at thick sections
Degas cycle before coating
Hot-dip galvanized steel
Air between zinc layers from the dip
Fine pinholes across the zinc surface
Surface prep per ASTM D7803, degas, low-cure powder
Zinc-plated parts
Gas or moisture in the plating
Pinholes and small blisters
Bake the parts, then coat warm
Rusted or contaminated steel
Moisture and volatiles in rust, oil, residue
Blisters, later delamination
Blast or chemically remove, do not coat over
Welds, seams, lap joints
Liquid or vapor from cleaning and pretreatment
Bubbles along the joint
Dry thoroughly, seal or redesign the joint
Castings: the “curse of the casting”
Castings are porous, and air near the surface expands and escapes during cure. Finishers have a nickname for the problem, and it is deserved. The air that matters is not on the surface you can see. It sits in pores a few thousandths of an inch below it, so cleaning alone cannot remove it.
Surface condition matters too. ASTM D7803, the standard practice for preparing hot-dip galvanized surfaces for powder coating, states that preparation is the key to adhesion on galvanized steel. Loose patina or moisture-retaining oxides left on the surface are a recurring cause of outgassing and pinholing, and in corrosive service those pinholes become routes for chlorides to reach the steel.
Rust, oils and moisture
Powder coating is generally not recommended over active rust. Rust and other contaminants hold moisture and volatiles. Coat over them and they keep outgassing, which causes blistering, intercoat adhesion loss and delamination, and cuts the life of the finish well below what the powder is rated for. Abrasive blasting to remove the contamination is the standard answer.
If a part is going to outgas, make it do so before the powder is on it. The industry practice for persistent outgassing is a dedicated degas or preheat cycle. The sequence looks like this:
Clean and prepare the part so the only volatiles left are the ones inside the metal.
Heat the part to at least 15°C (30°F) above the temperature you will use to cure the powder. Products Finishing gives this figure as the working guideline.
Hold long enough for the whole part, including heavy sections, to reach that metal temperature. Thick castings lag the oven air by a wide margin.
Remove the part and apply the powder while it is still warm but dry, before it cools to ambient and air can re-enter the pores.
Cure on the normal schedule.
One trap: a single pass through the oven is not a degas cycle. Many shops assume one bake does both jobs, and on complex castings it does not. A second point: oven setpoint and part temperature are different numbers. A large casting can take far longer than the oven air to reach temperature, which is why we explain oven temperature versus part metal temperature separately. If you only verify the oven setpoint, you do not know whether the degas actually happened.
Speed is the reason fast, high-temperature cures are popular. It is also a reason castings pinhole. Technical guidance notes that lower-temperature, longer-duration cycles often reduce outgassing because trapped gas has more time to leave before the film cross-links and locks in.
A practical set of adjustments, in the order most shops try them:
Preheat above cure temperature, then coat warm (above).
Use a low-temperature cure powder where the spec allows. The film gels later relative to when gas starts moving.
Hold film thickness to the technical data sheet.Over-building the film makes it harder for gas to escape during the gel phase. More powder is not more protection if the film is full of pinholes.
Slow the ramp so the surface does not skin over while the interior is still releasing gas.
The trade-off is real. Longer cycles cost oven time and throughput. On a large production run, that cost has to be weighed against rework, stripping and recoating, which cost more.
Preparation: removing what you can remove
The degas cycle handles the gas inside the metal. Preparation handles everything on it.
Abrasive blasting removes rust, scale and old coating, and gives the powder a profile to key into. It is the standard fix when rust is present.
Chemical cleaning and pretreatment remove oils and soluble salts. Parts must then be dried completely, including seams and blind holes, because trapped rinse water outgasses just like trapped air.
Paint stripping is needed when an earlier coating is failing or contaminated. Powder over a compromised coating inherits its problems.
An important caution on blasting castings: it can open porosity that was sealed at the skin. A blasted casting that looked fine can outgas worse than the as-cast part, which is one more reason to validate the process on the actual part instead of assuming.
When it is not the process: casting quality
Sometimes you have done everything right and the part still pinholes. At that point the problem is probably the casting. Guidance from the same source says that if outgassing stays uncontrollable after preheat and low-cure powder adjustments, the casting density is the likely cause. A standard benchmarking method in industry is to X-ray castings from different suppliers to see which die caster delivers lower-porosity parts.
