Powder Coating
Powder coating sprays electrostatically charged dry polymer powder onto an earthed part, then bakes it into a tough continuous film.
- Part
- Finishing
- Variants
- 2
- Revised
- 2026-08-11
At a glance
- Family
- Additive
- Typical tolerances
- Adds 2–4 mils (50–100 µm) per surface; budget 2× film thickness on a bore diameter and roughly 4× on thread pitch diameter, or mask
- Surface finish
- Smooth gloss through heavy texture depending on powder; the film smooths but does not fill substrate defects
- Typical volumes
- 1 to millions of parts; conveyorized lines are most economical above a few hundred parts per run
- Lead time
- 3–7 business days at a job shop; 24–48 hours expedited on stock colors; 2–3 weeks for a custom-matched powder
- Materials
- Metal
What it is
Powder coating applies a dry thermoset polymer powder to an electrically grounded part with an electrostatically charged spray gun, then fuses and cures it in an oven into a single continuous film. There is no solvent, so film builds that would sag in liquid paint are routine: a typical single coat runs 2–4 mils (50–100 µm) against roughly 1–2 mils for a sprayed liquid topcoat. Cure is typically 10–20 minutes at 350–400 °F (177–204 °C) part metal temperature.
The economics come from reclaim — overspray that misses the part is collected and re-sprayed, so material utilization can exceed 95% against 30–65% for conventional liquid spray, and VOC emissions are essentially zero. The constraints follow from the same physics: the part must conduct to hold a charge and must survive the bake, deep recesses starve from Faraday-cage shielding, and 2–4 mils lands on every reachable surface including threads and bores.
How it works
- Clean and pretreat. Degrease, then either chemically convert the surface or blast it. Iron phosphate is the common commercial pretreatment; zinc phosphate gives markedly better corrosion performance under the film. On structural steel the alternative is abrasive blasting to an SSPC preparation grade with a 1.5–3 mil (38–75 µm) anchor profile. Pretreatment, not the powder, determines how the finished part performs in salt spray.
- Dry off. A dry-off oven drives water out of seams, weld joints and blind holes. Trapped moisture flashes during cure and blisters the film.
- Apply. A corona gun charges powder to 30–100 kV as it leaves the nozzle; the charged particles follow field lines to the grounded part and cling electrostatically until cured. Tribo guns charge by particle-to-wall friction instead and penetrate recesses better because they produce no free-ion cloud. Fluidized-bed dipping — preheating the part and immersing it in aerated powder — is the alternative route for heavy builds of 10–40 mils (250–1000 µm), used mainly for electrical insulation.
- Cure. The part passes through a convection or infrared oven. The powder melts, flows out, and the thermoset chemistry crosslinks. Schedules are quoted as part metal temperature, not air temperature: 10–20 minutes at 350–400 °F (177–204 °C) is typical, and heavy sections need extra oven time simply to reach temperature. Low-temperature powders cure near 250–300 °F (121–149 °C); UV-cure powders melt thermally and then crosslink under UV in seconds, which is what makes powder coating of MDF and some plastics possible.
- Cool and inspect. Film thickness is verified with a magnetic or eddy-current gauge, adhesion by cross-hatch tape test to ASTM D3359. Reclaimed overspray from the booth cyclone is sieved and blended back into the feed.
Which powder chemistry?
Epoxy is the toughest and most chemically resistant but chalks quickly in sunlight, so it is an interior and primer material. Polyester — TGIC and TGIC-free — is the standard exterior choice. Polyester-urethane gives a smoother, thinner-looking film for appearance parts. Superdurable polyester and fluoropolymer powders serve architectural work specified to AAMA 2604 and AAMA 2605.
Design guidelines
Budget 2–4 mils on every reachable surface
The film goes everywhere the powder can reach. On a 0.250 in bore, 3 mils per wall closes the diameter by about 0.006 in. On a 1/4-20 thread the pitch diameter grows by roughly 4× the radial build — 3 mils becomes about 0.012 in, which no thread class accepts. Mask threads with silicone plugs or caps, or chase them after cure.
Break every sharp edge
Molten powder pulls away from a sharp edge by surface tension, so an unbroken edge ends up with a fraction of the film thickness of the adjacent flat and becomes the site where corrosion and chipping start. Break outside edges to at least a 0.020 in (0.5 mm) radius; 0.030–0.060 in (0.75–1.5 mm) is better on parts that must pass a long salt-spray requirement.
Design around the Faraday cage
Electrostatic field lines terminate on the nearest grounded surface, so powder starves in inside corners, deep pockets, channel interiors and slots. As a working rule, a recess deeper than its opening is wide will coat thin. Open up internal corners, provide gun access, or accept a thinner film in the recess and state it on the drawing.
Provide a hanging point, drainage and venting
The part hangs from a hook that must make bare metal-to-metal contact, so at least one location will carry a mark: add a dedicated hanging hole, typically 0.20–0.38 in (5–10 mm), in a non-cosmetic area. Cups, box sections and blind pockets trap pretreatment solution that boils out during cure and ruins the film, so add a drain hole at the low point in the hanging orientation and vent enclosed volumes.
