PL
ENThe thickness of a powder coating directly affects durability, corrosion resistance and production costs. Too thin a film does not protect the material, while too thick a film creates defects and wastes powder. Below we set out the subject of powder coating thickness – what the standards say, what it looks like in practice and which mistakes are the most common.
Why does coating thickness matter?
Coating thickness determines whether the powder coating will do its job throughout the part's intended service life. This is not just about the first few months after production, but about retaining its properties over the long term, often in demanding environmental conditions.
A reference point in quality control
It is one of the few parameters that can be easily measured and compared against the process specification. In a complaint, a quality audit or a failure analysis, thickness is always the first point of reference.
Standards governing powder coating thickness
Although coating thickness is sometimes treated as an ‘in-house' parameter, it rests on clearly defined technical standards. These do not set a single mandatory value, but describe the measurement methods, how to assess the results and how thickness relates to the durability of the protection. They are the reference point in audits, certification and quality disputes.
| Standard | Scope and application |
|---|---|
| ISO 2178 | Non-destructive measurement of non-magnetic coatings on ferromagnetic (steel) substrates, using the magnetic induction method. It does not state the ‘correct' thickness – it defines how to measure it correctly and reproducibly. |
| ISO 2360 | Thickness measurement on aluminium and other non-ferrous metals, based on the eddy-current method. |
| ISO 2808 | Sets out the methods for measuring the thickness of paint and coating films – both non-destructive and destructive (e.g. cross-sections). A reference point when establishing quality-control procedures. |
| ISO 12944 | Covers protective systems against corrosion. Developed with liquid coatings on steel structures in mind, but applied by analogy in the powder industry – it links thickness to the corrosivity of the environment and the intended service life. |
| QUALICOAT | A certification scheme for architectural applications (especially aluminium). It sets the minimum requirements for substrate preparation, coating thickness and visual quality. |
The effect of thickness on corrosion protection
A powder coating protects the substrate mainly by forming a barrier that limits the access of moisture and oxygen. The thinner the film, the shorter the path corrosive agents have to travel to reach the metal.
Critical zones
A film that is too thin loses its integrity sooner, especially in areas exposed to mechanical damage and changing weather. Edges, welds and pressings are the first zones where degradation occurs. Even a small reduction in thickness in these areas can shorten the real protection time by several years.
Thickness and mechanical strength
Thickness affects how the coating responds to impacts, point loads and movement of the substrate. A thin film cracks or is punctured more easily, particularly on sharp edges. A film that is too thick, on the other hand, becomes rigid and copes less well with movement of the material.
The coating should be matched to the function of the part. Components exposed to impact or assembly call for a different approach than enclosures or decorative items. Excessive thickness does not make up for application faults – it often makes them worse.
How does coating thickness translate into the look of the part?
An even thickness is a precondition for stable colour and gloss. Even small differences can cause changes in light reflection that are visible to the naked eye, especially on large, flat surfaces.
With textured and metallic powders the problem intensifies. Uneven thickness leads to local changes in texture or a ‘floating' of the visual effect. In the production of visible parts, thickness is often controlled far more closely than in purely technical applications.
Read also: Surface preparation for powder coating – stages, mistakes and best practice
What determines the final coating thickness?
The final thickness is not the result of a single setting. It is the outcome of a whole chain of events: from substrate preparation, through application, to curing. The chemistry of the powder affects how easily the film builds and how it behaves in the oven. The geometry of the part determines the distribution of the electrostatic field. The curing conditions affect flow-out and the final structure of the coating. Even small changes in one of these areas can push the thickness outside the intended range.
Measuring coating thickness in practice
The measurement should be taken on the finished, cooled coating – only then does the thickness reflect the actual amount of material protecting the substrate.
