Why Do Pinholes Appear? Outgassing on Castings and Galvanised Parts
Pinholing is usually a fault in the substrate rather than the coating: porous castings and hot-dip galvanised surfaces release gas as they heat up, and the film is punctured before it gels. This article settles the choice between a preheat zone, a degassing powder and a slower oven ramp on the basis of the line's temperature profile.
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The cause of pinholing in powder coating is usually not the coating but the substrate underneath it: porous castings and hot-dip galvanised surfaces release trapped air, moisture and volatiles from the zinc layer as they climb to the 180–200 °C curing band. When that gas breaks through a film that has not yet gelled, it leaves pin-head openings, most of them under 0.1 mm across. The answer is almost never "put more powder on". It is adding a preheat (degassing) stage to the line's temperature profile, slowing the oven's heat-up ramp, or moving to a gas-permeable degassing powder. Which of the three is right depends on the type of part, its wall thickness and the throughput of the line — doing all three is money spent for nothing.
What makes this defect hard to diagnose is that the result shows up in the coating while the cause sits beneath it. Most of the pinholing cases that people try to close by changing powder suppliers, tweaking gun settings or replacing a booth actually close when a single stage is added to the thermal layout of the line. The order below forces the measurements first and brings the investment decision last.
A pinhole, a crater and a pop are not the same defect
Three different defects get called by one name on the shop floor, and that is what sends people to the wrong part of the line. Make the distinction under a magnifier by looking at the bottom of the opening: a pinhole goes all the way down to the substrate, a crater has coating in its base.
| Defect | Appearance | Real cause | Where to look |
|---|---|---|---|
| Pinhole | Narrow, steep-sided hole down to the substrate | Outgassing from the substrate | Part type, preheat, oven ramp |
| Crater | Shallow, wide dish with coating in the base | Oil, silicone, compressed-air contamination | Oil trap, pretreatment, shop air |
| Popping | Burst bubble with a raised rim | Excessive film build or too fast a cure | Film thickness, conveyor speed |
A shop that reads this table wrong and treats a crater as a pinhole will pay for a preheat stage and still have the same defect. If the defect also gets confused with thickness problems, use the comparison table in our article on orange peel and surface defects alongside this one.
Where does the gas come from? Sources by substrate
Not every substrate releases the same amount of gas, and knowing the source is what sets the length of the preheat cycle.
| Substrate | Source of gas | Pinholing risk | Practical measure |
|---|---|---|---|
| Sand casting / porous castings | Air and moisture trapped in the pores | High | Preheat before coating; bare pass through the oven |
| Hot-dip galvanising | Volatiles and porosity in the zinc layer | High | Preheat; degassing-grade powder |
| Electroplated zinc | Hydrogen retained from the plating bath | Medium | Ageing after plating, preheat |
| Aluminium die casting | Mould-release residue, porosity | Medium | Pretreatment discipline, preheat |
| Hot-rolled steel sheet | Surface moisture, scale | Low | A dry-off stage that actually works |
| Cold-drawn sheet / profile | No meaningful gas | Very low | Standard pretreatment is enough |
The conclusion is blunt: if castings or hot-dip galvanised parts run on your line, pinholing is not a possibility but something the thermal layout has to prevent by design. On a sheet-metal line, check the dry-off stage first and only then move to the outgassing hypothesis.
The preheat (degassing) stage: get the gas out before the coating goes on
The most reliable fix is to run the part through a cycle slightly above curing temperature before it is coated. The logic is simple: let the gas escape while there is no film over it. That cycle can run in a separate batch oven, or as a dedicated stage on the conveyor of a tunnel line.
Time and temperature are set by the wall thickness of the part; there is no catalogue figure. In practice thin castings clear with a short cycle while heavy sections need a markedly longer one, and the right duration is found by increasing it step by step on a test part until no holes remain. A batch electric oven suits this work well: an 80–250 °C operating range and 16 baking profiles let the degassing cycle and the cure run on the same equipment under separate profiles, while ±3 °C uniformity means every part sees the same cycle.
The preheat stage costs energy and time. In volume production, running it on every part cuts throughput, so the decision follows the share of castings and galvanised work on the line. Where the part mix is varied, the most efficient answer is usually to leave the main line alone and preheat only the risky parts in a separate batch oven.
Oven ramp, uniformity and conveyor speed
If a preheat stage is not possible, the second lever is the oven's heat-up ramp. Under a fast ramp the powder melts and gels quickly, so gas leaving the substrate meets a closed film and punches through it. Slowing the ramp lets the gas escape while the film is still fluid enough to close behind it.
