It was a Friday afternoon. I had a set of premium wheels ready to hand over. The customer was waiting outside with a trailer. And then I saw it — dozens of tiny craters on every single wheel, like someone had peppered them with fine sand.
Panic? For about 30 seconds. Then the diagnostic process I’d built over 15 years kicked in.
Powder coating fish eyes — also called craters — are one of those defects that appear without warning and can ruin an entire day’s work in seconds. Small, round depressions with exposed substrate in the center. Like a meteor field on your coating.
In this article I’ll walk you through exactly where fish eyes come from, how to diagnose them systematically, and — most importantly — how to eliminate the root cause permanently. This is 15 years of hands-on powder coating shop experience, not theory.
What Are Fish Eyes in Powder Coating?
A fish eye (crater) is a small, circular depression in a cured powder coating where the coating has pulled away from the surface. The substrate — bare metal or primer — is visible at the center. The edges of the crater are typically raised, and the surrounding coating may be slightly wavy.
The key fact most coaters miss: fish eyes don’t form in the oven. The cause is already present before cure — on the metal surface, in the powder, or in the compressed air supply. The oven only reveals it. Searching for the cause “in the oven” is the most common diagnostic mistake.
Fish Eye vs. Crater — Same Defect?
In practice, both terms describe the same problem. Some coaters make a distinction:
- Crater — a depression with exposed substrate, complete loss of coating continuity
- Fish eye — a depression with a thin coating layer at the bottom, but a characteristic raised “ring”
The cause and fix are identical for both. I use both terms interchangeably throughout this article.
Why Does the Coating “Pull Away” and Form a Fish Eye?
Pure physics. When powder melts in the oven and begins to flow, it behaves like a liquid. Liquids spread across surfaces according to surface tension — the higher the surface tension of the metal, the better the liquid wets it and spreads uniformly.
Contamination (oil, silicone) drastically lowers surface tension at that point. The molten coating “runs away” from the contaminated spot — exactly like water running away from a greasy patch on a pan. A crater remains.
That’s why fish eyes are round with raised edges — the molten coating gathered around the contamination, not on it.
6 Main Causes of Fish Eyes in Powder Coating
Cause 1: Oil in Compressed Air — The #1 Culprit
This is number one on my list. In my shop, it caused fish eyes more often than anything else.
A compressor without a proper oil filtration system is a time bomb. Oil migrates into the lines, reaches the spray gun, and deposits as an invisible film on the part. After cure — fish eyes across the entire surface, evenly distributed.
How to check: Blow your gun onto a white sheet of paper. Greasy, yellowish spots? You have an oil problem.
Solution — complete air filtration system:
Compressed air in a powder coating shop must meet ISO 8573-1 standard. Minimum required class for powder coating:
- Oil: Class 1 — maximum 0.01 mg/m³ (essentially zero)
- Water: Class 4 — pressure dew point maximum +37°F (+3°C)
- Particulates: Class 2 — particles up to 1 µm
To achieve these parameters, you need a filtration system in this order:
- Cyclone separator — directly after the compressor, removes large water droplets and solid particles
- Air dryer — mandatory, not optional (details in Cause 2)
- Coalescing filter (oil filter) — Class H or better per ISO 8573, captures oil mist
- Final filter — directly before the gun, last line of defense
Coalescing filters have a clog indicator — check it weekly. Replace every 3 months under heavy use, regardless of how the element looks. A clogged filter passes just as much oil as no filter at all.
Cause 2: No Air Dryer — A Critical Mistake
Many coaters have filters but no dryer — and that’s a fundamental error. A coalescing filter removes oil mist and large water droplets, but it does not remove water vapor. Water vapor passes through every mechanical filter unimpeded.
A refrigerated dryer (most common choice) cools the air, water vapor condenses and drains. Dew point after a refrigerated dryer: +36°F to +50°F (+2°C to +10°C) — sufficient for a shop in normal conditions.
A desiccant dryer (silica gel or molecular sieves) achieves a dew point of -4°F to -40°F (-20°C to -40°C) — necessary when working in cold conditions or with particularly demanding coatings.
