Powder coating is usually a line item on a quote, but it decides whether a part survives its first winter outdoors. Two shops can spray the same RAL number on the same substrate and deliver finishes that fail at 300 and 1,200 hours of salt spray. The difference is pretreatment, powder family, film build, cure, and edge geometry, and each of those is specifiable.
Powder Coating at a Glance
Powder coating is a dry process: charged powder is sprayed onto a grounded, pretreated part, then melted and cross-linked in an oven into a thermoset film 60–100 µm thick. No solvent is involved, so full build arrives in one coat and overspray is recoverable.
One pass reaches that build, two to three times a liquid coat, without runs. The powder is only the barrier; the conversion coating beneath it does the electrochemical work. And every part must survive 180–200 °C, which rules out some assemblies and heat-treated tempers.
Pretreatment Decides the Corrosion Result
Pretreatment runs degrease, rinse, conversion coating, demineralized rinse, and dry-off. The conversion layer — iron phosphate, zinc phosphate, or chrome-free zirconium — passivates the surface and keys the powder. Most field failures trace to this line rather than the powder, and “powder coated” on a drawing leaves pretreatment to the supplier.
- Degrease. An alkaline bath removes drawing oils and stamping lubricants. A weak or cold bath leaves a film that survives rinsing and kills adhesion.
- Rinse. Cleaner carry-over neutralizes the conversion bath.
- Conversion coating. Iron phosphate suits indoor and light-duty steel, zinc phosphate suits outdoor steel and castings, and chrome-free zirconium is standard for aluminum.
- DI rinse. Tap water leaves calcium and chloride salts that blister and pit the film.
- Dry-off. Residual moisture flashes off in the oven as pinholes.
Galvanized and aluminum need chemistry that cold-rolled steel does not. One iron phosphate line run for every substrate produces finishes that look right and fail outdoors, so put the service environment and its test on the drawing.
Powder Chemistries and Where Each One Belongs
Six chemistries cover most industrial work: epoxy, polyester TGIC, polyester TGIC-free, hybrid epoxy-polyester, polyurethane, and fluoropolymer. They differ in UV stability, chemical resistance, flow, and cost. Choosing by color is the common error; choosing by service environment — sun, chemicals, abrasion, expected life — is the correct method.
Epoxy has the best chemical resistance and no exterior durability — it chalks within months in sun, so it belongs indoors on enclosures, valve bodies, machine frames, pipe, and rebar. Polyester TGIC is the default exterior finish; TGIC-free (HAA) matches it with a narrower cure window. Hybrid is the economical indoor decorative choice. Polyurethane gives thin, smooth exterior films for trim and wheels, while fluoropolymer (PVDF/FEVE) is the architectural grade for 20-year retention.
For aluminum, anodizing is the alternative rather than a substitute: an integral oxide that cannot chip, with less color range. See anodizing aluminum compared with powder coating.
| Powder chemistry | Typical application | Salt spray, ASTM B117 (60–80 µm on zinc phosphated steel) | Cost index (hybrid = 1.0) |
|---|---|---|---|
| Epoxy | Indoor enclosures, valve bodies, machine frames, pipe and rebar | 750–1,000+ h; no UV stability | 1.0–1.1 |
| Polyester TGIC | Exterior architectural, outdoor equipment, garden products | 500–1,000 h | 1.1 |
| Polyester TGIC-free (HAA) | Exterior parts for TGIC-restricted markets, appliance skins | 500–1,000 h | 1.2 |
| Hybrid epoxy-polyester | Indoor furniture, shelving, fixtures, appliance internals | 300–500 h; chalks outdoors | 1.0 (base) |
| Polyurethane | Automotive trim and wheels, thin-film decorative exterior | 500–750 h; primer for heavy duty | 1.6–2.0 |
| Fluoropolymer (PVDF/FEVE) | Curtain wall, facade panels, premium exterior | 1,000–4,000 h with primer | 4.0–6.0 |
Treat the column as a ranking, not a warranty: hours depend on pretreatment, film build, and scribing.
