Coating and Paint Defects

Fisheye and Crater Paint Defects: How to Prevent Coating Defects Caused by Film Dewetting, Fix Fisheyes and Choose Additives to Prevent Coating Defects

Fisheyes form when a spot on the surface, or a droplet landing on the wet paint, has a lower surface tension than the paint around it. Measure substrate contact angle and paint surface tension before you spray, so you find the contaminant instead of masking it with an additive.

Who this is for: Paint shop and refinish quality teams, automotive and industrial coating lines, and coatings formulators.

Where it fits: After surface prep and before spraying, on each paint batch, and when fisheyes or craters appear.

What it does not do: It does not identify the contaminant chemically, and it does not diagnose solvent pop, pinholes or orange peel.

Method

Sessile drop contact angle and pendant drop surface tension

Standard

Fully compliant with ISO 19403-3; supports ASTM D7541

Setup

About 2 minutes

Book a 15 minute demo Request a quote

Grey painted panel covered in small circular fisheye craters

Written by

Gurdeep Singh Saini

Holds a BASc in Mechanical Engineering from Ryerson University (now Toronto Metropolitan University) and an MASc from York University. He develops the automatic edge and baseline detection behind the Dropometer's analysis.

COO at Droplet Lab

How little it takes to cause a fisheye

1 to 2 mN/m

difference in surface tension is enough for a droplet landing on wet paint to spread and start a crater.

Bubat and Scholz, Chimia 56(5):203 (2002)

0.1 mg/ft²

of silicone oil on a surface was enough to affect bond strength for two epoxy adhesives and a paint in NASA backed tests.

Caldwell, Hudson and Cash, Adhesion Society (2003), NASA NTRS 20030064092

Sources: A. Bubat and W. Scholz, Silicone additives for paints and coatings, Chimia 56(5):203 to 209, 2002; G. A. Caldwell, W. D. Hudson and S. F. Cash, Bond sensitivity to silicone contamination, 26th Annual Meeting of the Adhesion Society, 2003 (NASA NTRS 20030064092). Figures come from these sources, not from Droplet Lab measurements.

Quick reference

What this fisheye check does and what it does not

A quick reference for paint shop and coatings teams checking fit before reading further.

Evidence Box

Problem this solves

Fisheyes, craters and crawling caused by surface contamination or a paint whose surface tension is too high for the substrate.

Standards

Fully compliant with ISO 19403-3 for paint surface tension by pendant drop and ISO 19403-2 for surface free energy; supports ASTM D7541.

Peer reviewed method

The Dropometer drop shape method was validated against a KRUSS DSA100E in two peer reviewed papers, in Review of Scientific Instruments and Colloids and Surfaces A.

Customer evidence

Zeus Industries, a polymer manufacturer, completed its own gage R&R study on the Dropometer and reported that it performed very well.

Customer publication

Researchers used the Dropometer to measure how added surfactants change the surface tension of polymer solutions (Pal and Sun, ChemEngineering, 2025).

Honest limit

The check shows that a wetting problem exists and where. It does not name the contaminant; that needs chemical analysis.

Who this is for

What fisheye problem are you trying to solve?

Pick the card closest to your role. Each one points to the part of this page that answers it.

Paint shop or refinish manager

Fisheyes keep appearing and each one means sanding and a repaint. A contact angle check after prep shows whether the panel is clean and free of silicone, wax or grease before the painter sprays.

Find the source before adding a fisheye eliminator; the additive hides the cause.

Coating line process engineer

Craters come in waves on an automotive or industrial line. Trending substrate contact angle, paint surface tension and air line checks shows whether the cause is the part, the paint or the air supply.

Check the compressed air line as well as the parts; oil in the air reaches every panel.

Coatings formulator

You set the paint's surface tension and choose flow and levelling additives. Measuring surface tension against the substrates your customers use shows how much margin the paint has against contamination.

Set targets from your own substrates and panels; there is no universal value.

