The crate lands at goods-in and the first cabinet is out of the foam before the paperwork is signed. Someone wipes the packing dust off the front panel, runs a thumb along the corner seam, and stops. Nothing is cracked, nothing leaks, the door closes with the right sound. The thumb has found a line of hard grit beside the seam: weld spatter. The weld behind it is sound, and the part is about to be rejected anyway. Weld appearance acceptance criteria exist for this moment — a written description of what a finished seam is allowed to look like, agreed before the first arc is struck.
What an appearance criterion actually is
A welded assembly passes two gates, and they are not the same gate. Structural acceptance asks whether the joint carries its load, holds dimension and stays sealed. Appearance acceptance asks what the finished seam may look like. Drawings usually specify the first in detail and the second not at all, so the argument lands at goods-in.
An appearance criterion has to be observable. "Good workmanship" means one thing to the welder, another to the night-shift inspector, another to the buyer at the dock:
- Seam continuous; no start/stop ridge on faces visible in service.
- Weld spatter removed from all surfaces before coating.
- Corner seam ground to a smooth transition on the front and sides; internal seams as-welded.
- Stainless: heat tint removed from the weld and the heat-affected band either side.
Each line can be checked by eye against a sample. One that needs an instrument belongs in the dimensional or NDT section.
Appearance criteria are not process parameters either. Current, travel speed, shielding gas, wire feed and joint preparation decide what the weld becomes; we cover that side in our sheet metal welding process guide. The criterion describes the result and leaves the method to us.
The three quality levels in plain language
ISO 5817 sets three quality levels for imperfections in fusion-welded joints: B, C and D. B is the most demanding, D the least, and C is where most general fabrication drawings land when they say anything at all. For laser and electron beam welds the parallel family is ISO 13919, built on the same idea.
What changes between levels is how much imperfection is tolerated and how much finishing it implies. Level B asks for a seam that runs smoothly into the parent metal: minimal undercut, no open crater, consistent bead width, blended stop/start points. Level D accepts a visibly hand-made weld; level C covers most ordinary fabrication work.
The cost difference is nearly all labour: more grinding, more re-inspection, slower travel, more parts scrapped because the repair looked worse than the original weld. Several metres of visible seam moved from D to B can double the finishing time.
We learned that the expensive way. Our first attempt at settling a cosmetic dispute on one enclosure program was to specify the top level for the whole product. It worked technically and failed commercially — the price stopped the program. We now assign levels per zone: front face and door opening at one level, internals at another.
A quality level is not a strength rating. A level D weld on a correctly sized joint carries its load; a level B weld on an undersized one does not. Where a joint is fatigue-loaded or sits in a lifting path, imperfection level and joint sizing are separate decisions.
Nine appearance defects and what they look like
Names first, because half the arguments at a receiving dock are arguments about vocabulary. The "what you see" column is what matters when you write a criterion.
| Defect | What you see | Usual cause | Fix before coating |
|---|---|---|---|
| Weld spatter | Hard droplets beside the seam; catches a fingernail | Long arc, dirty plate or wire, poor gas cover | Scrape or flap-disc off; use anti-spatter compound |
| Undercut | Groove along the weld toe, eaten into the parent metal | Current too high or travel too fast; poor torch angle | Blend smooth; if deep, cover pass and re-blend |
| Overlap | Weld metal lying on the plate without merging into it | Too little heat for the wire feed, travel too slow, torch off the joint line | Grind out and re-weld; a cover pass only hides it |
| Burn-through | Hole or sunken dark spot where the pool dropped through | Too much heat for the thickness, root gap too wide, no backing | Cut out and re-weld with backing or lower heat |
| Porosity | Pinholes and pits in the bead face, sometimes in a line | Oil, paint, rust or moisture; shielding gas blown away by draught | Grind out and re-weld; clean joint, check gas cover |
| Crater | Depression where the bead stopped, sometimes with a hairline crack | Arc broken abruptly at the end of a run | Fill it; on crack-sensitive material grind out and re-weld |
| Uneven bead / excess reinforcement | Bead width wanders; height stands proud in places | Inconsistent travel speed or torch angle, varying gap, poor fit-up | Grind to consistent width, then fix the fit-up |
| Misalignment | One edge above the other, a step that survives grinding | Poor clamping, unequal part lengths, distortion before tacking | Re-fit and re-weld; blend long on cosmetic faces — weld distortion control guide |
| Heat tint (stainless) | Straw to blue to grey bands beside the seam | Oxidation of the hot surface when trailing shield gas is lost | Mechanical removal, chemical cleaning, passivation — stainless weld cleanup guide |
Which defects are cosmetic and which are structural
Cosmetic defects are surface-condition problems: spatter, heat tint, uneven bead width, stop/start ridges, excess reinforcement on a joint that already has full throat. They cost finishing time and become coating problems. They do not remove load-carrying section.