Electrostatic Spray vs Fluidized Bed
Electrostatic spray charges powder with a gun and sprays it onto a grounded part. Fluidized bed preheats the part above the powder’s melting point and dips it into air-suspended powder, which fuses on contact. Spray suits complex geometry at 60–120 µm; the bed suits simple, high-volume parts at 250–1,000 µm.
Corona guns run 30–100 kV and wrap powder around the part, at the cost of the Faraday effect. Triboelectric guns charge by friction and reach recesses better, but are weaker on some substrates.
Fluidized bed has no charging and no Faraday limit, coats edges uniformly, and reaches thicknesses no gun can match. Its limits: parts must tolerate preheat above 200 °C, masking is nearly impossible, and thick films are brittle. It suits racks, busbars, pipe, and rebar, not a machined housing.
Both recover overspray, so on high-volume sheet metal fabrication powder utilization is a real cost lever.
Cure Schedules: Metal Temperature, Not Oven Temperature
Cure is set by the temperature the substrate reaches and how long it holds, not by the oven set point. Standard exterior polyesters specify 180–200 °C metal temperature for 10–20 minutes, and a 6 mm bracket plus a 0.8 mm panel in one load do not arrive together.
- Under-cure passes a fingernail test and fails later: poor impact, adhesion loss after weeks, solvent rub failure.
- Over-cure yellows light colors, embrittles the film, and can destroy adhesion on post-formable parts.
- Low-temperature powders cure at 160 °C or below for heat-sensitive assemblies, at higher cost and lower chemical resistance.
- Thick sections need longer dwell. Verify with a thermocouple or data-logging puck on the part.
Without a cure chart for the load, the finish is unverified regardless of the data sheet.
Film Thickness, Edge Coverage and the Faraday Cage Limit
Typical industrial build is 60–100 µm (2.4–4.0 mil), architectural 60–80 µm, and heavy-duty epoxy 100–150 µm or more. Below about 50 µm, coverage over a blasted or phosphated surface turns marginal and salt spray performance falls sharply. Above about 120 µm, impact and flexibility drop and the film chips instead of deflecting.
Measure with a magnetic or eddy-current gauge, several readings per face, recorded. Substrate gauge is separate — see the sheet metal gauge chart. Two effects limit any gun:
- Edge coverage. Powder pulls back from a sharp edge as it melts, leaving a thin or bare line that corrodes first. A 0.5–1 mm radius, deburring, and higher build help.
- Faraday cage effect. Surrounding grounded metal shields the field, so powder barely enters deep channels and tube interiors. Cut kV, use a smaller or tribo gun, increase distance, rotate the part, or coat the recess in a second pass.
Both are geometric, so both can be designed out: a drawing demanding coverage inside a 6 mm wide, 30 mm deep slot asks for the impossible.
Outgassing on Castings and Galvanized Parts
Outgassing is gas escaping from the substrate while the powder melts, producing pinholes, bubbles, and blisters that break the barrier. It is the most common defect on castings and hot-dip galvanized steel, and it is a substrate problem rather than a powder problem.
Castings. Porosity, mold release agents, and absorbed moisture release gas at cure temperature. Countermeasures: a bake-out at or above cure temperature, a gas-proof primer, a degassing powder grade, and vacuum impregnation in severe cases. Thick sections and visible porosity carry the highest risk, and a machined face is not automatically safe. See the surface notes under investment casting.
Galvanized steel. The zinc layer is micro-porous and traps moisture and passivation residues. Coating too soon, or without bake-out, produces pinholes and adhesion loss that surfaces days later. Wash off white rust, bake out, then prime before the polyester topcoat; skipping that primer is why galvanized work blisters at the first freeze-thaw cycle.
