Powder Coating
Also: powder coat, electrostatic powder coating, polyester powder coat, epoxy powder coat, TGIC polyester, powdercoating
Powder coating sprays dry resin powder onto grounded metal and bakes it into a tough 50-100 µm film: the durable colour finish for steel and aluminum parts.
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Powder coating applies a dry thermoset powder, usually polyester, epoxy or a hybrid, with an electrostatic gun onto a grounded, pre-treated metal part. The part then goes into an oven at 160-200 °C (325-400 °F) for 10-20 minutes, where the powder flows and cross-links into a continuous film. There is no solvent, transfer efficiency is high, and a single pass builds 50-100 µm (2-4 mils), two to three times a coat of wet paint.
Pre-treatment does the corrosion work. A blasted surface with an iron or zinc phosphate, or a zirconium conversion coat on aluminum, under a polyester topcoat gives hundreds of hours of salt spray; an epoxy or zinc-rich primer coat under the colour coat gives more. Skip the pre-treatment and the coating will chip and undercut at the first scratch. Polyester (TGIC or TGIC-free) is the outdoor chemistry; epoxy is harder and more chemical resistant but chalks in sunlight; hybrids sit between; polyurethane and super-durable polyesters are for architectural gloss retention.
Powder is thick and it builds unevenly: edges pick up extra, deep recesses get less because of the Faraday cage effect, and a masked thread or bore near an edge often needs a plug. It cannot be applied to anything that will not survive the oven or cannot hold a charge, so plastics, rubber, magnets and assemblies with seals are out unless a low-temperature powder is used.
At a glance
| Typical tolerances | Film build 50-100 µm (2-4 mils) per coat, commonly held to ±25 µm (±1 mil); 100-200 µm for two-coat primer plus colour systems. The film adds to every coated surface, more on edges and less in recesses, so mask fits, threads and datums. A masked hole loses no size; an unmasked 6 mm (0.25") hole shrinks by 0.1-0.2 mm. Do not powder coat a surface that carries a dimensional tolerance tighter than ±0.1 mm (±0.004"). |
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| Size limits | Batch ovens in job shops commonly take 2-3 m (7-10 ft) long parts up to 300-500 kg; large shops run 6-10 m (20-33 ft) architectural sections and vehicle frames. Conveyorized lines take parts small enough to hang on the line. Sheet under 0.5 mm (0.020") can warp in the oven; heavy castings need a longer bake to reach cure temperature. |
| Surface finish | A smooth, slightly orange-peeled film in gloss, satin, matte, textured or wrinkle finishes, in any RAL or stock colour, including metallics and clears. Pencil hardness 2H typical, better chip and abrasion resistance than wet paint, and 500-1,000 h salt spray with proper pre-treatment. The film hides fine machining marks but telegraphs weld spatter, grind marks and dents. |
| Lead time | 2-5 business days for stock colours on a batch of parts; 1-2 weeks when a custom colour, a primer coat or extensive masking is involved. Custom colours have a minimum powder purchase. |
Typical of U.S. job shops; confirm with the shop for your part.
Materials
Steel, stainless, aluminum, galvanized steel and zinc die castings, all grounded and pre-treated. Cast parts must be degassed (pre-baked) or they pinhole. Plastics and composites only with special low-cure or UV-cure powders and a conductive primer; most job shops do not run them. Brass and copper take it but rarely need it.
What drives the cost
- Pre-treatment: blasting, phosphating or chemical conversion, and outgassing of castings
- Masking: every threaded hole, bore, ground point and datum that must stay bare is labour
- Part size and rack density: the oven and the line are paid for by the hour
- Colour: stock colours are cheap; custom RAL matches, metallics and clears carry setup and minimum-buy charges
- Two-coat systems (primer plus topcoat) roughly double the labour
- Quantity: small lots pay a minimum lot charge; hundreds of parts drop the unit price steeply
- Testing and certs: film thickness reports, cross-hatch adhesion, salt spray, AAMA 2604 or 2605 conformance
When to use it
- A durable, chip-resistant colour finish on steel or aluminum enclosures, frames, brackets, guards and furniture
- Outdoor and architectural parts where gloss and colour retention matter (super-durable polyester)
- Thick, edge-covering corrosion protection on welded steel fabrications that will be handled roughly
- Volume runs where the low material waste and single-pass build beat wet paint on cost
- A finish that hides minor surface flaws on castings, laser-cut sheet and weldments
When not to
- Parts with tight fits, precision threads or datums that cannot be masked or post-machined
- Assemblies containing plastics, seals, magnets, lubricants or electronics that will not survive 180 °C (350 °F)
- Very thin films or exact colour matches to a wet-painted mating part
- Parts that see temperatures above about 150-200 °C (300-400 °F) in service; the resin degrades
- Deep, narrow recesses where the powder cannot reach; consider e-coating first or instead
Design tips
- Specify the chemistry (polyester for outdoors, epoxy for chemical or indoor use), the colour code (RAL or a manufacturer number), the gloss and the film thickness range.
- Specify the pre-treatment: "blast to SSPC-SP 6 and iron phosphate" or "zinc phosphate" or "chromate-free conversion coat on aluminum". It determines corrosion life more than the powder does.
- Mark every surface that must stay bare, and give the shop plugs or masking dimensions; put grounding studs where a mask is cheap.
- Add a hanging hole or a tab: the part must hang on a rack and be electrically grounded through it.
- Vent closed tubes and box sections with a hole of at least 6 mm (0.25") so trapped air and moisture escape in the oven.
- Grind welds smooth and remove spatter and slag before coating; the powder will not fill or hide them.
- Radius sharp edges to at least 0.5 mm (0.02"); powder pulls thin over sharp edges and corrosion starts there.
- For castings, request an outgassing pre-bake at or above cure temperature to stop pinholing.
Powder Coating by material
Frequently asked questions
- How thick is powder coating?
- A single coat is typically 50-100 µm (2-4 mils), with 60-80 µm the usual target; a primer plus topcoat system runs 100-200 µm. It is two to three times as thick as a coat of wet paint, which is why it needs masking on fits.
- Is powder coating better than paint?
- For durability, yes: it is thicker, harder and more chip and chemical resistant, with no solvent. Wet paint wins on thin films, exact colour matching, touch-up in the field, and parts that cannot go through an oven.
- Can you powder coat aluminum?
- Yes, and it is common on extrusions and machined enclosures. Aluminum needs a chromate or chromate-free (zirconium) conversion coat first for adhesion and corrosion resistance; a light etch alone is not enough outdoors.
- Does powder coating prevent rust?
- It does when the steel is properly blasted and phosphated underneath and the film is unbroken. A scratch to bare metal will undercut over time, so for harsh service add a zinc-rich primer or galvanize first and powder coat over it.
- What temperature does powder coating cure at?
- Most powders cure at 160-200 °C (325-400 °F) part-metal temperature for 10-20 minutes. Low-temperature powders cure near 120-140 °C (250-285 °F) for heat-sensitive substrates, at some cost in hardness.