Structural defects do. Undercut removes parent metal beside the weld. Burn-through leaves a hole. Incomplete fill leaves the groove partly empty, so the throat is smaller than designed. Lack of fusion means weld metal sitting against the parent metal, not joined to it. Overlap belongs here too, despite usually being written up as cosmetic: that metal lies on the surface, unfused. Crater cracks belong here, and so does clustered porosity.
Thin sheet makes the boundary sharper. Undercut that would pass unremarked on a 12 mm bracket takes a real fraction out of a 1.5 mm wall, and a burn-through repair on a cosmetic face stays visible for the product's life. Write it as two lists: repair before coating, repair before shipping.
Evaluation beyond the eye — penetrant, magnetic particle, radiographic or ultrasonic testing — is a different activity from appearance acceptance. We build and finish; formal NDT, procedure and welder qualification records, and any certification declaration belong to the customer's or a third party's scope and live in the product's test report.
Why a sound weld still gets rejected
Three mechanisms account for most of what we see.
The coating shows the seam through. Powder film is thin — typically 60 to 120 µm depending on powder and specification — and follows the substrate rather than filling it. Even a seam ground flush differs from the parent metal in the heat-affected band and in how it takes a blast profile, so after coating it often reads as a faint line or gloss change. Large flat panels make it more obvious.
Spatter under paint is worse. Each droplet is a peak with a shadow: powder builds on the peak and pulls thin around its base, leaving a bump on top of a thin spot that becomes the first rust point outdoors.
Sharp edges get thin film. Coating draws back from a sharp corner as it cures, so the edge ends up with less film than the flat — precisely where corrosion protection matters most.
None of it is fixed by adding powder — see our powder coating defect troubleshooting guide for the remedies, and the cosmetic surface class guide for which faces may show anything at all. Judge the weld as the customer will see it.
Before the weld reaches the paint line
The order we work in at our benches:
- Spatter first, mechanically. Scraper or chisel for loose droplets, flap disc where there is a field of them; anti-spatter compound before welding removes most of it.
- Grind the seams the criterion names, and no others. Flush-grinding every seam is expensive and thins the wall.
- Break the edges. Sharp corners from shearing, punching or grinding cause the thin-film problem above; see our edge break and deburring guide.
- On stainless, remove heat tint mechanically, then chemically, then passivate. Mechanical-only leaves a smeared surface that stains in service.
- Clean, then check. ISO 8501-1 names blast-cleaning grades and hand and power tool grades; which one your parts need is for you to agree with your applicator.
Fit-up is the cheapest of these steps. A joint that closes before tacking needs little grinding; one that gaps needs a lot. The appearance problem usually starts at the brake and the laser, not at the welding bench.
Inspection: what to look at, in what order
ISO 17637 is the reference for visual examination of fusion-welded joints. Pass or fail comes from the quality level you specified plus your own criterion.
Conditions we hold to: even, diffused light with no glare off polished stainless; the part at working distance and eye level; no magnification, because the receiving dock will not use any either; the surface in the state it will be judged.
The sequence:
- Look at the whole assembly first: alignment, squareness, twist, whether the seam runs where the drawing puts it.
- Follow each seam end to end: continuity, start/stop points, craters, width changes.
- Look at the weld toes for undercut, overlap and spatter fields.
- Look at the surrounding surface for arc strikes, grinding gouges, heat tint, edge condition.
- Confirm the surface is ready for coating: no oil, marker pen, fingerprints or blast dust.
- Compare anything borderline against the limit sample, then mark, photograph and record it.
Two timings matter more than most people expect. Look after grinding and again after blasting, because blasting opens pinholes and laps a grinder has smeared over. Look after coating on first articles — the only moment you learn whether the seam reads through. Our inspection flow and FAI and PPAP documentation show where these sit.
Limit samples: the argument stopper
A limit sample is a physical part, or a defined area of one, showing the agreed boundary between pass and fail. Photographs help but do not replace it: gloss, step height and the feel of a bump do not survive a camera.
How we build one:
- Take it from real production. A coupon made by the best welder on a good morning is a sales sample, not a limit sample.
- Mark the exact area, with part number, revision, date and specified level on the label.
- Make two where coating is involved: one pre-coat for weld finish, one coated for finished appearance.
- Show both sides of the line where you can: worst-acceptable beside just-rejected settles more than one sample.
- Get it approved by both parties, reference recorded on the drawing and in the first article pack.
- Store it so it survives: sealed, dated, re-checked — rust and fingerprints change what it shows.
Re-issue it when material, thickness, joint design, weld process or level changes, when the coating or RAL colour changes — dark texture hides more than light gloss — or when the sample is damaged.
Our first attempt at this was wrong. On one enclosure program we issued bare steel weld-finish samples, had them approved and shipped. The first coated batch was rejected for seam read-through and spatter bumps the bare samples never predicted. They answered the question we asked; the customer asked a different one. Coated samples are standard for us now.
