Coatings Defects: Cratering, Fisheyes, Pinholes

How to use this guide

Craters, fisheyes, and pinholes are among the most expensive coating defects because they create a high rework rate and are often intermittent (hard to reproduce). This guide is built as a shop-floor diagnostic: identify the defect type, isolate the most likely cause category (contamination vs process vs outgassing), and apply corrective actions that reduce recurrence—not just “mask” the issue.

Use it to align production, quality, EHS, and procurement on what to check, what to measure, and what documentation to request (SDS/COA/technical data) before changing solvents, cleaners, or additives.

Where it fits

• Coating types: solventborne, waterborne, high-solids, UV/EB, powder + liquid topcoats (where applicable).
• Substrates: steel, aluminum, galvanized, castings, plastics/composites (substrate porosity matters).
• Process steps: pretreatment/cleaning → drying → application → flash → cure (oven) → handling/packaging.
• High-risk interfaces: compressed air, silicone-containing maintenance products, release agents, lubricants, conveyor oils, packaging dust, and recycled solvent streams.

Quick diagnosis: tell them apart in 60 seconds

The mechanics (what’s happening in the wet film)

All three defects are controlled by a small set of variables: surface energy, wetting, evaporation rate, viscosity/leveling, and gas generation/escape. When any variable moves outside your stable window, defects appear—often only at certain humidity, temperature, or contaminant load.

Key decision factors

• Film formation & cure schedule: flash time, solvent blend, oven ramp rate, peak metal temperature, and cure time.
• Substrate preparation: cleaner type, rinse quality, drying effectiveness, pretreatment chemistry, and handling practices.
• Contamination sources: silicone, oil mist, release agents, lubricants, compressor oil/water, packaging dust, and recycled solvents.
• Application controls: viscosity, spray parameters, atomization air quality, gun condition, and filtration.
• Chemical exposure class: acids/alkalis/solvents in service can influence additive selection and cure robustness.

Root-cause map (most common in industrial lines)

1) Contamination-driven fisheyes / craters

The number-one driver is contamination with materials that dramatically lower surface energy (especially silicones, some lubricants, waxes, and certain surfactant residues). A trace amount can cause visible defects.

• Silicone sources: mold release agents, polishes, “anti-stick” sprays, sealants, hand creams, some defoamers, and gasket greases.
• Oil sources: compressor oil carryover, mist lubricators, hydraulic leaks, conveyor oils, and cutting oils on incoming parts.
• Cleaner residue: poor rinse, overconcentrated detergent, incompatible additives, hard-water salts.
• Airborne: shop aerosols, nearby maintenance work, forklift exhaust particulates, packaging fibers.

2) Process-driven cratering (surface-tension gradients, foam collapse)

Cratering can occur even without classic silicone contamination if the formulation and process create surface-tension gradients (Marangoni flow) or if foam collapses and leaves voids.

• Over/under defoamer: too much can cause craters; too little causes foam entrainment and later collapse.
• Flow/leveling mismatch: wrong solvent blend, viscosity out of spec, or too fast “skin” formation trapping defects.
• High shear and air entrainment: aggressive mixing, improper pump selection, or cavitation pulling air into the paint.
• Dirty filtration or bypass: gels/particles seed defects and disrupt leveling.

3) Pinholes (outgassing / solvent popping / moisture)

Pinholes are fundamentally gas escape problems. The gas can be air trapped in porosity, solvent vapor, water vapor, or reaction gas (some systems).

• Porous substrates: castings, sintered parts, welds, galvanized with trapped volatiles.
• Film build too high: thick coats skin over, trapping solvent; pinholes appear during bake.
• Flash time too short: insufficient solvent release before oven; ramp rate too aggressive.
• Substrate temperature: cold parts condense moisture; hot parts flash too fast → skinning and popping.
• Humidity and dew point: moisture causes microbubbles, poor wetting, and pinhole formation (especially waterborne).

Corrective actions by defect type

Fisheyes

• Eliminate the source: remove silicone/wax products, isolate maintenance activities, control lubricants.
• Upgrade cleaning & rinsing: validate cleaner concentration, rinse quality, and drying (water breaks indicate oils).
• Air quality: coalescing filters, correct dryer, routine drain checks; verify at spray booth drop.
• Handling discipline: gloves, no hand creams, controlled staging area, clean racks and hooks.
• Formulation options (last resort): anti-crater/flow additives can widen the window but may introduce long-term contamination risks if silicone-based—use carefully and document.

