Powder coating: surface prep, thickness and inspection

The durability of a powder coating depends on surface preparation, not on the paint itself: powder does not stick to grease, rust or mill scale. The second rule is measuring thickness as you go, because a coating that is too thin or too thick is a reject, and the third is checking adhesion and appearance after the oven.

How powder coating differs from spray painting

In powder coating, dry powder is applied to the part and clings to it electrostatically, then melts and cures in an oven. In spray (wet) painting, liquid paint is applied and dries or cures after the solvent evaporates.

The methods for measuring wet film, dry coating and uncured powder layer are described in the PN-EN ISO 2808 standard. For powder, you measure the layer before the oven and the dry coating after the oven.

How steel, zinc and aluminium surfaces are prepared

Steel, zinc and aluminium are prepared chemically in several stages. The typical order looks like this:

1. Degreasing, which removes oil and grease left after machining.
2. Rinsing.
3. Phosphating (often combined with degreasing), which prepares the surface for paint.
4. Rinsing again.
5. Passivation.
6. Drying before the part enters the booth.

For steel structures, abrasive blast cleaning (sand blasting or shot blasting) to grade Sa 2.5 according to ISO 8501-1 is often added, preceded by washing and degreasing. After cleaning, steel starts to rust again, so the plant's documentation limits the time before the primer is applied. In one such specification it is up to 6 hours, but the instruction of the particular plant is what applies.

On aluminium, poor degreasing shows immediately: water does not wet the surface evenly but runs off and collects in droplets. A dull finish after etching, in turn, points to too weak etching. A part with such a symptom goes back to preparation before it reaches the booth.

What powder coating thickness is required and how it is measured

There is no single thickness for all uses. The target value is given in the technical data sheet of the specific powder and in the customer's specification. For example, manufacturers give a range of 60-100 µm for one polyester powder and 60-80 µm for another. The coating system is also affected by the environment where the structure will stand: an element for indoor use is painted differently from a structure for an industrial atmosphere with high humidity (corrosivity category C5 according to PN-EN ISO 12944).

Dry coating on steel is measured with a magnetic gauge (PN-EN ISO 2178), and on aluminium and other non-ferromagnetic substrates with an eddy current gauge (PN-EN ISO 2360). The measurement is taken at several points on the part, because thickness differs on edges, in corners and on flat surfaces.

A coating that is too thin protects worse against corrosion, and one that is too thick wastes powder and can interfere with assembly on threads and in holes. Both results are rejects, so the operator corrects the gun settings or the application method straight away, not after the whole batch.

How adhesion and coating quality are checked

Adhesion is tested, among other methods, by the cross-cut test (PN-EN ISO 2409): a knife with blades cuts a grid down to the substrate, tape is stuck on, and you assess how much of the coating came off. The other method in use is pull-off (PN-EN ISO 4624), which measures the force needed to tear off a glued-on dolly. For aluminium, the powder manufacturer gives a boiling water test and a tape adhesion test as the minimum.

Visual inspection catches the typical coating defects:

• orange peel, meaning a wavy, uneven surface,
• craters and small pits, often caused by surface contamination,
• missed spots in corners and recesses,
• chipping and poor adhesion, which usually result from surface preparation.

An adhesion defect rarely comes from the paint itself. First you check degreasing, rinsing and the time from cleaning to painting.

Does a powder coating operator need qualifications

In general there is no mandatory certificate for a powder coating operator, and the requirements are set by the plant. They most often include workstation and health and safety training, operating the gun and the thickness gauge, and knowing the technology instruction. Experience with lacquers and drying ovens also helps in work in the furniture industry, where substrate preparation matters in a similar way.

What hazards come with powder coating

A powder booth can be a dust explosion hazard zone, so some plants require protective clothing and antistatic footwear there, marked with the symbols described in the guide to work clothing and protective equipment. Respiratory protection in line with the instruction is used against dust and vapours at the workstation, and protection against high temperature near the oven.

Frequently asked questions

Do I need to know Polish to work in powder coating?

Language requirements are set by the plant. At the workstation it helps to understand the technology instruction, the markings on powder data sheets and quality control instructions.

Why degrease if the part is blasted anyway?

Oil and grease stay under the abrasive and contaminate the cleaned steel. That is why washing and degreasing are done before blast cleaning.

What happens to a part that failed inspection?

It depends on the defect and the plant's instruction. The part may go back for repair, for repainting, or for stripping of the coating and full preparation from scratch.

Is powder coating a good start without experience?

Hanging and unloading parts on the line is often where people start without experience, and the gun and gauge are learned at the workstation, as at most workstations in production work.

This text describes the general rules. What you are entitled to depends on your type of contract: an employment contract (umowa o pracę) or a contract of mandate (umowa zlecenie). Your own contract and payslip are what count.