Paste-ready drawing / RFQ callout
WELD APPEARANCE — DRAWING / RFQ CALLOUT (copy, fill in, delete lines that do not apply)
Zone / location: ........................ (e.g. front face and door opening, visible in service)
Surfaces covered: ........................
Quality level: ISO 5817 level ___ (B / C / D) — ISO 13919 level ___ for laser or beam welds
Seam finish: as-welded / ground flush / blended transition / seamless appearance required
Weld spatter: removed from all surfaces before coating — yes / n/a
Undercut: not permitted / permitted if removed by blending / repaired before coating
Craters and start/stop: filled and blended on faces visible in service
Misalignment: maximum step before finishing: ____ (agree per project)
Stainless: heat tint removed mechanically and chemically, then passivated
Edge condition: edges broken / rounded before coating
Coating readiness: preparation grade per ISO 8501-1: ____ (agree with applicator)
Judged in this state: after welding / after grinding / after coating
Limit sample reference: ____ (part no., revision, date, approved by)
Inspection: visual examination per ISO 17637; unaided eye, normal working distance
Records required with shipment: ____ (photographs, FAI report, weld map)Frequently asked questions
Which ISO 5817 quality level should I specify for an enclosure?
Specify per zone rather than per product. Most enclosure exteriors work at level C, with visible faces finished further than the level alone requires; internal brackets, stiffeners and rear faces sit comfortably at D. Reserve B for faces at eye height under direct lighting where appearance is part of what is being sold — retail housings, kiosks, laboratory fronts. Ask for the price at each level before committing the whole product to one, because the finishing hours are where the difference lands.
Can I just write "welds to be visually acceptable" on the drawing?
You can, and everyone who reads it will read it differently. The welder hears no burn-through, the inspector hears no spatter, the buyer hears no visible seam at all, and the applicator hears clean and ready to coat. That sentence has generated more rework in our shop than any other single line on a drawing. Replace it with a named level, a short list of prohibited defects and a limit sample reference: three lines, all of them checkable.
Does specifying a higher quality level make the weld stronger?
No. The level governs the size and frequency of tolerated imperfections, not joint strength. Strength comes from joint design, throat thickness, filler and parent material — a separate calculation and a separate line on the drawing. A level D weld on a correctly sized fillet carries its load; a level B weld on an undersized one does not. Where fatigue, cycling or lifting is involved, get the joint sized by an engineer, then choose the appearance level on top of it.
Will powder coating hide weld defects?
Usually the opposite. Powder follows the substrate instead of filling it, so a seam visible as a step or a line before coating stays visible as a line or gloss change afterwards. Spatter becomes a bump sitting on a thin spot, and sharp weld edges and ridges end up with less film than the flat areas around them. If appearance matters, agree the seam finish before coating and judge the first article after coating; the coating-side symptoms are listed in our powder coating defect troubleshooting guide.
Who approves a limit sample, and what happens if we disagree later?
Both parties: the fabricator and whoever signs off at your end, with the reference written onto the drawing and into the first article pack. While the sample is current it governs — the argument is settled by holding the part next to it. If a dispute arrives that the sample cannot answer, re-issue it for the changed condition rather than argue from text. We date every sample and re-issue on any change to material, thickness, joint design, process or coating.
Do you provide certified weld inspection or NDT reports?
We carry out visual examination and dimensional checks as part of our own inspection, and we supply the records, photographs and first article documentation behind them. Formal NDT such as penetrant, magnetic particle, radiographic or ultrasonic testing, along with weld procedure and welder qualification records and any certification declaration, is the customer's or a third party inspector's scope, and the results belong in the product's own test report. We do not claim certifications we do not hold, and would rather say so at quotation stage than at goods-in.
The short version
- Appearance acceptance and structural acceptance are two gates. Write them separately.
- Name a quality level per zone, not per product: ISO 5817 B, C or D, or ISO 13919 for beam welds.
- List the defects you will not accept, split into must-fix-before-coating and must-fix-before-shipping.
- Judge appearance in the state the customer sees it — after coating, for painted parts.
- Remove spatter, break edges, and on stainless remove heat tint before passivating.
- One signed limit sample settles more than ten paragraphs of written description.
Where we fit
We are Sanjun, a sheet metal OEM and ODM shop in Shenzhen building custom enclosures and cabinets. We weld, grind, clean, coat and assemble under one roof, so seam finish and coating are decided by the same people rather than argued across two suppliers. What helps us most on a new enquiry is the callout above filled in — or a note on which faces are visible in service. If you have neither, we will propose zones and levels and put a limit sample in front of you at first article.
If you are still designing: our material selection guide covers what welds cleanly, welded versus modular enclosure construction asks whether that seam needs to exist, our surface finishing guide sets out what each finish hides, and the hygienic washdown design guide covers enclosures washed down regularly — continuous welds, no crevices — worth reading before the IP and IK ratings are set.