Color, Gloss and Texture Specifications
Color is a standard reference plus a measured tolerance: RAL Classic or RAL Design, a Pantone number, or a physical master panel, with limits of ΔE ≤ 1.5 for close match and ΔE ≤ 2.0 for general industrial work under D65. “RAL 7016” alone is not a specification, since manufacturers’ versions of the same number differ visibly.
Gloss is measured at 60° per ASTM D523 in gloss units: matte below 10, satin 20–35, semi-gloss 50–70, high gloss 80–95. Asking for “gloss” with no number invites a 90 GU part rejected against an 85 GU sample.
Texture is a separate axis: smooth, fine, sand, wrinkle, hammered. It hides substrate defects and orange peel, which is why visible castings and weldments use it, but it traps dirt.
Salt Spray Performance by System
ASTM B117 holds a scribed or unscribed panel in continuous 5% sodium chloride fog at 35 °C and records hours to blistering or scribe creep per ASTM D1654. It ranks systems and does not predict service life, so treat the hours as a comparison between candidate finishes, not a warranty period.
Architectural aluminum under AAMA 2605 requires 4,000 hours, reachable only with full pretreatment plus primer.
Pair B117 with a cyclic test — ASTM G85 or ISO 12944 categories C3 to C5 — and with adhesion after exposure. A finish that loses adhesion at the scribe has failed even if the flat face is intact. A finish that is not measured is not controlled, the same principle behind quality assurance.
Masking, Racking and Contact Points
Masking protects features that must stay conductive, threaded, or dimensionally clear: threaded holes, mating faces, datums, grounding pads, and bearing bores. Racking decides where the part is held and where bare contact marks appear. Both belong on the drawing, because the coater cannot see intent.
- Threads. Mask internal threads; chasing powder out ruins the thread. Mask external threads too, or accept a build that changes the fit.
- Datums and mating faces. A 30 µm film on a datum is a 30 µm dimensional error. Mask it or adjust the tolerance.
- Contact points. Rack marks are bare metal, so put them on a non-visible face and say so.
- Drainage and grounding. Hang parts so powder and rinse water cannot trap, and give every part a clean ground path; a coated hook starves the film.
Defects, Causes and Fixes
Most defects fall into four families: surface tension (orange peel, craters), gas and moisture (pinholes, blisters), electrical (back-ionization), and process drift (color, gloss, adhesion). Each has a known cause and a correction that belongs on the line rather than in the reject bin.
| Defect | What it looks like | Typical cause | Fix |
|---|---|---|---|
| Orange peel | Pebbled surface, poor distinctness of image | Film too thick, high-viscosity powder, gun too close, low cure temperature | Cut build, adjust distance and kV, verify metal temperature |
| Pinholes | Fine holes through the film, often clustered | Outgassing, moisture, oily air line | Bake-out, gas-proof primer, verify dry-off, filter compressed air |
| Craters | Round depressions with raised rims | Silicone, oil, or release agent | Fix degrease, ban silicone, replace powder, clean booth |
| Poor adhesion | Film lifts at the scribe or flakes on impact | Incomplete cleaning, weak conversion coating, under-cure | Rebuild pretreatment, check bath chemistry, confirm cure |
| Back-ionization | Star-shaped bare spots, usually in corners | Excess kV or film build; the layer repels charge | Reduce kV and current, increase distance, two thinner passes |
| Thin or bare edges | Substrate visible along sharp edges and punched holes | Powder pull-back, no radius, film at the low end | Radius and deburr edges, raise build, edge-capable powder, primer |
| Blisters | Raised bubbles, sometimes only after weathering | Trapped moisture, salt contamination, galvanized substrate | DI final rinse, dry-off, bake-out, primer on galvanized |
| Color or gloss drift | Batch mismatch against the master panel | Different powder lot, overbake, film thickness variation | Retain a master panel, measure ΔE and GU per lot |
Specifying Powder Coating on a Drawing
A callout needs seven elements: substrate and pretreatment class, powder chemistry, color reference with ΔE tolerance, gloss range in GU, film thickness limits, masked features, and acceptance tests. A finish is a tolerance like any other, so omit one element and the supplier picks the cheapest compliant option.