Why this matters: Water vapor condenses in cooler sections of the air lines and reaches the gun as microdroplets. On the part, they create microscopic “islands” — after cure, the powder has nothing to wet in those spots. Fish eyes remain.
Fall and winter are peak problem seasons — outside temperatures drop, the compressor works harder, condensation spikes. Fish eyes appearing seasonally? Moisture is the first suspect.
How to check: Point the gun airstream at your dry palm for a few seconds. Feel dampness or cold? Check the condensate separators — when was the last time you drained them?
An air dryer pays for itself after just a few rework jobs. It’s not a luxury — it’s the foundation of a fish-eye-free shop.
Cause 3: Silicone Contamination — The Worst Enemy
The nastiest type of fish eye. Silicone cannot be removed with ordinary degreaser and can contaminate an entire spray booth for weeks.
Silicone enters your shop from places you’d never suspect:
- Maintenance sprays (standard WD-40 contains silicone)
- Hinge and gasket lubricants
- Car care products and polishing pastes
- A worker’s hand lotion — touching a part after applying it
Silicone is surface-active — it spreads in ultra-thin layers and “infects” neighboring surfaces. Once it’s in the booth, it can cause problems for weeks.
How to identify: Fish eyes keep returning despite repeated cleaning. The center of the crater is smooth and “repellent.” Rub the suspect area with your finger, then put a drop of water on it — on a silicone-contaminated surface, water beads up instead of spreading.
Solution: Dedicated anti-silicone cleaners (not ordinary solvent). Thoroughly clean the entire booth, hooks, and air lines. And an iron rule — absolutely no silicone-containing products anywhere in the surface prep or application area. This isn’t a suggestion — it’s an operating condition.
Cause 4: Inadequate Degreasing
A classic cause. Oil from machining, coolant from CNC operations, rust-preventive coating from shipping — or simply fingerprints — remain on the metal.
Particularly problematic parts:
- New CNC-machined parts — covered in cutting fluid that appears dry but leaves an oil film
- Customer-supplied parts — you don’t know how they were stored or treated
- Stamped or bent sheet metal — die lubricant stays on contact areas
- Post-weld parts — welding flux, torch guide oil
How to identify: Fish eyes in repeatable locations — where the part was handled, where it contacted machinery. Clusters, not random distribution.
Solution: Two-stage degreasing — strong initial degreaser, followed by clean isopropyl alcohol. After degreasing — no bare hand contact, ever. Always disposable nitrile gloves.
Proper surface preparation is the foundation of every quality coating. For a full breakdown of mechanical and chemical prep methods, read: Sandblasting Before Powder Coating: What You Need to Know
Cause 5: Outgassing from Aluminum and Castings
Aluminum — especially die castings — contains microporosity filled with trapped gas. When heated in the oven, these gases escape through the molten coating and create craters from the inside out.
This is why aluminum wheels require outgassing before coating: 356°F–392°F (180°C–200°C) for 30–45 minutes in the oven before applying powder. This drives out the gas before you lay down the coating.
How to identify: Craters only on aluminum and castings, not on steel sheet. Often slightly larger and deeper than oil-contamination craters.
Skipping outgassing on wheels is a guaranteed problem — especially on refinished wheels where the metal structure has already been stressed by previous processing.
Cause 6: Contaminated Powder or Moisture-Damaged Powder
Powder stored in poor conditions absorbs atmospheric moisture. Clumped, damp powder doesn’t atomize properly and leaves defects after cure.
How to check: Pinch a small amount of powder between your fingers. Should be free-flowing like flour. If it clumps or feels damp — discard it.
How to identify: Fish eyes appear suddenly after opening a new bag of powder, with nothing else changed.
Solution: Store powder below 77°F (25°C) in tightly sealed containers. Powder has a shelf life — after one year in poor storage conditions, it loses its properties.
Step-by-Step Diagnosis — The STOP Method
When you see fish eyes — don’t act blindly. Changing everything at once guarantees you won’t know what fixed it. The problem will return.
STOP — Halt production. Don’t coat more parts.
OBSERVE — Describe exactly what you see: how many fish eyes, how they’re distributed, what’s at the center.