Check that everything in the assembly survives the oven
Cure at 350–400 °F (177–204 °C) rules out most thermoplastics, elastomeric seals, bearings, adhesives, electronics and some pressed-in inserts. Coat before assembly, or move to a low-temperature or UV-cure powder.
Do not expect powder to hide the substrate
A 2–4 mil film smooths but does not fill. Weld spatter, deep tool marks, casting porosity and blast profile telegraph through. Textured and wrinkle powders exist specifically to disguise a poor substrate.
| Feature | Recommended | Limit | Why |
|---|---|---|---|
| Single-coat film thickness | 2–4 mils (50–100 µm) | 1.5 mils (38 µm) minimum | Below ~1.5 mils coverage becomes discontinuous |
| Outside edge radius | 0.030 in (0.75 mm) | 0.020 in (0.5 mm) | Molten powder pulls back from sharp edges |
| Bore diameter allowance | 2 × film thickness | — | Film builds on both walls |
| Thread allowance | 4 × film thickness on pitch dia. | Mask instead | Thread geometry multiplies radial build |
| Recess depth vs. opening width | ≤ 1 : 1 | — | Faraday cage starves deeper recesses |
| Hanging hole | 0.20–0.38 in (5–10 mm), non-cosmetic face | — | Grounding contact always leaves a mark |
| Component temperature rating | Above cure temperature | 350–400 °F (177–204 °C) | Seals, bearings and plastics fail in the oven |
Cost drivers
Powder coating is priced per part or per rack over a lot minimum, and the powder itself is rarely the dominant cost.
- Racking and line density. Cost tracks how many parts fit on a linear foot of conveyor. A part that hangs flat and nests closely runs at a fraction of the cost of a bulky weldment.
- Pretreatment level. A five-stage zinc phosphate washer or an abrasive blast to a specified SSPC grade is a separate operation with its own labor and consumables — and it is the single biggest determinant of corrosion performance.
- Color changes. Reclaim booths must be blown down and cleaned between colors, so a short run in a non-stock color pays for that cleanout.
- Masking. Silicone plugs, caps and high-temperature tape are applied and removed by hand, twice per part.
- Rework rate. Powder cannot be spot-repaired the way liquid paint can. A defect usually means chemical or burn-off stripping and a full recoat.
Cost-reduction tactics:
- Consolidate to a single stock color — black and white are the cheapest almost everywhere.
- Design in a hanging hole and orient the part so it drains, which eliminates rework for runs and trapped solution.
- Replace masked threads with post-cure thread chasing where the thread is accessible.
- Break edges during machining or deburring rather than paying for hand work at the coater.
- Release in full-line-load batches — the oven and booth cost the same whether the conveyor is full or half empty.
Variants
2 named
Electrostatic Spraying
Fluidized Bed Powder Coating
Questions
7 questionsHow thick is powder coating?
A typical single coat is 2–4 mils (50–100 µm), roughly double a sprayed liquid topcoat. Below about 1.5 mils (38 µm) coverage becomes patchy. Fluidized-bed dipping is used when a much heavier build of 10–40 mils (250–1000 µm) is needed, mainly for electrical insulation.
What temperature does powder coating cure at?
Most thermoset powders cure at 350–400 °F (177–204 °C) part metal temperature for 10–20 minutes. Low-temperature powders cure near 250–300 °F (121–149 °C), and UV-cure powders melt thermally then crosslink under UV light in seconds, which is what allows MDF and some plastics to be powder coated.
Will powder coating close up threads and bores?
Yes. At 3 mils per surface a bore loses about 0.006 in on diameter, and a thread's pitch diameter grows by roughly 4× the radial build, or about 0.012 in — enough to bind any standard thread class. Mask threads with silicone plugs and caps, or chase them after cure.
Can plastic be powder coated?
Only with a low-temperature or UV-cure powder, and only if the part can be made conductive — typically with a conductive primer, because electrostatic application needs a ground path. Standard thermoset powders cure at 350–400 °F (177–204 °C), which destroys most thermoplastics.
Why does powder coating come out thin on sharp edges?
Molten powder pulls away from a sharp edge by surface tension during flow-out, leaving a fraction of the film thickness found on the adjacent flat. That thin spot is where chipping and corrosion begin. Break outside edges to at least a 0.020 in (0.5 mm) radius, and 0.030 in or more for demanding corrosion requirements.
Is powder coating more durable than liquid paint?
For impact and abrasion resistance, generally yes — the film is thicker and fully crosslinked. But corrosion performance is set mostly by pretreatment: zinc phosphate under the powder outperforms iron phosphate by a wide margin. Liquid paint still wins where a Class A automotive appearance, field touch-up or a heat-sensitive substrate is required.
Which powder chemistry should I specify?
Epoxy for interior and primer use — tough and chemically resistant, but it chalks in sunlight. Polyester for general exterior work. Polyester-urethane for appearance parts. Superdurable polyester or fluoropolymer for architectural aluminum specified to AAMA 2604 or 2605.