Fisheyes and Craters in Powder Coating: Causes and Fixes

Provided by Lin Cong, Technical Director, MUSI Technology · Published October 2, 2026 · Last reviewed October 2, 2026

Quick answer: Craters and fish eyes in powder coating are round depressions formed when the molten film retreats from a low surface tension contaminant; a fisheye is a crater open to bare metal. Silicone is the classic cause and acts at concentrations as low as 0.001 %, while an incompatible powder such as acrylic can crater a batch at about 0.1 %. Oil carried over in compressed air is the most common source. Gun makers publish limits such as 0.1 mg/m³ oil vapour (Gema) and a dew point below 10 °C (Nordson). The cure is to isolate the source step by step, then clean and segregate.

MUSI Technology, a powder and liquid spray coating line manufacturer in Changzhou, China, designs powder lines with compressed air treatment specified to the gun maker’s limits, booths and recovery systems that open fully for clean-down, conveyor lubrication kept away from open product and pretreatment stages with controlled final rinse quality.

Large cartridge filter unit, cyclone and red fan housing for powder handling on a powder coating line
Reclaim equipment returns overspray to the hopper, so any contamination in the recovery path comes back to the gun.

What do fisheyes and craters look like in powder coating?

Craters in powder coating are round depressions where the molten film has pulled away from a point, and fish eyes in powder coating are the severe form in which the film withdraws completely and leaves bare metal at the centre. Both are wetting failures. They differ from pinholes, which are gas pores tied to porous substrates and thick film, and are covered in the powder coating defects overview.

Feature Craters Fisheyes Pinholes
Shape Circular depression Circular depression open to the substrate Very small pore
Film at the centre Thin film remains None, bare metal visible Open channel through the film
Distribution Random, often denser on horizontal faces Random, follows the contaminant Follows castings, galvanized faces, thick film
Mechanism Low surface tension contaminant Same, heavier contamination Gas or vapour leaving the part during cure

What causes craters and fish eyes in powder coating?

Powder coating craters are caused by a contaminant whose surface tension is far lower than that of the molten powder. Silicone-based materials measure about 15-20 dynes/cm against about 40-50 dynes/cm for powder coatings, so the melt retreats from every contaminated spot. Silicone contamination in powder coating is damaging at concentrations as low as 0.001 %, and an incompatible powder can crater a batch at about 0.1 %. The Powder Coating Institute names unclean compressed air as the most prominent cause.

Source of contamination How to confirm Fix
Oil or water in compressed air Blow air through a clean white cloth at the gun; open filter drains Service dryer, replace coalescing elements, drain receivers
Silicone sprays, sealants, lubricants, mould release Audit maintenance stores and nearby processes; defect appears plant-wide Remove silicone products from the building; clean air handling, booth and guns
Hand creams, antiperspirants, cosmetics Defect follows one operator or manual touch-up areas Clean gloves, no personal care products at the booth
Forming oil, rust preventative or grease left by pretreatment Water-break test after final rinse; craters follow part geometry Correct cleaner concentration, temperature and rinse quality
Incompatible powder (acrylic or silicone high-heat grades in polyester, epoxy or hybrid; different suppliers) Spray virgin powder from a clean box unit on a clean panel Dedicated hoses, pumps and hoppers; full clean-down before changeover
Conveyor chain lubricant drip Craters on top faces under the track Drip trays, correct lubricant dose, clean chain
Contaminated reclaim powder Defect stops with reclaim by-passed Discard reclaim, clean cyclone, sieve and filters
Oven condensate Defect appears only after cure, heaviest near exhaust or roof Clean oven, increase exhaust air

What compressed air quality does powder coating need?

Powder coating needs compressed air that is dry and practically oil-free at the point of use, and gun makers state this as limit values in their manuals, not as a single ISO 8573-1 class. The Gema oil limit of 0.1 mg/m³ equals the ISO 8573-1 class 2 limit for total oil. Nordson’s Econo-Coat dew point range reaches up to about the class 4 level of +3 °C.

Published requirement Water Oil Particles
Gema OptiFlex Pro manual unit 1.3 g/m³ water vapour maximum 0.1 mg/m³ oil vapour maximum Not stated in the data used here
Nordson Encore HD manual system Dew point below 10 °C Not stated in the data used here Below 5 µm
Nordson Econo-Coat Series II Pressure dew point -6.7 to 3.4 °C or lower at 6.89 bar 0.1 ppm oil vapour or aerosol maximum 99 % removal above 0.3 µm
ISO 8573-1 class 2 -40 °C pressure dew point 0.1 mg/m³ total oil Class table in the standard
ISO 8573-1 class 4 +3 °C pressure dew point 5 mg/m³ total oil Class table in the standard

The Powder Coating Institute describes the basic treatment train as an air cooler followed by two-stage filtration: a 5 µm filter and a coalescing filter. The limit that applies to a given line is the one in the manual of the installed guns and pumps.

