Powder Coatings

Powder coating has become a standard finish for metal products of all kinds: appliances, furniture, automotive parts, agricultural equipment, architectural aluminum, electrical enclosures and more. In protective coatings, fusion-bonded epoxy (FBE) powders are a principal external corrosion coating for steel pipelines and are widely used on reinforcing bar.

The process differs fundamentally from liquid painting. There is no solvent to evaporate and no mixing of components on site. Instead, the coating is applied as a dry powder and transformed into a continuous film by heat, which also means the substrate must be able to withstand oven temperatures.

How powder coating works

A powder coating is manufactured by premixing resin, curing agent, pigments and additives, melt-compounding them in an extruder, then cooling, breaking and grinding the material into a fine powder. Each particle therefore contains all of the coating’s ingredients.

Most powder is applied by electrostatic spray. A corona gun charges the particles with a high-voltage electrode, or a tribo gun charges them by friction, and the charged powder is attracted to the grounded part. The part then enters an oven where the powder melts, flows into a smooth film and, for thermoset powders, cross-links. A common cure schedule is on the order of 10–20 minutes at roughly 350–400 °F (177–204 °C) part metal temperature, though low-cure powders and specific products vary; always follow the product data sheet.

Thermoplastic powders and some functional coatings are applied by dipping preheated parts into a fluidized bed, where air keeps the powder suspended like a liquid. This produces thicker films.

Types of powder coatings

TGIC-free polyesters commonly use hydroxyalkylamide (HAA) curing agents. Fluoropolymer powders bring the weathering performance of fluoropolymer coatings to factory-applied finishes, while epoxy powders share many of the strengths and weaknesses of liquid epoxy coatings.

Fusion-bonded epoxy

FBE is a thermosetting epoxy powder applied to blast-cleaned, preheated steel. The powder melts on contact, flows and cures within a short time, often with little or no post-cure. It is applied in coating plants to line pipe, girth-weld joints in the field (using induction heating), rebar, valves and fittings. Single-layer and dual-layer FBE systems are common, and FBE is also used as the bottom layer of three-layer polyolefin systems. See pipeline coatings.

The powder coating process

1. Clean. Remove oils, soils and mill residues by alkaline washing or solvent cleaning.
2. Pretreat. Apply a conversion coating, such as iron or zinc phosphate or a zirconium-based pretreatment, or abrasive-blast for heavy-duty service. Rinse and dry thoroughly.
3. Mask and hang. Mask threads and contact surfaces, and hang parts so they are well grounded. Clean hooks and racks regularly, because built-up coating insulates the part.
4. Apply. Spray in a booth with recovery, adjusting voltage, airflow and gun distance for even coverage.
5. Cure. Bring the part, not just the oven air, to the specified temperature for the specified time. Heavy parts take longer to reach metal temperature.
6. Inspect. Check film thickness, appearance, adhesion (for example ASTM D3359 cross-cut) and degree of cure, often with a solvent rub test.

Film thickness is normally measured on the cured coating with a magnetic or eddy-current gauge, following a procedure such as SSPC-PA 2 or ISO 2808 where the specification calls for one. Uncured powder can also be checked with non-contact gauges, which allows gun settings to be adjusted before parts enter the oven. Both too little and too much powder cause problems: thin films may show substrate or give poor corrosion resistance, while excessive thickness wastes material and can cause orange peel, reduced flexibility and chipping at edges.

Common defects

• Outgassing: castings, galvanized steel and porous metals release trapped gases during cure, leaving pinholes and bubbles. Pre-baking the part or using degassing-formulated powders helps.
• Faraday cage effect: electrostatic fields concentrate on edges and outer surfaces, so inside corners and recesses receive less powder. Lowering voltage or adjusting gun technique can improve coverage.
• Orange peel: excessive texture from heavy film, poor flow or incorrect cure profile.
• Poor adhesion: usually traced to inadequate cleaning or pretreatment.
• Color or gloss variation: over-bake, under-bake or mixing reclaimed powder from different batches.

Advantages and limitations

Frequently asked questions

Is powder coating better than paint?

For parts that can be oven-cured, powder often gives a tougher, more uniform finish with lower emissions. For large structures, field work or heat-sensitive substrates, liquid coatings remain the practical choice.

Can powder coating be applied over galvanized steel?

Yes, and the combination is common, but galvanized steel needs suitable cleaning or sweep blasting and often pre-baking or a degassing powder to avoid pinholes.

Why did my outdoor epoxy powder coat fade?

Epoxy powders chalk and fade in sunlight just like liquid epoxies. Exterior parts should use polyester, super-durable polyester or fluoropolymer powders.

How thick is a powder coating?

Decorative powders are commonly applied at about 2–4 mils (50–100 µm), while functional coatings such as FBE and thermoplastic powders are often considerably thicker. Measure dry film thickness against the specification.