Fit check

Is this the right check for your fisheyes?

It helps when wetting is the cause. It helps less when the defect comes from trapped solvent, air or moisture.

Good fit if

Small circular craters show bare substrate or a lower coat in the centre.

Paint pulls back or starts to crawl away from an edge or corner.

Defects follow a substrate lot, a cleaner change or a new silicone product in the building.

You want to prove prep is working before the clear coat goes on.

You change paint batch, reducer or additive and want to know the effect.

Less relevant if

Defects are solvent pop or pinholes from trapped solvent or air.

The problem is orange peel texture rather than craters.

You need to identify the contaminant chemically.

The substrate is too rough or porous to hold a drop, such as raw fiberglass mat.

Summary

How to prevent fisheyes in paint

The answer in under a minute.

Fisheyes form where a contaminant gives the surface, or a droplet on the wet paint, a lower surface tension than the paint. To prevent them, measure water contact angle on the prepared substrate against a clean reference, and paint surface tension against a good batch. Spray only when both are inside your gates; otherwise clean again and find the source.

The Dropometer contact angle and surface tension kit runs both checks beside the booth; our contact angle measurement guide and surface tension measurement guide explain the methods. Critical surface tension estimates follow ASTM D7541. Zeus Industries ran its own gage R&R on the instrument; read the Zeus case study and our analysis of Pal and Sun (2025). Our paint guide covers the wider context.

Expert Quote

Expert perspective on coating defects

“

Craters, fisheyes and orange peel are wetting stories told after the fact; measured surface tension and contact angle let you read the ending before you commit the coat.

The problem

What causes fish eye craters in paint

Fisheyes are small circular craters or depressions in a wet paint film, often with a raised rim and bare substrate or a lower coat in the centre. They are a wetting defect. Wet paint flows away from any spot with a lower surface tension than itself, so a speck of silicone, wax, grease or lubricant on the substrate pushes the paint away. Droplets landing on the wet paint do the same. Bubat and Scholz report that a difference of 1 to 2 mN/m is enough. Sources are everywhere in a paint shop: silicone polish and silicone in the air from nearby products, oil or water in the air line from the compressor, detergent or cleaner left after prep, a dirty paint gun, adhesive residue from masking tape, fingerprints, and release agents on plastic or fiberglass parts. There are different causes, but the mechanism is the same, which is why surface tension and contact angle measurements find them. Paint makers advise that the first fix is to thoroughly clean the surface, not to change the paint.

Small circular craters appear minutes after spraying.

Paint crawls back from edges, corners or one panel area.

A blemish or crater-like openings show up in the clear coat over a good base coat.

Defects come and go with the shift, line or time of day.

Defects follow one batch of parts, cleaner or paint.

Troubleshooting

Different causes of fisheyes and how to correct them

Why:

These films lower the surface energy of the substrate below the paint's surface tension, so the paint dewets. Our dewetting glossary entry explains the mechanism.

How to detect:

Water contact angle on the prepared panel is well above the clean reference, at one area or across the part.

Corrective action:

Clean with a wax and grease remover, wipe off before it dries, and recheck. Our silicone contamination detection page covers finding the source.

Why:

A worn compressor, full moisture traps or a failed air dryer put oil and water into the air supply, which reaches the spray gun and every panel.

How to detect:

A clean glass slide blown with line air reads a higher contact angle than an unexposed slide.

Corrective action:

Replace the filter element, drain moisture traps, check the air dryer and replace contaminated hose sections.

Why:

Degreaser, detergent or a remover left to dry on the surface leaves a film that changes how paint wets.

How to detect:

Contact angle differs from the reference after cleaning, often in streaks. Our contact angle reference values for common surfaces give a sense check.

Corrective action:

Wipe cleaner off while wet, use clean cloths, and let the surface flash off fully before spraying.

Why:

Silicone in the air from nearby products, or overspray of a lower surface tension, lands on wet paint and spreads.