A workable note reads:
FINISH: POWDER COAT, POLYESTER TGIC, EXTERIOR GRADE. PRETREATMENT: ZINC PHOSPHATE ON STEEL, CHROME-FREE ZIRCONIUM ON ALUMINUM. COLOR: RAL 7016, ΔE ≤ 1.5 vs MASTER PANEL. GLOSS: 25–35 GU AT 60°, ASTM D523. FILM: 60–80 µm, MIN 60 µm ON VISIBLE SURFACES. MASK: M8 THREADS, DATUMS A, B, C, GROUNDING PADS. ACCEPTANCE: 720 h ASTM B117, NO BLISTERING, ≤ 2 mm SCRIBE CREEP PER D1654; ADHESION D3359 CLASS 4B; IMPACT D2794. NO TOUCH-UP ON VISIBLE SURFACES.
Supporting standards to name: ASTM D3451 (powder tests), D523 (gloss), D2244 (color), D3359 (adhesion), D2794 (impact), D522 (bend), B117, plus AAMA 2603/2604/2605 for architectural aluminum and ISO 12944 for steel.
State which surfaces are cosmetic: a rack mark on a hidden flange is acceptable, one on a front panel is not. Coating normally follows welding and forming, as assumed in steel surface finishing. Rework means stripping and recoating, which removes a little substrate each cycle, so state whether rework is allowed.
YC Prime Precision runs powder coating in Qingdao alongside anodizing, plating, passivation, and blasting, on steel, stainless, aluminum, and cast parts, under ISO 9001 process control with inspection records on request. If you have a drawing with a finish callout, request an instant quote and include the service environment — indoor, outdoor, coastal, or chemical — because that decides which powder system is quoted.
Frequently Asked Questions
How thick should powder coating be on an industrial part?
Most industrial parts run 60–100 µm (2.4–4.0 mil), architectural work 60–80 µm, and heavy-duty epoxy 100–150 µm. Below about 50 µm, coverage over a phosphated or blasted surface becomes marginal; above about 120 µm, impact and flexibility drop and the film chips rather than deflecting.
How long does powder coating last outdoors?
Exterior polyester on properly pretreated steel holds color and gloss for years, and fluoropolymer systems are specified where 20-year retention is required. The usual benchmarks are AAMA 2604, about five years of South Florida exposure, and AAMA 2605 at ten. Real life depends more on pretreatment and film build than on the brand.
Can you powder coat over galvanized steel or a casting?
Yes, with extra steps. Galvanized steel needs white rust and passivation residues removed, a bake-out, and usually a primer, because the zinc layer is porous. Castings need a bake-out or a gas-proof primer to stop outgassing. Coating either without those steps produces pinholes and blisters.
Why does powder coating not cover the inside of a recess?
That is the Faraday cage effect: surrounding grounded metal shields the field, so charged powder is repelled from deep recesses, inside corners, and tube interiors. Lower voltage, a smaller or triboelectric gun, greater distance, part rotation, or a second pass all improve penetration.
Is powder coating better than anodizing for aluminum?
They solve different problems. Anodizing is an integral oxide that cannot peel and suits wear surfaces and thin decorative films. Powder coating gives a thicker, more color-flexible barrier but can chip. Thick-film corrosion protection favors powder; hardness and dimensional stability favor anodizing.
What salt spray hours should I require?
Match it to the environment: about 500 hours for indoor and sheltered parts, 720–1,000 hours for exposed outdoor steel, and 1,500 hours or more for coastal or chemical service. Specify scribe and pass criteria per ASTM D1654, because hours without an acceptance definition mean nothing.
