Dirt and nib (inclusion) in automotive coatings is a defect in which a still-open wet film captures and buries a foreign solid - airborne dust, fibre, dried overspray particles, sanding swarf, water scale or leftover masking adhesive - leaving a raised point or embedded speck in or between the cured coats. Its essence is 'an external particle caught by a wet film', and it is not the same failure as a crater or fisheye (where silicone or oil locally lowers surface tension), a pinhole (where a bubble breaks the surface), or orange peel and sag (flow and levelling faults). A wet film is a natural dust catcher, so dirt control spans four fronts: booth cleanliness, part cleaning before loading, wet working on the line, and the polishing budget.
TL;DR — Accept one physical fact first: a foreign particle around 5 micron and above is enough to leave a detectable, rework-required bump in a wet film, which is exactly what booth filtration tries to suppress. The control has four levers. One, booth air: keep the downdraft face velocity near 0.2-0.3 m/s, run two- or three-stage supply filtration (pre, medium, high-efficiency) at a slight positive pressure, and manage air cleanliness on the ISO 14644-1 framework. Two, before loading: neutralise surface static with ionised air and remove loose dust with a tack roller or tack cloth. Three, wet working: prefer wet sanding between coats, or clear the swarf and dry before recoating. Four, find and fix: log defect density in specks per square metre and each particle's size, colour and location to trace the source, then after curing use graded sanding (about P1500-P2000) and polishing - but rework is limited by the 2K clearcoat dry-film budget of 35-55 micron. Microscopic identification of pinholes and craters is on our site, as are wet-sanding and polishing parameters.
Key figures and diagnosis anchors
- Defect-sensitive size: booth air handling targets particles 5 micron and above, because a speck of that scale in a wet film is enough to form a bump you can see at a raking light and must level out. Bigger particles, and ones caught in the final clearcoat, raise the rework probability.
- Laminar supply velocity: automotive downdraft booths are typically held near 0.2-0.3 m/s face velocity - fast enough to carry overspray and airborne dust down and away, slow enough not to disturb the wet film or trap solvent. The exact figure follows the line design and the customer's process specification.
- Filtration stages: the supply path usually runs two- or three-stage filtration (coarse pre-filter, medium filter, high-efficiency terminal filter), with ceiling media, floor grating and the overspray arrester acting as first-pass capture. Filter media must be changed on pressure-drop and schedule, or it becomes a secondary dust source.
- Clearcoat film budget: a 2K clearcoat is commonly applied to a dry film of about 35-55 micron. Each polish pass removes a few microns, so the number of reworks is finite; below the customer's minimum remaining thickness the part is re-sprayed, not polished again.
- Cleanliness framework: booth air cleanliness is managed against ISO 14644-1 particle-concentration classes, set by product, colour and customer; dark and high-gloss finishes are more dirt-sensitive and need higher cleanliness and tighter air-change control.
- Measurement and traceability: quantify with defect density (specks per square metre) and first-time-through yield, and record each particle's size, colour, hardness and location - which panel, which gun, which station - to reverse-engineer the source rather than 'just clean more'.
Where the dirt comes from: sources, diagnosis and differentiation
Environment: supply air, ceiling media, floor and positive pressure
Booth air is a continuous, often the largest, external source. Damaged or overdue ceiling HEPA media, or a leaking seal, re-blow captured fines back into the zone; floor grating, water-wash or dry overspray arresters that have dried to powder; peeled old paint from walls and deflectors lifted by the airflow; and lost positive pressure or open doors letting dirty shop air in - all rain onto the wet film. Signature: dirt is scattered, random, across parts, unrelated to any one part, pointing at air or the booth.
Parts and incoming material: loose dust, swarf, mould-release dust
Dust, paper and plastic trim, machining or press swarf, and talc or silica from mould release ride along on pressed and moulded parts. Body pretreatment such as phosphating or zirconium conversion mainly removes oil and builds a conversion coat on metal, so loose dust and electrostatically held fines still need a de-staticing and tack-off before loading. Plastic parts such as bumpers attract dust by static, so skipping pre-clean makes dirt almost unavoidable.