How to detect:

Craters appear on panels whose substrate contact angle passed.

Corrective action:

Check the spray booth, air filtration and any silicone product used nearby; separate the areas.

Why:

A changed batch, reducer or additive raises the paint's surface tension, so it has less margin against contamination.

How to detect:

Pendant drop surface tension of the paint as sprayed is above the reference batch.

Corrective action:

Check the batch and mix with the supplier; use an additive only on their advice.

Why:

Mould release on plastic and fiberglass parts has low surface energy and is hard to see.

How to detect:

High contact angle on new parts, which falls after sanding or a dedicated cleaner.

Corrective action:

Add a release agent removal step to prep and recheck each new parts lot.

Not sure where your fisheyes come from?

Bring a clean panel and a contaminated one to a call, and we will compare them on screen.

Book a 15 minute demo

Quality records

What a fisheye prevention record contains

Each check produces a record you can file with the job, part or paint batch. These records can feed into your existing quality records under IATF 16949, ISO 9001 or your shop's own process.

Part and prep

Substrate, finish, prep steps, cleaner and degreaser used, and flash off time.

Paint

Product, batch, reducer and ratio, and any additive.

Method settings

Drop volume, reading time, spots per panel, needle size and drop age.

Readings

Substrate contact angle median and spread, paint surface tension, and any air line check.

Decision

Spray, clean again or hold, with the operator and date.

Drop images

Automatic edge and baseline detection gives the same reading from the same image, so a reviewer can recheck any result.

Measurements

What to measure to prevent fisheyes

Substrate

Water contact angle on the prepared surface

Why it matters: Silicone, wax, grease and release agents raise it, which is how they cause dewetting.

How to interpret: Compare with a clean reference of the same substrate and prep. There is no universal pass value; set the gate from your own panels.

When it is not enough: It does not catch droplets that land on the wet paint.

Paint

Surface tension of the paint as sprayed

Why it matters: The paint must have a surface tension at or below the substrate's to wet it.

How to interpret: Compare with a defect free reference batch; differences of 1 to 2 mN/m matter.

When it is not enough: Surface tension changes as solvent evaporates; it is a snapshot.

Margin

Substrate surface free energy

Why it matters: Two probe liquids give the substrate's surface free energy, which you can compare directly with paint surface tension.

How to interpret: A wider gap between substrate energy and paint surface tension means more margin.

When it is not enough: It needs a second probe liquid and more time.

Air supply

Contact angle on a slide blown with line air

Why it matters: Oil in compressed air contaminates every panel.

How to interpret: Any rise against an unexposed slide means oil or water in the line.

When it is not enough: It does not show which part of the air system is at fault.

Validation

Validated measurement approach for coating defects

How the Dropometer itself has been validated, and where customers have used it.

Peer reviewed method

The Dropometer drop shape method is published in Review of Scientific Instruments and Colloids and Surfaces A. Contact angle was validated against a KRUSS DSA100E, with a published accuracy of 0.35°.

See the validation papers

Customer evidence

Zeus Industries, a polymer manufacturer, completed its own gage R&R study on the Dropometer and reported that it performed very well. Pal and Sun used the same pendant drop method to measure how surfactants change surface tension (ChemEngineering, 2025).

Read the case study

QC Protocol

How to prevent fisheyes before you spray

Six steps for one substrate, prep and paint. Repeat for each new substrate, cleaner or paint system.

1

Lock the method

Fix drop volume, reading time, spots per panel, needle size and drop age. Our guide to reproducible contact angle measurement explains each setting.

2

Baseline a clean panel and a good batch

Measure a clean reference panel after normal prep, and the surface tension of a defect free paint batch.

3

Set gates from test panels

Lightly contaminate panels with silicone, wax or fingerprints, spray them, and set the gate below the point where fisheyes start.

4

Check each job after prep

Measure the prepared surface at the set spots, including edges and corners, and the paint as sprayed.

