Fisheyes & Cratering
Why wet coatings pull back into fisheyes and craters, how to trace silicone, oil and other contamination to its source, and how to repair and prevent it.
Key takeaways
- Fisheyes are round spots where wet coating pulls away from a contaminated area, leaving a thin or bare center ringed by a raised edge; craters are bowl-shaped depressions with a similar origin.
- Both are caused by low-surface-energy contamination — silicone, oil, grease, wax or release agents — on the surface, in the air supply or in the coating.
- The pattern tells you the source: one area, every surface, one batch, or one spray line.
- Repair means removing the contamination, not just covering it; recoating over an uncleaned surface usually repeats the defect.
Fisheyes and craters are surface-wetting failures. A liquid coating spreads over a surface only when the surface energy of the substrate is higher than the surface tension of the liquid. A tiny spot of silicone or oil has very low surface energy, so the wet coating retracts from it, forming a circular void. The defect appears within seconds to minutes of application, which makes it one of the few failures that applicators see as it happens.
What fisheyes and craters look like
- Fisheyes — circular openings, typically a few millimeters to a centimeter across, where the film has receded to leave the substrate or previous coat exposed or barely covered. The rim is often slightly thicker.
- Craters — shallow, bowl-shaped depressions, sometimes with a small particle or droplet at the center, where the film is thin but not fully open.
- Crawling — a severe form where the film draws back into islands or beads across large areas, typical of a heavily contaminated or very smooth, glossy surface.
These defects are different from pinholes and outgassing, which are tiny holes from escaping air or gas, and from orange peel, which is a continuous textured film. A crater with a hard particle at its center may simply be dirt, but most craters are surface-tension driven.
Causes
Contamination on the surface
- Silicone from lubricants, sprays, polishes, release agents, sealants or hand creams.
- Oil and grease from machining, handling, equipment leaks or fingerprints.
- Waxes and mold release agents on fiberglass and plastic parts.
- Residues from incompatible cleaners, or contaminated rags and solvent.
- Amine blush or other films on a previous coat. See amine blush.
Contamination from equipment or air
- Oil or water in compressed air from compressors without adequate separators.
- Lubricants on spray equipment, hoses or packing that contact the coating.
- Airborne aerosols — silicone sprays, overspray from nearby operations, or fumes from adjacent processes.
Contamination in the coating
- Incompatible additives, the wrong thinner or residue in mixing containers.
- Gel particles or incompatible material from incompletely cleaned equipment.
Diagnosis
The distribution of the defect is the strongest clue. Map where it occurs before cleaning anything.
| Cause | Clues | Check | Prevention |
|---|---|---|---|
| Surface contamination | Defects grouped in specific areas, handprints, drip lines | Water-break test; wipe with clean white cloth | Solvent or detergent clean; control handling |
| Silicone | Persistent fisheyes that recur after recoating | Site survey for silicone products; lab analysis | Ban silicone products near work |
| Compressed air oil | Defects across all work from one compressor | Blotter test per ASTM D4285 | Separators, filters, regular draining |
| Equipment residue | Defects begin after equipment change | Inspect hoses, lines, cleaning solvent | Dedicated, cleaned equipment |
| Coating contamination | Defects from one batch or mix | Spray batch on clean test panel | Clean mix containers; correct thinner |
| Amine blush / intercoat film | Fisheyes on second coat only, greasy feel | Water wipe test of previous coat | Wash blush before recoat |
Useful field tests
- Water-break test. Spray or flood the surface with clean water. A continuous sheet that holds for several seconds indicates a clean surface; beading or breaks indicate hydrophobic contamination. ASTM F22 describes the method.
- Blotter test. Discharge compressed air onto a clean white blotter for a set time and check for oil or water marks. See compressed air cleanliness.
- Test panel. Apply the same batch to a known-clean panel to separate surface contamination from coating contamination.
For recurring or high-value cases, laboratories can identify residues on swabs or on the defect itself using infrared spectroscopy or surface analysis.
Silicone is notoriously hard to remove and spreads easily on rags, gloves and tools. One aerosol can of silicone lubricant used near a paint shop can cause fisheyes for days.
Repair
- Stop and investigate. Do not keep spraying if fisheyes appear. Identify the source first.
- Allow the coating to cure enough to sand, or remove wet coating with the manufacturer’s recommended solvent if the area is small and practical.
- Remove the defect. Sand affected areas to sound, uniform coating, or blast if contamination is extensive or reaches the substrate.
- Decontaminate. Clean with an appropriate detergent or solvent per solvent cleaning (SSPC-SP 1), using clean rags and the two-rag method; change rags frequently.
- Verify. Repeat the water-break test before recoating.
- Recoat within the recoat window, applying a test area first.
“Fisheye eliminator” additives are used in some refinish work. They reduce surface tension so the coating flows over contamination, but they can introduce silicone to the shop and mask an unresolved problem. They are not appropriate for protective or immersion coatings unless the manufacturer approves.
Prevention
- Prohibit silicone-containing products in and around coating areas.
- Degrease before abrasive blasting, since blasting can spread oil rather than remove it.
- Maintain air compressors with separators and filters, and test air cleanliness at the start of each shift.
- Handle prepared surfaces with clean gloves and protect them from drips and aerosols.
- Use dedicated, clean mixing equipment and the specified thinner.
Frequently asked questions
Why do fisheyes appear in the second coat but not the first?
Contamination landed on or formed on the first coat, such as amine blush, overspray, handling residue or airborne silicone, before the second coat was applied.
Can I just apply a heavier coat to cover fisheyes?
Usually not. The coating continues to pull away from the contaminant and adhesion over it will be poor.
Do fisheyes affect corrosion protection?
Yes, when they leave thin or bare spots. In immersion service they can act as holidays and should be repaired.
Educational reference. Coating performance varies by formulation. Always follow the manufacturer’s product data sheet, safety data sheet and your project specification.