People and consumables: garments, fibre and adhesive residue
Operators are the main internal generator. Ordinary workwear sheds fibre, gloves shed powder, wipes shed lint, masks and shoe soles carry grit, and masking film or tape leaves adhesive and shreds as it is pulled. Fibrous dirt reads as elongated or curled specks, often tinted with the basecoat, and points at personnel or masking consumables.
Equipment and process: dry overspray and line scale
Atomised overspray whose solvent flashes off becomes dry overspray - a typical internal source in fast robot or electrostatic-rotary-bell work; when it falls back on the wet film it becomes a same-colour particle. That is why higher transfer efficiency and finer atomisation must be paired with overspray suppression and correct air velocity. Scale flaking from the paint-supply and circulation loop, or a poorly cleaned gun or colour-change valve, gives hard same-colour specks.
Sanding swarf: cross-contamination between coats
Swarf from wet- or dry-sanding an intermediate coat is buried by the next wet coat if it is not fully cleared, giving clusters of same-colour particles along the sanding path - the same lesson as intercoat dust control in wood finishing. Prefer wet sanding, or de-static, blow and tack the panel and let it flash off before recoating.
Telling dirt from craters, pinholes and orange peel
Dirt stands up: a hard inclusion you feel at a raking light or under a fingertip, and light sanding exposes a core. Craters, fisheyes and pinholes are recessed - a crater is a volcano where silicone or oil has pulled the coating back and exposed the substrate; a pinhole is a bubble escape. Treat one like the other and you go the wrong way; see our microscopic pinhole-and-crater identification.
Finding it and quantifying it
Judge dirt by defect density in specks per square metre and by first-time-through yield, and build a particle 'fingerprint' from size, colour, hardness and location: same-colour hard specks point at the supply loop or a dirty gun; tinted curling fibre points at garments or masking; grey powder points at sanding or mould release. Read the ledger with the density and yield trend to spend the cleaning and filtration effort where the true source is.
The control loop: air, part prep, wet working and rework
Booth air and filtration
Hold the downdraft face velocity in the design band (about 0.2-0.3 m/s), keep the air-change rate and a slight positive pressure, monitor filter life by pressure drop and change ceiling, medium and high-efficiency elements on time, and clean the booth walls, floor grating and overspray arresters on schedule so the filters never turn into a dust source. Manage the door and load-change window with an air curtain or delayed extract to keep dusty shop air out.
Part cleaning and de-staticing before loading
Before a part enters the booth, run three steps - de-static, blow, tack. An ionised air bar or ion gun neutralises surface charge and lifts loose dust, a tack roller or low-lint, non-residue tack cloth removes the rest, and spraying follows immediately to avoid re-attraction. Plastic parts and dark, high-gloss finishes gain the most from this step.
Wet working and 5S on the line
Sand intermediate coats wet, or after dry sanding fully de-static, blow and tack and let the panel flash before recoating. After pulling masking film, dust off before entering the booth. Choose low-shed garments, lint-free gloves and non-linting wipes, keep tools and parts off the floor, and put 5S cleaning on a fixed frequency with named owners. On robot lines, run overspray suppression and an electrostatic capture wall to hold down the internal load.
Rework: graded sanding, polishing and the film-thickness budget
Dirt that is already cured and buried can be levelled with graded sanding (about P1500-P2000) and then polished out. But every pass spends the 2K clearcoat dry-film budget of 35-55 micron, so decide rework against the measured remaining thickness - below the customer limit, stop and re-spray locally or in full rather than polish through the clearcoat and hit the basecoat. See our wet-sanding and polishing guide for grit, machine settings and hologram avoidance.
Dirt versus other wet-film coating defects
| Defect | How it forms | Typical size / shape | Main sources | Correction and rework |
|---|---|---|---|---|
| Dirt / nib (inclusion) | A foreign solid is caught and buried by the wet film | 5 micron and above; a raised speck, visible at raking light, sandable off | Booth air, part dust, personnel fibre, dry overspray, sanding swarf | Cleanliness and filtration control, part de-static and tack, wet working; after cure, graded sand and polish |
| Crater / fisheye | Silicone or oil locally lowers surface tension so the coating pulls back | Recessed, substrate-exposed volcano with a raised rim | Mould release, wax, oil, oily compressed air, skin oil | Cut off the contamination and clean the interface; formed craters usually need local or full re-spray |
| Pinhole | Solvent, air or substrate gas escapes before the film closes | Fine round holes, early in film formation | Insufficient flash-off, wet film too thick, porous substrate | Longer flash-off, thinner coats, sealed substrate; polishing shallow but rarely fully removes them |
| Orange peel | Under-levelling leaves a continuous peel texture | Continuous ripples, not point defects | Wrong viscosity, air pressure, distance or temperature | Tune the process window; rework by wet-sanding and polishing |
| Sag | Excess wet thickness flows under gravity | Drips and curtains, usually at lower edges of vertical panels | Too much wet film, insufficient flash-off | Control per-pass thickness and flash-off; level and recoat once dry |
FAQ
How do I tell dirt from a crater or fisheye at a glance?