5

Spray, clean again or hold

Spray inside the gates. Clean again outside them, and hold parts that fail twice.

6

Check the air and trend

Run the air line slide check weekly to help prevent repeat defects, and trend readings by line, shift and paint batch.

We completed our gage R&R study on the unit and it performed very well.

Brandon Barbee

Corporate Quality Engineer, Zeus Industries, Polymer Manufacturing

Download the fisheye and crater prevention SOP

A two page SOP you can adapt for your substrates and paints, with the method lock, gate setting steps, an air line check, a troubleshooting table and a record sheet.

Contact angle measurement with the Dropometer

Example output

Example fisheye prevention log

Example data only. Your values and gates will differ and must come from your own references.

How to read this log

Example gates: substrate contact angle within 8° of the clean reference, and paint surface tension within 1 mN/m of the reference batch. Panel 2 was handled near a silicone polish, so it was cleaned again and passed. Batch B was held because its surface tension rose after a reducer change.

Sessile drop contact angle measurement: DI Water on Teflon surface, left contact angle 44.9°, right 45.7°

Example fisheye prevention log

Item	Prep or change	Substrate contact angle (°)	Paint surface tension (mN/m)	Versus reference	Result
Clean reference panel	Sanded, degreased	58.0	26.5	Reference	Reference
Panel 1	Sanded, degreased	60.5	26.5	Inside	Spray
Panel 2	Handled near silicone polish	86.0	26.5	+28° substrate	Clean again
Panel 2	Degreased again	61.0	26.5	Inside	Spray
Batch B	Reducer changed	59.5	29.8	+3.3 mN/m paint	Hold

Example data, not customer data. Water contact angle on sanded and degreased primer, read at a fixed time; paint surface tension by pendant drop as sprayed, at 23 °C.

Signal order

The order to check signals when fisheyes appear

Check these in order. Each one rules out a source before you look at the next.

1

Defect pattern

Where the craters are: one part, one area, or every panel.

2

Substrate contact angle

Shows whether the prepared surface is contaminated.

3

Air line slide check

Shows whether oil or water is coming through the air supply.

4

Paint surface tension

Shows whether the paint batch, reducer or additive changed.

5

Booth and nearby products

Silicone in the air, overspray and new products used near the spray booth.

FAQ

Common questions about fisheyes and craters

Questions painters, paint shop managers and coatings teams ask about fisheye defects.

What causes fisheyes in paint?

Fisheyes are small circular craters caused by surface contamination with a lower surface tension than the paint: silicone, wax, grease, lubricant, oil from the air line or cleaner residue. The paint flows away from the spot. Contaminated droplets landing on wet paint cause the same defect.

How do you fix fisheyes once they appear?

Let the paint cure, sand out the affected area, thoroughly clean the surface with a wax and grease remover, and repaint. Then find the source, or the next refinish job will fisheye too. A contact angle check after cleaning confirms the surface is clean before you repaint.

Should I use a fisheye eliminator?

Adding a fisheye eliminator lowers the paint's surface tension so it can wet a contaminated spot. It is a short term solution that treats the symptom, not the cause. PPG's refinish guidance limits anticissing additives to extreme circumstances and advises checking with the paint manufacturer first. Clean the surface and find the source to avoid repeat defects.

What is the difference between a fisheye, a crater, a dimple and solvent pop?

Fisheyes and craters are wetting defects: the paint pulls away from a low surface tension spot. A dimple is a shallow depression, often with the same cause. Solvent pop is a small blister or hole from solvent trapped under a skinned surface, so contact angle will not help there.

How do contact angle and surface tension measurements prevent fisheyes?

Paint wets a surface when its surface tension is at or below the surface's critical surface tension. Contact angle shows whether the prepared surface has dropped below that, and pendant drop surface tension shows where the paint sits. Our article on why contact angle matters for coatings and adhesives QC explains more.

