OEM

Automotive paint defects: types, causes and how they’re caught

Oct 09, 2026

9 min read

Automotive paint defects are flaws in a vehicle’s paint film that show up as uneven texture, trapped particles, holes or blisters, runs, or patches of the wrong colour. The ones paint shops deal with most are orange peel, dirt and dust inclusions, craters (also called fisheyes), runs and sags, solvent pop, pinholes, mottling and dry spray. Most trace back to one of three causes: contamination reaching the surface, the wrong amount of paint, or application and curing conditions outside their window. On a production line they are found by inspectors working under controlled light and, increasingly, by deflectometry and AI, which measure the surface instead of relying on a tired eye.

Defects that start on the production line

Orange peel

An uneven, bumpy texture that looks like the skin of an orange. PPG notes that some orange peel is common in most factory finishes, so the question is how much. Causes include spray settings that stop the paint flowing out (low air pressure, a wide fan pattern, too much distance from the panel), paint that is too thick, solvents that flash off too fast, and a booth that is too hot. Because orange peel is a matter of degree, OEMs measure it with instruments rather than judging it by eye (see below).

Dirt and dust inclusions

Solid particles trapped in the paint, usually millimetre-sized and visible without magnification. In OEM paint shops BASF lists the usual sources as the line itself: body sealers, metal flakes, weld balls, dried paint particles, sanding dust and filter bag fibres, plus lint and fibres from clothing. Its prevention list reads like a housekeeping checklist: lint-free coveralls, filter changes on schedule, and sanding dust cleared from body cavities before the next step.

Craters and fisheyes

Small circular depressions where the wet paint has pulled away from a spot of contamination, sometimes deep enough to show the layer below. BASF puts their diameter at about 0.5 to 3 mm and the cause at a low surface tension contaminant: silicones, greases and some stearates. The sources are surprisingly personal as well as industrial, from lubricants and mould-release agents to skin and hair care products on the people in the booth. The paint industry also calls them cissing.

Runs and sags

Wet paint that flows downwards before it sets, leaving drips or a curtain. The usual causes are too much paint in one coat, too little flash time between coats, the gun too close or moving too slowly, and cold surfaces or slow solvents.

Solvent pop

Blister-like bubbles or small open craters, caused when solvent or air is trapped under a surface that has skinned over and then escapes during the bake. BASF names high film build as the normal cause, made worse by short flash times, the oven’s heat-up profile, and booth temperature or humidity outside the design window.

Overspray and dry spray

Overspray on a production line is paint from one body landing on the next. BASF links it to car bodies spaced too close for the line speed. The result can look like dirt, metallic flakes in a solid colour, or a hazy, rough patch. Dry spray is a grainy, low-gloss finish where paint dried before it reached the surface, from paint that is too thick, air pressure too high, or the gun held too far away.

Mottling

Patches of lighter and darker colour in metallic paint, often in streaks that follow the spray direction. The cause is uneven application of the basecoat: a poor spray pattern, paint mixed wrongly, or a surface that is too hot or too cold.

Defects more common in refinish and repair

These turn up mainly in body shops, but inspectors at the end of the line and at the dealership meet them on repaired panels too.

Pinholes: tiny holes from the undercoat that reach the surface, often from filler or primer that wasn’t sanded or mixed properly.
Blistering: small bubbles of about 0.5 to 1.5 mm in the topcoat, from incompatible materials or water, oil or dirt in the air lines.
Lifting and wrinkling: the surface shrivels or swells because solvents in the new coat attack the layer beneath.
Sand scratch swelling: sanding marks that show through the topcoat, from paper that was too coarse or undercoats that hadn’t dried.
Water spotting: dull spots where water dried on paint that had not fully cured.

Why paint defects escape inspection

A painted body is a curved mirror. Many defects are only visible when light hits them at the right angle, and the human eye resolves only part of the range that matters: BYK, which makes appearance instruments, notes that structures of 10 to 30 mm are seen best from about 3 metres, while fine structures of 0.1 to 1 mm can only be recognised close up.