For engineers, this means the coating specification and the casting specification are linked. If a part has a demanding finish requirement, say so in the casting purchase order, and share defect photos with the foundry or die caster when outgassing appears.
Is it outgassing, or something else?
A frequent question on finishing forums is whether small spots are outgassing or dirt. Some quick checks help, though none replace inspection under magnification:
Observation
Points toward
Pinholes concentrate on castings, welds or thick sections
Outgassing
Craters with a visible particle or fiber at the center
Contamination (dirt, fibers, oil)
Spots that follow the same pattern part after part
Substrate or process cause
Random spots that vary with the day or booth conditions
Airborne contamination or compressed air quality
Defects that vanish on a re-run after a bake before coating
Outgassing confirmed
The last test is the most useful. Take a failed part, strip it, bake it at or above cure temperature, and recoat it warm. If the defect disappears, you have confirmed the cause and found the fix at once.
Specifying outgassing control for production parts
If you are a quality manager or design engineer, do not leave this to a generic finishing cycle. Guidance from the Association for Materials Protection and Performance on selecting powder coating systems for protective purposes covers surface preparation and application, and the practical lesson is the same: the performance of the finish depends on the part’s metallurgy, so each critical component needs its own validated steps.
A useful specification package includes:
The substrate and process (die cast, sand cast, galvanized, plated) and any known porosity concerns.
Surface preparation method and cleanliness requirement.
Whether a degas cycle is required, with the metal temperature and hold time, not just oven setpoint.
Powder chemistry and cure schedule, including whether low-cure is acceptable.
Film thickness range.
Inspection criteria for pinholes, and how defects are classified.
What records you need with each lot.
On the last point, documented control is what makes a good result repeatable. A process under an ISO 9001:2015 framework gives you traceable steps across a large run instead of a result that depends on who was on shift.
Our perspective
We see outgassing as a process-control problem before it is a coating problem. Heavy, complex parts are our daily work, and they are the parts where a standard cycle fails: thick sections lag the oven, castings carry hidden porosity, and the cost of a pinholed lot is amplified by size and by the logistics of moving it.
So we start with feasibility. Before a production run we want to know the substrate, the part geometry and the environment it will serve in, then build the preparation, thermal path and powder choice around those. We back it with in-house laboratory testing and quality documentation, so a reviewer can see what was done, not just that the part looks good when it leaves.
Our advice is to raise outgassing risk early, in the quote stage, not after the first rejected batch. A conversation about casting source and finish expectations costs nothing. A recoat of a pallet of large castings costs a great deal.
— PPC Technical Team
How we can help
Professional Powder Coating Ltd. finishes large, heavy and complex metal components, with surface preparation, sandblasting and paint stripping handled alongside coating, in-house quality testing, and protective packaging and logistics across North America. If you have a casting or galvanized part with a history of pinholes, send us the drawing and substrate details and we will review technical feasibility with you before production. See our services or our capabilities and facility. For a related read, see powder coating cast iron without pinholes.
FAQ
What is outgassing in powder coating?
It is the release of trapped air, moisture or other volatiles from the part as it heats in the cure oven. The escaping gas pushes through the soft film and leaves pinholes, bubbles or craters. It is most common on castings and galvanized or zinc-plated parts.
Can you powder coat over rust?
It is generally not recommended. Rust holds moisture and volatiles that keep outgassing, which leads to blistering, adhesion loss and delamination. Remove it by abrasive blasting before coating.
How much hotter should the degas cycle be than the cure?
The guideline is at least 15°C (30°F) above the cure temperature, measured at the part, not the oven air. Apply the powder while the part is still warm and dry so air does not re-enter the pores. Your powder supplier’s data sheet and the part’s heat tolerance set the limits.
Why is my powder coat flaking off?
One common cause is outgassing from contaminated or porous substrates, which creates pinholes and weakens adhesion once moisture gets in. Poor pretreatment and coating over rust are other causes. Inspect a failed area under magnification to see whether pinholes or blisters preceded the flaking.