Which standards apply?

ISO 19403-3 covers the surface tension of paints by pendant drop, ISO 19403-2 surface free energy from contact angles, and ISO 19403-6 dynamic contact angle. ASTM D7541 estimates critical surface tension; the Dropometer supports that practice.

What instrument do we need?

Contact angle and pendant drop surface tension in one kit that works beside the booth. Our guide to the best contact angle goniometer for industrial QA compares options. Adhesion problems after painting are covered on our peeling and adhesion failures page.

Business impact

What changes when you check wetting before you spray

Typical changes in how teams work. Benchmarks are given only where a published source exists.

Before and with a wetting check

Metric	Before Dropometer	With Dropometer	Indicative Benchmark
Finding fisheyes	After spraying, as defects	Before spraying, as a contact angle reading	No published benchmark; track on your own jobs
Source of contamination	Guesswork and trial	Substrate, air line and paint checked separately	No published benchmark; track on your own line
Paint batch changes	Found when craters appear	Surface tension checked against a reference	1 to 2 mN/m difference can start a crater (Bubat and Scholz, 2002)
Additive use	Fisheye eliminator added by default	Used only on supplier advice	Anticissing additives limited to extreme circumstances (PPG refinish guidance)

See the check on your own panels

Bring a clean panel and one that fisheyed. We will show the difference on a call.

Book a 15 minute demo

Estimate the cost of fisheye rework

Enter your own numbers. The result is an estimate, not a Droplet Lab claim.

Instrument cost ($) Each Dropometer unit is $5,000; the default models 1 unit.

Rework events / month Repaint events per month that your own records trace to fisheyes, craters or dewetting.

Avg rework cost per event ($) Your own average cost to sand, clean and repaint one part or panel.

Failures preventable (%) Use your own failure analysis; start low if unsure.

Scrap cost per event ($) Scrap cost per event from fisheyes or craters only, from your own records.

Other monthly savings ($) Other monthly savings you can show from your own records. Set to 0 if unsure.

Result

~0

Monthly savings

~0

Payback period

~0

First year net benefit

Monthly savings = preventable rework cost + preventable scrap cost + other monthly savings.

Limits

What this check cannot tell you about paint defects

Honest limits, so you know when to reach for another test.

It does not identify the contaminant chemically.

It cannot catch droplets that land on the wet paint after the check.

Very rough or porous surfaces give readings that are hard to compare.

Paint surface tension changes as solvent evaporates; measure as sprayed, at a fixed drop age.

Gates do not transfer between substrates, prep methods or paint systems.

It does not diagnose solvent pop, pinholes or orange peel.

Contact angle and surface tension show wetting problems and where they come from. Naming the contaminant, and defects such as solvent pop, pinholes and orange peel, need other methods.

Book a 15 minute demo Request a quote

How this page was created

Editorial and technical transparency notes for this page.

Transparency Details 4 checklist items

01

Drafting assistance

Drafted with Claude Opus 5.5 (Anthropic) using web search for sources, then edited by the Droplet Lab team.

02

Technical review

Reviewed and edited for technical accuracy by the Droplet Lab Team.

03

Verification steps

Standard identifiers, units, thresholds and key procedural claims are checked against cited sources before publication.

04

Updates

Reviewed every 12 months or when the underlying method changes.

Report a correction

Spotted an issue in this summary? Send a correction request and our team will review it.

Correction Request

We work hard to keep this page accurate and up to date. If you spot an error (wrong revision/year, missing requirement, incorrect interpretation, or broken link), tell us and we'll review it.

Contact us to report a correction

Sources

References

1.

Bubat, A., Scholz, W. Silicone additives for paints and coatings. Chimia 56(5):203 to 209, 2002. https://chimia.ch/chimia/article/download/2002_203/2872/13557

2.