The inspector also works against the clock. A literature review by Sandia National Laboratories found that error rates of 20% to 30% are frequently quoted for visual inspection, that inspectors are more likely to miss a defect than to flag a good part, and that detection can fall by up to 40% within 30 minutes on the task. We look at those findings in more detail in automated visual inspection in manufacturing.

How plants detect paint defects

Light tunnels and trained inspectors

The traditional method is a highlight booth with banks of lights, where inspectors look for disturbances in the reflection, mark the defects and send the body to sanding and polishing. It works, but the result depends on the person, the shift and the line speed.

Instruments for orange peel

Appearance is measured with wave-scan instruments. BYK’s version shines a laser at 60° and records how the reflected light varies as the instrument rolls along the panel, splitting the waviness into long-wave (1.2 to 12 mm) and short-wave (0.3 to 1.2 mm) values. Lower values mean a smoother, more brilliant finish, and each OEM sets its own scales. These are spot measurements: they tell you how good the finish is on a sample, not where every defect on every body is.

Deflectometry

Deflectometry projects a known pattern, usually stripes, onto the paint and watches how its reflection is distorted. Fraunhofer IOSB describes it as the automated version of what a skilled inspector does: holding the surface in the right light and looking for disturbances in the mirror image. Because it measures how the surface bends light, it finds dents, craters and inclusions that are hard to see in a plain photograph, which is why it suits reflective parts such as car bodies.

AI classification

Detection is half the job. The other half is saying what each defect is and where it sits, because a crater calls for a different fix from a dust inclusion or a run. AI models trained on labelled defect images classify each finding, locate it on the body and pass the result to the line, so rework starts with the defect already documented.

CamCom’s Modulus system combines the two approaches in an inspection tunnel fitted at the end of the paint shop: up to 18 or more cameras with synchronised LED and UV lighting take around 8,000 images of each vehicle, deflectometry finds defects down to 50 microns, and each defect is classified by type and placed on a 3D map of the body, with a pass or fail decision sent to the plant’s manufacturing execution system. In CamCom’s phrase, the AI doesn’t see the defect directly; it sees the physics of how light bounces off it. See AI paint and body defect inspection for details.

From detection to root cause

The value of recording every defect, rather than just repairing it, is the pattern. A cluster of craters points to a contamination source such as silicone in the booth. A rise in dirt points to filters or cleaning. Runs on the same panel of every body point to film build or a robot path. Overspray points to the spacing between bodies at the current line speed. When defects are logged by type and position against each vehicle, quality engineers can trace a problem upstream before it turns into thousands of reworked bodies or warranty claims.

Common questions

What are the most common automotive paint defects?
Orange peel, dirt and dust inclusions, craters (fisheyes), runs and sags, solvent pop, pinholes, mottling, dry spray and overspray. In refinish work, blistering, lifting, sand scratch swelling and water spotting are also common.

What causes orange peel in car paint?
Paint that cannot flow out smoothly before it sets. Common causes are spray gun settings (low air pressure, wide fan pattern, too much distance), paint that is too thick, solvents that flash off too quickly and high booth temperatures. Some orange peel is normal in factory finishes, so OEMs measure it against their own limits.

What causes craters or fisheyes in automotive paint?
A contaminant with low surface tension, such as silicone, grease or some stearates, that makes the wet paint pull away from that spot. Sources range from booth lubricants and mould-release agents to skin and hair care products.

How are paint defects detected on a car production line?
Traditionally by inspectors in highlight booths under controlled lighting. Plants also measure orange peel with wave-scan instruments, and increasingly use deflectometry and AI, which scan every body, classify each defect and record where it is.

Sources: PPG common paint defects guide: orange peel, dirt in clear, fisheyes, runs and sags, solvent pop, pinholes, mottling, dry spray, blistering, lifting and wrinkling, sand scratch swelling, water spotting. BASF, automotive OEM coatings defects (craters, dirt, overspray, pop). BYK-Gardner, Appearance of a class A surface: orange peel and brilliance. Fraunhofer IOSB, Deflectometry for the inspection of specular surfaces. Micro-Epsilon, paint defect inspection. Judi E. See, Visual Inspection: A Review of the Literature, Sandia National Laboratories, SAND2012-8590, October 2012.