Feel it and look at an angle. Dirt stands up, is hard, and sands off with the coating beneath intact. Craters and fisheyes are sunken, often showing the substrate with a raised rim, and will not sand away - they need re-spray. Colour helps too: same-colour hard specks point at the supply line or a dirty gun, tinted fibre points at garments or masking, grey powder points at sanding or mould release. When unsure, cut a section and look under magnification - the method in our pinhole-and-crater identification.
Would a higher booth air velocity not just mean less dust?
Not necessarily. Air velocity carries overspray and airborne dust downward, but beyond the design band (about 0.2-0.3 m/s) it disturbs the wet film, skews the spray pattern, flashes surface solvent too fast and drives dry overspray back onto the part - so it can raise defects or cause orange peel. The right move is to hold the design velocity, keep the filters effective and control positive pressure and the door window, not simply to blow harder.
What happens if I recoat without clearing sanding dust?
The next wet coat buries the swarf, giving clusters of same-colour particles spread along the sanding path - stubborn, because they sit at the layer interface. Sand wet where possible; if dry-sanding, then de-static, blow and tack thoroughly, flash off, and recoat. Writing 'clear dust after sanding' into the work standard, and tracking it with first-time-through yield, is far cheaper than polishing later.
Why do automated and robot lines care more about cleanliness and dry overspray?
Robots and electrostatic bells atomise finely and run fast with high transfer efficiency, producing a lot of overspray that dries into fine dry overspray and falls back as same-colour particles. Automation lowers the human dust load but raises the dry-overspray internal source, so it leans harder on overspray suppression (water-wash walls, dry capture, electrostatic capture walls), correct velocity and ceiling/floor maintenance. The glossier and darker the clearcoat, the more this matters.
Can polishing remove all the dirt, and is there a rework limit?
No, and yes. Cured, buried dirt can be levelled with graded sanding and polished out, but each pass spends the 2K clearcoat dry-film budget of 35-55 micron. Once the remaining film falls below the customer's floor you cannot polish again without breaking through the clearcoat into the basecoat, so you must re-spray locally or in full. Treat rework as a film-thickness account, not a free second pass. See our wet-sanding and polishing guide for settings.
How do I trace the source from the particle instead of just cleaning?
Keep a particle fingerprint: size distribution, colour, hardness, and where it appears (which panel face, whether it clusters at a gun, a station or a shift). Same-colour hard specks point at the supply loop or colour-change cleaning; curled tinted fibre at garments or masking; grey powder at sanding or mould release; random fine specks across parts at booth air or filtration. Read that ledger against defect density and yield to aim your cleaning and filtration at the real source.
Last updated: 2026-10-07 | Basis: '5 micron and above as the booth capture target, downdraft face velocity about 0.2-0.3 m/s, two- to three-stage supply filtration' from public automotive paint-shop air-purification engineering material (e.g. MayAir, Automotive Paint Shop Air Purification Solution); air-cleanliness classes framed by ISO 14644-1; 2K clearcoat dry film 35-55 micron, defect density in specks per square metre, first-time-through yield, and wet/dry sanding and polishing rework as general automotive-finishing engineering practice. Actual velocity, filter class, cleanliness level, film thickness and rework limits follow the line design and the customer's current specification, and shift with colour, gloss, season, humidity and equipment state; confirm on the first article and by shop-floor measurement. Related: pinhole and crater microscopic identification, automotive wet-sanding and polishing, body pretreatment, plastic bumper primer. Author: Kexin New Materials (Guangdong) Co., Ltd. technical team. Trade terms: EXW/FOB only.