Caldwell, G. A., Hudson, W. D., Cash, S. F. Bond sensitivity to silicone contamination. 26th Annual Meeting of the Adhesion Society, 2003. NASA NTRS 20030064092. https://ntrs.nasa.gov/citations/20030064092

3.

Zisman, W. A. Relation of the equilibrium contact angle to liquid and solid constitution. Advances in Chemistry 43, pp. 1 to 51, American Chemical Society, 1964. https://doi.org/10.1021/ba-1964-0043.ch001

4.

PPG Refinish. Fisheyes: causes, prevention and repair. https://www.ppg.com/en-AU/refinish-anz/fisheyes

5.

Pal, R., Sun, C. C. Influence of added surfactants on the rheology and surface activity of polymer solutions. ChemEngineering 9:105, 2025. Customer publication that used the Dropometer; the link goes to our analysis of the paper. https://dropletlab.com/validation/citations/analysis/influence-of-added-surfactants-on-the-rheology-and-surface-activity-of-polymer-solutions/

6.

ISO 19403-3. Paints and varnishes. Wettability. Part 3: Determination of the surface tension of liquids using the pendant drop method. https://dropletlab.com/industry-standards/iso-19403-3/

7.

ASTM D7541. Standard practice for estimating critical surface tensions.

https://dropletlab.com/industry-standards/astm-d7541/

8.

Chen, X. et al. Contact angle measurement with a smartphone. Review of Scientific Instruments 89(3):035117, 2018.

https://pubs.aip.org/aip/rsi/article-abstract/89/3/035117/368179/Contact-angle-measurement-with-a-smartphone

9.

Surface tension measurement with a smartphone using a pendant drop. Colloids and Surfaces A: Physicochemical and Engineering Aspects.

https://www.sciencedirect.com/science/article/abs/pii/S0927775717307744

10.

Droplet Lab validation papers

11.

Customer publications that cite the Dropometer

12.

ISO 19403-3 explained

13.

Zeus case study

14.

Paint guide

Fisheye and Crater Paint Defects: How to Prevent Coating Defects Caused by Film Dewetting, Fix Fisheyes and Choose Additives to Prevent Coating Defects

Featured Studies & Publications

How little it takes to cause a fisheye

1 to 2 mN/m

0.1 mg/ft²

What this fisheye check does and what it does not

Evidence Box

What fisheye problem are you trying to solve?

Paint shop or refinish manager

Coating line process engineer

Coatings formulator

Is this the right check for your fisheyes?

Good fit if

Less relevant if

How to prevent fisheyes in paint

Expert perspective on coating defects

What causes fish eye craters in paint

Different causes of fisheyes and how to correct them

Not sure where your fisheyes come from?

What a fisheye prevention record contains

Part and prep

Paint

Method settings

Readings

Decision

Drop images

What to measure to prevent fisheyes

Water contact angle on the prepared surface

Surface tension of the paint as sprayed

Substrate surface free energy

Contact angle on a slide blown with line air

Validated measurement approach for coating defects

How to prevent fisheyes before you spray

Lock the method

Baseline a clean panel and a good batch

Set gates from test panels

Check each job after prep

Spray, clean again or hold

Check the air and trend

Download the fisheye and crater prevention SOP

Example fisheye prevention log

Example fisheye prevention log

The order to check signals when fisheyes appear

Defect pattern

Substrate contact angle

Air line slide check

Paint surface tension

Booth and nearby products

Common questions about fisheyes and craters

What changes when you check wetting before you spray

Before and with a wetting check

See the check on your own panels

Estimate the cost of fisheye rework

Estimate avoided repaint and refinish cost from fisheye and crater defects.

What this check cannot tell you about paint defects

Standards, evidence and related coating defect workflows

ISO 19403-3

ASTM D7541

Zeus polymer extrusion

Pal and Sun (2025): our analysis

Dropometer

Coating and paint defects

Silicone contamination detection

Fix orange peel

How this page was created

Drafting assistance

Technical review

Verification steps

Updates

Report a correction

References