ANALYZE — Pattern interpretation:
| What you see | Likely cause |
|---|---|
| Fish eyes evenly distributed across the entire part | Oil or moisture in compressed air |
| Fish eyes in random clusters | Silicone contamination |
| Fish eyes where the part was handled | Inadequate degreasing, fingerprints |
| Fish eyes only on top surfaces | Contamination falling from hooks or booth ceiling |
| Fish eyes only on aluminum / castings | No outgassing |
| Fish eyes after opening new powder | Moisture-damaged powder |
| Fish eyes appearing seasonally (fall/winter) | Moisture in air system |
TEST — Verify your hypothesis on a test panel, not on a customer’s part. Blow the gun on a white card. Re-clean a test piece. Replace the filter and run another test.
IMPLEMENT — Only once the test confirms your diagnosis, act on production.
Back to the Friday wheels: fish eyes evenly distributed on every wheel — a systemic pattern, not localized contamination. Gun test on white card — greasy yellow spots. Filter was overdue. New filters, anti-silicone system flush. Wheels delivered Monday morning. Customer never knew.
Buy the Complete Powder Coating Manual – $27 →
What to Do With a Part That Already Has Fish Eyes
This is what most articles skip entirely.
Don’t put it back in the oven — re-curing will not fix fish eyes. The coating is already too hard to flow back into the depressions.
Repair procedure:
- Identify and eliminate the cause first — before touching the part
- Sand off the defective coating with P120–P180 grit (or down to bare metal for severe fish eyes)
- Re-degrease thoroughly
- Apply powder and cure per parameters
For isolated fish eyes — localized spot repair is possible. For fish eyes scattered across the entire part — full strip and re-coat is the only sensible solution.
Prevention Schedule
Daily:
- Drain condensate from compressor tank and separators
- Check coalescing filter clog indicator
- Nitrile gloves before handling cleaned parts
- No silicone products anywhere near the coating area
Weekly:
- Gun test on white card (oil and moisture check)
- Assess powder condition (flowability, color)
- Clean hooks
Monthly:
- Full air filtration system inspection
- Clean booth with anti-silicone cleaner
- Verify dryer is functioning (condensate being discharged)
Quarterly:
- Replace coalescing filter elements — regardless of appearance
- Compressor service inspection
With every new batch of parts:
- Ask the customer how parts were stored and treated
- CNC-machined parts — assume coolant presence, degrease aggressively
- Aluminum and castings — always outgas before coating
Fish Eyes vs. Orange Peel — How to Tell the Difference
New coaters often confuse these two defects. The difference is fundamental — different causes, different fixes.
| Feature | Fish Eye (Crater) | Orange Peel |
|---|---|---|
| Appearance | Round depression, exposed substrate | Uneven texture across entire surface |
| Distribution | Random or clustered | Uniform across the whole part |
| Edge | Raised, sharp | No distinct edge |
| Cause | Contamination (oil, silicone, moisture) | Application or cure parameters |
| Fix | Cleanliness — air, parts, booth | Application technique and oven settings |
What NOT to Do When Fish Eyes Appear
- Don’t put the part back in the oven — it won’t fix fish eyes
- Don’t change everything at once — you won’t know what worked
- Don’t ignore recurring fish eyes — it’s a systemic signal, not a fluke
- Don’t use WD-40 or silicone sprays anywhere near the coating area — ever
- Don’t skip filter replacements — elements have a service life independent of appearance
Summary
Powder coating fish eyes always have a specific cause. In 7 out of 10 cases it’s a compressed air problem — oil or moisture. The rest is surface contamination or silicone.
Three things that eliminate fish eyes permanently: a complete air filtration system with a dryer, a zero-silicone policy in the coating area, and systematic pattern-based diagnosis.
The complete troubleshooting chapter — diagnostic table for 95% of coating defects, full application parameters, and step-by-step repair procedures — is in my powder coating manual. 130 pages of knowledge from 15 years of daily shop work.
Buy the Complete Powder Coating Manual – $27 →Have a fish eye problem that isn’t covered here? Leave a comment — I’ll do my best to help.
