Weld finish and visible welds

At a glance

Sources: Euro Inox Vols. 1, 3, 6 and 15 and an IMOA brochure. They cover stainless steel: for brass, bronze and mild steel ask the partner workshop. A finish name is not a standard, so approve a sample.

The weld-appearance ladder

Most of the cost in a visible weld is not the welding, it is what happens afterwards. Euro Inox says the amount of final grinding, polishing and buffing on decorative stainless can be kept down if the cutting, bending and welding before it are done correctly, and that the neater the welding, the less finishing effort and cost. The drawing shows one butt joint at four levels of finish. Each step up asks for more skilled hand work, so each is a choice for the specification to make, joint by joint.

• Heat tint always comes off. Euro Inox says that for the best corrosion resistance a welded joint must have its heat tint removed and be finished as smoothly as the intended look allows. IMOA says the corrosion resistance of a weld should be similar to the surrounding metal, and that its roughness should not exceed that of the surrounding finish.
• Expressed is a decision, not a default. A bead left on show is part of the design, so it has to be even and the same all along the joint. It needs the same sample approval as a ground weld.
• Flush needs a finish to go with it. Grinding a weld level removes the original surface, and the area around it has to be blended back. IMOA says a weld can often be blended to match the surrounding finish and become invisible; Euro Inox shows hand-finished joints in handrails and street furniture with no evidence of the weld. How far that goes depends on the finish and the light, so approve a sample.

What grinding a weld flush involves

This is Euro Inox's guidance for decorative stainless steel. It is the reason a flush weld takes a skilled finisher and costs more than a cleaned one, and the reason to ask for a sample rather than a promise.

• Start with less weld. TIG welding is slower than MIG but, on balance, the best choice for decorative work, where finish matters (Euro Inox). Avoid thick weld seams, which distort the work and add grinding. A preformed elbow joint needs only two straight butt joints with good access, where a sharp mitre is harder to weld and finish.
• Keep the heat down. Austenitic stainless expands more and conducts heat less than carbon steel, so grinding heat stays in the metal. Euro Inox says to keep heat input low and to adjust tool speed and pressure, or heat tint and distortion follow. Heat discolouration appears when the surface passes about 200 °C.
• Do not start too coarse. Too coarse a grit leaves depressions, and the metal beneath can end up too thin. A 120 grit is usually the coarsest used on this kind of work. For mirror finishes each grit should be no more than twice the previous one, or traces of the coarser stage show.
• Blend along the grain. Work in the direction of the existing polish. Use discs rather than belts, and the smallest disc that will do, to keep the ground area small. Flap discs reduce the risk of grinding marks and undercuts that are hard to blend in later.
• Buy pre-finished and protect it. Pre-finished sheet, tube and bar, kept under plastic film, limit the work on the assembly to dressing and blending the joints. Directional finishes need their restoration planned at every welded junction (Euro Inox).
• Poor work cannot be hidden. Euro Inox notes that, unlike steel that is finally paint finished, it is difficult to remedy or hide poor workmanship on mechanically finished stainless.

For brass, bronze and mild steel we found no equivalent sourced guidance. Our journal notes on MIG and TIG welding and welding brass explain why heat is the issue in brass; the grit and temperature points above are for stainless and should not be carried over without the finisher's advice.

Stitch, seal and expressed welds

A stitch weld is a row of short welds with gaps between them; a continuous weld is unbroken; a seal weld closes a joint against dirt or water; a tack weld only holds the fit-up. The choice is about heat, sealing and how the joint will be seen.

• Continuous or sealed where the joint will get wet, dirty or cleaned with chemicals. Euro Inox says joints should be sealed, with closing welds or mastic fillers, or made wide enough to avoid crevices where dirt collects and the metal becomes open to corrosion. In one Euro Inox case study a corner joint is sealed by welding for hygiene and to reduce contact injuries, not for strength.
• Stitch for hidden frames and dry interiors, where less heat per length helps keep the work flat. That is the studio's reading of Euro Inox's advice to keep heat input low: no source we read gives a pitch or length. Each gap is a crevice, so close the gaps if the joint will see damp or cleaning.
• Tack only where something else carries the joint, such as a backing strip, a fold or a fixing. Euro Inox describes tack welds holding the fit-up before the seam is made.
• Expressed where the weld is a feature. Draw the weld on the drawing, say how it is finished, and state the ripple, length and pitch you want.
• No weld at all where the metal is thin. Capacitor-discharge stud welding attaches panels to a sub-frame with no weld clean-up or surface marking, on sheet at least 1 mm thick (Euro Inox).

ISO 5817 and EN ISO 13920: what they cover

Two standards turn up in weld notes. Both are paywalled and we have not read them, so this table gives only what their published listings say, and quotes no limits.

• A quality level does not describe a look. A weld could meet the level and still leave a visible bead. Write the finish as a finish, in words, with a sample.
• Metals. The listing names steel, nickel, titanium and their alloys. Brass, bronze, copper and aluminium are not named, so ask the fabricator which acceptance criteria they work to for those metals and to put them in writing.
• Structural welds in stairs, balustrade posts and the like are for the engineer. BS EN 1090 (execution of steel and aluminium structures) may apply; it is named in our research and has not been read here. See the balustrade and handrail rules sheet.