How do you find the source of fisheyes step by step?

The source is found by removing one possible contaminant at a time and spraying a test panel after each change. Start with clean powder on a clean panel, then add the line back in stages.

Step Test Result and meaning
1 Solvent-wipe a steel panel, spray virgin powder from a clean box unit, cure in a laboratory or separate oven Craters: powder lot or compressed air. Clean: go on
2 Repeat step 1 using a different compressed air source or after new filter elements Craters gone: compressed air was the source
3 Spray virgin powder through the production booth with reclaim by-passed Craters return: booth, hoses or pumps hold contamination
4 Switch reclaim back on Craters return: reclaim powder is contaminated
5 Run a pretreated production part instead of the wiped panel Craters return: pretreatment is leaving oil or rinse residue
6 Hang panels on the conveyor through the full line, uncoated zones included Craters on top faces: chain lubricant or airborne fallout
7 Cure identical panels in the production oven and a clean oven Craters only from the production oven: oven condensate

Powder compatibility can be tested in advance by spiking the production powder with 0.1 % of the suspect powder and curing a panel.

Which equipment and line settings prevent fisheyes in powder coat?

Fisheyes in powder coat are prevented by four equipment measures: compressed air treatment sized for the booth, a booth that can be cleaned completely, separate powder paths for incompatible chemistries, and pretreatment that delivers a water-break-free surface. Air treatment means an aftercooler, a refrigerant or desiccant dryer and coalescing filtration close to the booth. Recirculated deionised water in the final rinse with a fresh deionised mist reduces rinse residue. Key components are available from international brands such as Atlas Copco, Kaeser and Parker domnick hunter for dryers and coalescing filters, Gema, Nordson and Wagner for guns and powder pumps, and Henkel Bonderite and Chemetall (BASF) for pretreatment chemistry (configurable per customer specification). Booth design for clean-down is covered in the quick colour change booth article, and cleaner and conversion stages in the pretreatment comparison.

Area Prevention check
Compressed air Dew point and filter differential pressure logged; drains tested; elements changed on schedule
Plant rules No silicone sprays, sealants or polishes on site; maintenance lubricants approved before use
Operators Clean lint-free gloves; no hand cream or aerosol products near the booth
Powder handling Boxes kept closed; acrylic and high-heat silicone powders on dedicated equipment; hoses replaced after a contamination event
Reclaim Cyclone, sieve and recovery filters cleaned at every chemistry change
Conveyor Lubricant dosed away from the booth and oven entry; drip trays over open product
Pretreatment Bath titrations on schedule; final rinse conductivity monitored
Oven Walls, roof and ducts cleaned of condensate; exhaust flow verified

FAQ

What is the difference between a crater and a fisheye in powder coating? A crater is a round depression in which a thin film still covers the substrate. A fisheye is the severe form: the molten powder has withdrawn completely and bare metal shows at the centre. Both come from low surface tension contamination.

How much silicone does it take to cause craters? Very little. PCI Magazine reports that silicone-based products at concentrations as low as 0.001 % can cause widespread cratering, because their surface tension of about 15-20 dynes/cm is far below the 40-50 dynes/cm typical of powder coatings.

Can mixing two powders cause fish eyes? Yes. Acrylic and high-heat silicone powders are the usual offenders when they contaminate polyester, epoxy or hybrid powders, and about 0.1 % can be enough. Powders from different suppliers can also be incompatible, so equipment is dedicated or fully cleaned between them.

How do I know whether compressed air is causing powder coating fish eyes? Spray virgin powder on a solvent-wiped panel using the production air, then repeat with a different air source or new filter elements. If the craters disappear, the air was the source. Check the dryer, coalescing filter elements and drains.

Where MUSI Technology fits

MUSI Technology supplies complete powder lines in which air treatment, booth, recovery, conveyor and pretreatment are specified together, so contamination paths are closed at the design stage. See the electrostatic powder coating booth and the powder coating line guide for the equipment scope.

Provided by Lin Cong, Technical Director, MUSI Technology

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