Gas-fired ovens typically reach temperature in 35–45 minutes; on a system such as an overhead pallet oven working in the 140–220 °C band, the ramp is set together with burner capacity and loading pattern. The critical number is not the set point but the curve by which the part surface reaches it: the oven can read 180 °C while a heavy casting is still climbing. That curve cannot be argued about without a data logger, and we set out the method step by step in oven temperature uniformity.
On a tunnel line the same adjustment is made with conveyor speed. Slowing a conveyor that runs in the 1–6 m/min range lengthens dwell time in the oven, but it also directly reduces output within the line's 500–5,000 parts/hour band. In other words the ramp adjustment is not free; it is paid for in throughput.
Degassing powder or a change to the line?
The answer on the coating side is a gas-permeable degassing formulation. These powders let gas pass through the film, but they carry three costs: a limited choice of colour and gloss, a generally higher unit price, and a limit above which very heavy outgassing defeats them. The decision table looks like this:
| Situation | Correct answer | Effect on throughput | Note |
|---|---|---|---|
| Whole line is castings/galvanising | Preheat stage on the line | Permanent, plannable slowdown | Most reliable answer |
| Occasional risky batch | Preheat in a separate batch oven | Main line untouched | Flexible, low investment |
| Moderate gas, colour is free | Degassing-grade powder | None | If the colour/gloss limit is acceptable |
| Puncturing caused by heavy film | Bring film thickness to target | None | Measure first, reformulate later |
| Oven heats up too fast | Slow the ramp and the conveyor | Output falls | Requires a measured curve |
| Defect started after pretreatment | Audit the dry-off stage | None | Wet parts must not enter the oven |
Do not break that order. Changing the powder formulation on a line that has never been measured is an attempt to mask the symptom without closing the cause, and it becomes a permanent colour restriction.
What the pretreatment side contributes
A part that leaves the dry-off stage still damp produces the same defect through water evaporating in the oven. Strictly speaking that is not substrate gas, but it is indistinguishable by eye. On a pretreatment line built with 3–7 stages, the final dry-off has to be set to remove not only surface water but the water sitting in the pores — on a casting that takes markedly longer than on sheet.
On a dip-type pretreatment line, a porous part carries liquid out of the bath, and any liquid left in the pores boils off in the oven and punctures the film. Drain holes, drip time and jig angle are therefore discussed at the project stage. For the method comparison, see dip pretreatment.
Frequently asked questions
How do I tell a pinhole from a crater?
By the bottom of the opening. A pinhole runs down to the substrate and has steep walls; a crater is shallow, has coating in its base and forms a wide dish. If you are seeing craters, the cause is contamination rather than gas: oil, silicone, or oil and water carried in the compressed air. Misreading this is the most expensive mistake available here.
How many minutes should the preheat cycle be?
There is no catalogue figure; it follows wall thickness and is found by trial. The method is this: run a representative part bare through a cycle slightly above curing temperature, then coat and bake it, then count the holes. If holes remain, increase the time step by step. Record the duration you find per part family on a setting card so it is not rediscovered every shift.
Will a heavier coat close the holes?
No, it usually makes things worse. A heavy film makes it harder for gas to escape, and instead of pinholes you get popping. The right approach is to bring film thickness into the range on the powder data sheet and solve outgassing thermally. Two thin coats are also better than one heavy coat, though the first coat must cure fully or you introduce the second defect described in diagnosing under-cure.
Can hot-dip galvanised parts be baked without trouble?
They can, provided the thermal layout accounts for it. A galvanised surface both releases gas and behaves differently at high temperature as the zinc layer changes. In practice the safe route is to hold the cure at the lower end of the band, extend the time, and add a preheat cycle where needed. Outgassing varies markedly with coating thickness and the age of the coating; freshly galvanised parts are the riskier ones.
The defect appears in the same area every time — why?
That is the fastest way to narrow the diagnosis. If the holes cluster on a heavy section of the part, there is a thermal mass difference: that section heats late, the gas comes out late, and by then the film has gelled. If they cluster at one position in the oven, uniformity is the problem. If they cluster near the hanging point, carried-over rinse water is the suspect.
Summary
Pinholing has less to do with the quality of the powder than with what the substrate does once it gets hot. The diagnostic order is fixed: first separate the defect from craters and popping, then identify the substrate type, then verify dry-off, then judge the oven ramp against a measured curve, and only then touch the powder formulation. The lasting answer lives in the line's temperature profile: where castings and galvanised work dominate, a preheat stage is not a luxury but a line item to plan from the start. If you are laying out the whole investment, our powder coating plant setup guide is the right starting point; every oven and line we build is manufactured to the customer's largest part and delivered with a 2-year manufacturer warranty.
Send us your part type, wall thickness and current oven temperature record — we will work out together whether a preheat stage is necessary.