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Home / News / Industry News / Powder Coating Hides Well at Normal Thickness — Why Does It Show Through at Thin Film?
Industry News
Powder Coatings · Appliances · Aluminium Profiles · Engineering Machinery
Powder coating with normal film thickness gives good colour and hiding power — but when film thickness is reduced to lower material consumption, the substrate colour bleeds through, local colour mottle appears, and overall hiding can be insufficient. This problem is common in dark, light, and high-decorative powder coating formulations.
Many people's reaction is to increase the powder application quantity — but if the product's thin-film hiding capability is itself insufficient, simply increasing film thickness does not truly solve the problem. Improving thin-film performance requires investigation across powder formulation, pigment dispersion, and spray process together.
1 · Why Is Thin Film Particularly Prone to Show-Through?
After powder coating forms a film, whether the substrate can be completely hidden is related to both film thickness and the hiding capability of the coating itself. When film thickness decreases, the amount of pigment and filler per unit area also decreases — the coating's ability to cover the substrate colour naturally declines. This is especially true for light-colour powder coatings: if the substrate colour is deep, show-through is more likely after thin application. Thin-film show-through does not necessarily mean poor powder quality — it may mean the film thickness has already dropped below the range the current formulation can stably cover.
2 · Poor Pigment Dispersion — Problems Are More Obvious in Thin Film
Pigment in powder coating is not simply added in and ready. If pigment is insufficiently dispersed during extrusion and mixing, forming local agglomeration, non-uniform pigment distribution in the film results. At normal film thickness, this difference may not easily be noticed — but when film thickness decreases, areas with locally insufficient pigment become more likely to show through the substrate. Therefore, when improving thin-film hiding capability, pigment dispersion uniformity needs focused attention. For formulations using high-loading titanium dioxide, carbon black, or other high-tinting-strength pigments, the dispersion state has a particularly significant effect on final hiding performance.
3 · Pigment System Is Not "More Is Better"
Increasing pigment addition may appear to improve hiding power, but this does not mean more pigment is always better. When the pigment ratio is too high, system viscosity, melt flow, and mechanical properties may all change. At the same time, if pigment has not been sufficiently dispersed, increasing the quantity may instead cause more agglomeration. What truly needs attention is: pigment type + addition ratio + dispersion state + resin system — not simply raising pigment content.
4 · Powder Particle Size Also Affects Thin-Film Performance
Powder particles that are too large or have an unreasonable particle size distribution may cause non-uniform powder deposition during spraying. At lower film thickness, differences in powder coverage in different zones become more pronounced. If powder particle size is unstable, powder pick-up rate, melt spread, and final film thickness distribution may also change. Therefore, for thin-film products, the powder particle size itself cannot be overlooked.
5 · Poor Melt Flow Can Also Cause Local Show-Through in Thin Film
After powder is sprayed onto the workpiece surface, it does not directly form the final film. After baking, the powder needs to melt, flow, and gradually form a continuous coating. If melt flowability is insufficient and powder particles cannot fully fuse together, the film may develop locally thin zones. In a thin-film state, this type of problem is more easily amplified. Therefore, powder coating flow performance does not only affect surface appearance — it also affects the final continuity of the film.
6 · Spray Process Itself May Also Cause "Apparent Show-Through"
Sometimes the formulation has no obvious problem, but show-through still occurs after actual spraying. At this point it is necessary to check workpiece grounding, gun parameters, spray distance, and powder pick-up state. If powder deposits unevenly at different workpiece positions, it may form: locally insufficient film thickness → reduced hiding power → substrate colour shows through. Especially at edges, grooves, and corners of complex workpieces, insufficient film thickness is more likely to appear. Therefore, thin-film products typically have higher requirements for spray uniformity.
7 · How Can Additives Help Improve This?
If spray film thickness and equipment problems have been eliminated, the formulation side can focus on additive matching in the powder system. For example, improving pigment dispersion state so that the colouring components are more uniformly distributed in the resin system; adjusting the leveling system so that molten powder can more fully spread into a film. For powder systems that are prone to cratering and other surface defects, degassing and surface defects also need to be balanced. In other words, thin-film performance is not determined by any single additive — it is the combined result of dispersion, leveling, degassing, and the pigment system working together.
8 · How to Diagnose Thin-Film Show-Through?
If the product has normal hiding at standard film thickness but show-through occurs only at thin film, the investigation can follow this sequence: first confirm actual film thickness → check spray uniformity → check pigment dispersion → analyse the pigment system → check powder particle size → then adjust melt flow and additive system. If all areas show uniform show-through, the problem is more likely related to formulation hiding capability; if only localised areas show through, spray, powder pick-up, and workpiece structure need priority investigation. This type of investigation is more likely to find the true cause than simply increasing powder consumption.
Summary
Thin-film show-through in powder coating is not just "too thin a film." Pigment hiding capability, dispersion uniformity, powder particle size, melt flow, and spray uniformity all affect the final thin-film effect. If good hiding is to be maintained while reducing film thickness, both formulation and process need to be optimised simultaneously. For powder coating, a true thin-film system does not rely simply on increasing pigment or raising film thickness — it means that even a limited film thickness can form a uniform, continuous coating layer with sufficient hiding capability.
Thin film ≠ insufficient hiding always formulation fault
Pigment dispersion uniformity is critical at thin film
Pigment is not better when used in larger amounts
Powder particle size affects thin-film coverage uniformity
Poor melt flow causes locally thin film zones
Spray uniformity affects thin-film result significantly
Frequently Asked Questions
Is there a minimum film thickness threshold below which any powder formulation will show through?
Yes — every powder coating formulation has a minimum film thickness below which hiding is insufficient, regardless of pigment loading or quality. This threshold is determined by the optical properties of the pigment system (scattering coefficient and absorption coefficient), the uniformity of pigment distribution in the cured film, and the film continuity. The minimum hiding thickness can be reduced through better pigment dispersion and optimised melt flow, but cannot be eliminated entirely — it is a fundamental characteristic of the optical system.
Why does thin-film show-through often appear worse at edges and corners than on flat areas?
Edges and corners are inherently zones of lower powder deposition due to electrostatic field distribution in the spray process — the Faraday cage effect reduces powder penetration into concave geometry. Combined with the surface tension effect during melt flow that draws coating material away from edges, film build at these zones is typically lower than adjacent flat areas. This makes edge and corner zones the first locations where thin-film hiding fails, even when the average film thickness across the flat panel is above the hiding threshold.