Cratering

• Control viscosity and solvent blend: verify viscosity at application temperature; avoid “too fast” reducers that skin early.
• Optimize defoamer and mixing: ensure correct addition point; avoid over-shearing; confirm defoamer compatibility with your binder.
• Improve filtration and cleanliness: remove gels/skins; keep containers covered; use appropriate mesh strainers.
• Adjust application settings: atomization pressure, gun distance, overlap, and wet-film thickness; avoid dry spray and overspray contamination.

Pinholes

• Increase flash time / reduce ramp rate: let solvents escape before bake; soften the oven ramp to avoid “solvent pop.”
• Reduce film build: apply multiple thinner coats rather than one heavy coat if possible.
• Pre-bake / de-gas porous parts: preheat castings to drive off volatiles; keep parts clean after pre-bake.
• Control humidity and part temperature: avoid coating below dew point; keep parts at stable temperature entering the booth.
• Check cleaner residues: trapped water or surfactants can vaporize during bake, creating pinholes.

Process control checks (what to measure and log)

If defects are intermittent, measurement discipline is usually the fastest way to stop them. Track a small set of high-signal variables per shift (or per batch).

Specification & acceptance checks (procurement-ready)

When you change cleaners, solvents, or additives to address defects, the risk is replacing one problem with another. Ask for data you can verify on receipt and that supports stable line control.

• Identity: product name, grade, manufacturer, batch/lot traceability.
• Cleaner chemistry (if relevant): surfactant type, builder/alkalinity, recommended concentration window, rinseability notes, foam tendency.
• Solvents/reducers: composition class, water content limit (where applicable), evaporation profile guidance, compatibility statement for your resin system.
• Additives (flow/defoamer/anti-crater): chemistry family (silicone vs non-silicone), recommended dose range, compatibility, and known side effects (recoat, adhesion, printability).
• Quality (typical COA items): appearance, density, viscosity (if applicable), nonvolatile content (for additives), water content where relevant, and impurity limits as applicable.
• Packaging: liner type, closures, labeling, contamination control (tamper seals), and storage instructions.
• Safety: current SDS, VOC/flammability classification, PPE and ventilation requirements.
• Logistics: lead time, Incoterms, shelf life, and temperature constraints (freeze/heat sensitivity).

Troubleshooting signals (what to check first)

• Defect appears suddenly across all parts: air/oil/water issue, new maintenance product, batch change, booth contamination.
• Defect localized to edges/handling points: glove/handling oils, rack contamination, poor rinse, residual cleaner.
• Defect only after oven: outgassing, solvent pop, film build too high, ramp rate too aggressive.
• Defect worse on porous substrates: pre-bake needed, moisture in parts, trapped volatiles.
• Defect linked to humidity/temperature swings: dew point control, part temperature stabilization, flash time adjustments.

If you share your coating type (waterborne/solventborne/powder topcoat), substrate, pretreatment steps, and a few process notes (viscosity, flash, cure profile, air system setup), we can usually narrow down the most likely root cause quickly.

RFQ notes (what to include)

• Coating system: resin type, solventborne/waterborne, single/two-component, pot life, target appearance (gloss/matte), color sensitivity.
• Application: spray type (air/airless/air-assisted/electrostatic), target wet/dry film build, viscosity spec, reducer ratio, filtration mesh.
• Substrate & pretreatment: material, porosity (castings), cleaning chemistry, rinse stages, drying method, handling/racking.
• Cure schedule: flash time, oven type, ramp rate, peak metal temperature, cure time.
• Environment: booth type, airflow, dust control, humidity/dew point control, nearby contamination sources.
• Utilities: compressed air dryer and filter train, oil carryover control, maintenance frequency.
• Defect details: frequency, when it appears (wet vs after bake), affected areas, and any recent changes (batch, maintenance, cleaners, operators).
• Volumes: monthly consumption and packaging preference; delivery location; documentation requirements (COA/SDS).

Educational content only. Always follow site EHS rules and the supplier SDS/technical data sheet for safe use. Validate any formulation or process change with controlled trials before full-scale production.