Day-to-day inspection uses non-destructive methods. On steel (ferromagnetic) substrates the measurement is taken with magnetic induction gauges, and on aluminium and other non-ferrous metals with eddy-current gauges. Many modern instruments detect the substrate automatically, but it is worth choosing the measurement mode deliberately to avoid false readings. Variation in method can introduce a greater error than the application itself.
Read also: The role of the oven in the powder coating process
Thickness ranges by application
There are rough ranges that work well when the process is correctly set up. These are not rigid values, but sensible points of reference.
Indoor decorative parts – 40–65 µm
Metal furniture, fittings, enclosures. This thickness provides good looks and sufficient durability in a low-corrosivity environment.
Outdoor parts – 65–100 µm
Architectural elements and automotive components. The greater material reserve protects the part against moisture, UV and temperature changes.
Heavy industry – 100–150 µm
Structures exposed to heavy use or highly corrosive environments. These are often multi-coat systems with a protective primer.
Cost control
Every 10 µm above target is a real waste of powder per part. Controlling thickness means directly controlling material costs – on larger runs the difference can be very noticeable.
The most common coating-thickness problems
Even with a well-designed process and modern equipment, powder coating thickness can ‘drift'. Usually this is not the result of one spectacular mistake, but of the sum of small deviations that, over time, begin to affect quality and consistency.
A coating that is too thin
A film thinner than intended does not provide adequate corrosion protection and loses its performance sooner. The problem often comes down to inconspicuous process settings.
The most common causes are too great a distance between the gun and the part, too low an electrostatic voltage or a restricted powder feed. In these conditions the powder does not deposit evenly, and edges and recesses are left under-coated. The fix is to fine-tune the application parameters precisely and guide the gun deliberately over areas with difficult geometry.
Excessive thickness
Too thick a film in a single coat does not automatically increase durability. It leads to surface defects and process problems that are hard to put right later. In multi-coat systems a higher total thickness makes sense – but only if each coat is correctly applied and cured.
Typical consequences of exceeding the thickness in a single pass are an unstable surface texture, orange peel and increased internal stress. These often result from moving the gun too slowly or working too close to the part. Rather than ‘adding' more powder, it is more effective to tidy up the spray path and adjust the line speed.
Uneven thickness on a single part
An uneven thickness distribution reduces both the durability and the appearance of the coating. It usually stems not from the powder itself, but from the conditions in which it is applied.
Problems are caused by poor electrical conductivity of the substrate, inconsistent pre-treatment or an incorrect spray angle. The powder deposits selectively, creating zones that are too thin and too thick. Better surface preparation and deliberate control of gun movement can significantly reduce these differences.
The Faraday cage effect
Narrow gaps, corners and deep recesses are naturally difficult to coat. The electrostatic field ‘closes off' at the edges, limiting the supply of powder to the inside of the part. It helps to lower the gun's main voltage and use staged spraying – this lets you work the powder into the recesses without overloading the flat surfaces.
Large variation between parts
If the thickness varies not only on a single item but also between successive pieces, the cause usually lies beyond the spray station itself. The most common culprits are fluctuations in conveyor speed, an unstable powder flow or changes in material feed. Even small differences in the rate at which parts are conveyed translate into noticeable changes in thickness. Stabilising the line parameters matters more than adjusting the gun settings.
Cracking after curing
Coating cracks are rarely visible immediately on leaving the oven. More often they appear after cooling or during use. The cause is often applying too thick a film in one go, or a poorly chosen curing cycle. Better results come from building thickness in stages, while ensuring full melting and cross-linking of each coat. The coating then keeps its flexibility and integrity, even at a higher total thickness.
Read also: The most common problems in powder coating and how to solve them
Test before you start a production run
If you want to stabilise your coating thickness without adding more powder or complicating the process – get in touch with the TOPO team.
We will match a powder coating to the working conditions of your line.
We offer free samples of our powders.
Match the finish to your project
Choose a gloss level: matt, semi-matt, semi-gloss, gloss, high gloss.
Available surface textures: smooth, fine texture, coarse texture.
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