What to write on the drawing

• Mark every visible weld with its finish (for example W1, W2), not just “weld all round”.
• Say how it is judged. Name the sample, and the viewing distance and lighting if you want to fix them. Approve the sample in raking light as well as face-on, because a ripple that disappears face-on can show along the surface.
• Name the grain direction on brushed finishes, and say the blend follows it. The sheet on stainless finish codes shows how to name the finish itself.
• Fix the sequence. Weld, grind and polish first; then coat, plate or lacquer. Our PVD sheet says all welding and polishing is done before coating, because the coating is far too thin to hide a weld.
• Name the quality level and tolerance class only where you want them checked, and leave the choice of level to the fabricator and the engineer.
• Ask for a weld sample on the approved material and finish before production, and keep it as the reference.

Welds on visible faces are marked [W1] to [W3] on the drawings. [W1] ground flush: the weld is ground level with the parent surface, heat tint removed, and blended into the [brushed 2J] finish along the grain, with no depressions, undercuts or grinding marks visible at [1 m] in [normal room lighting], matched to sample [ref. __]. [W2] expressed: TIG welded, even ripple, heat tint removed, bead dressed smooth and left visible, to sample [ref. __]. [W3] hidden: not seen once assembled; spatter and heat tint removed; no finishing to the face.

Welds in steel and stainless steel to [BS EN ISO 5817 (edition [to be confirmed]), quality level [ ]], as confirmed in writing by the fabricator's welding coordinator. Copper-alloy and aluminium welds to the acceptance criteria the fabricator states in writing [ ]. General tolerances of welded assemblies to [BS EN ISO 13920 (edition [to be confirmed]), class [ ]]. Structural welds as the engineer specifies [ ].

All welding, grinding and polishing is to be completed before any PVD, plating, lacquer or paint. Pre-finished sheet, tube and bar are to stay under protective film until final assembly. Tools, abrasives and brushes used on stainless steel are to be kept for stainless steel only. Heat tint is to be removed from every weld and the surface cleaned and passivated as [the fabricator advises], to restore corrosion resistance.

Before production, a sample of each weld type [W1, W2], [250] mm long, is to be made on the approved material and finish, and approved by [name] in [raking and face-on light]. The approved sample is kept as the reference for the work.

Standards and sources

• Euro Inox, Vol. 6, The Mechanical Finishing of Decorative Stainless Steel Surfaces (1st ed. 2005)
• Euro Inox, Vol. 3, The Welding of Stainless Steels (2nd ed. 2007)
• Euro Inox, Building Series Vol. 1 (3rd ed. 2005) and Vol. 15 (2nd ed. 2011)
• IMOA, Which Stainless Steel Should Be Specified for Exterior Applications? (brochure)
• ISO 5817:2023, quality levels for imperfections in fusion-welded joints: listing read; text not read; confirm the BS EN ISO edition.
• EN ISO 13920:2023, general tolerances for welded constructions: listing read; text not read.

Questions we are asked

Can a weld be made completely invisible?

On stainless, a skilled finisher can get very close. IMOA says a weld can often be blended to match the surrounding finish and become invisible, and Euro Inox shows hand-finished joints with no evidence of the weld. The result depends on the finish and the light, and the more reflective the finish the harder any difference is to hide (the studio's reading), so approve a weld sample in the light the work will be seen in.

What ISO 5817 quality level should I specify?

That is not usually the designer's choice. ISO 5817 gives levels B, C and D for weld imperfections, with B the highest requirement on the finished weld, according to its listing. The level should come from the fabricator's welding coordinator, and from the engineer where the weld carries load. A level does not describe how a weld looks, so specify the appearance separately, by finish and sample.

What is the difference between a stitch weld and a continuous weld?

A continuous weld is an unbroken seam: it seals the joint but puts in more heat, so more distortion and more finishing. A stitch weld is a row of short welds: less heat per length, but each gap is a crevice. Euro Inox says joints should be sealed or made wide enough to avoid crevices, so in wet, hygienic or chloride-laden places use a continuous weld or seal the gaps.

Why does a ground weld sometimes look different from the sheet beside it?

Usually the grit or the direction is wrong. Euro Inox says to blend along the existing polish direction, that too coarse a grit leaves depressions and damage that can be slow or impossible to put right, and that 120 grit is usually the coarsest used on decorative stainless work. Grinding heat can also tint the metal, which Euro Inox puts at a surface temperature above about 200 °C. Approve a sample first.

Should metal be welded before or after it is polished or coated?

Weld first, then finish. For PVD, all welding and polishing is done before coating, because the coating is far too thin to hide a weld. Euro Inox advises buying stainless pre-finished and protected by film where possible, so that finishing is limited to blending the joints.

Do ISO 5817 and EN ISO 13920 cover brass, bronze and aluminium?

ISO 5817's listing names steel, nickel, titanium and their alloys, so brass, bronze, copper and aluminium are not named. Ask the fabricator which acceptance criteria they use for those metals, and which standard they come from. EN ISO 13920 is about the dimensions, angles and shape of welded assemblies: check its material scope in the standard.