Stainless Steel: Finishes and Fabrication
Two years ago I attended the opening of a 50-story “LEED Gold” office tower. After the mayor and officials left, a group of us walked the ground floor of the brand new building. Because we were all in the fabrication business, we couldn’t stop looking at the details. I found a shocking number of deficiencies in the stainless steel work.
Following is a partial list:
Stainless elevator door frames with visible polishing marks at the miters.
Cove base with visible polishing marks at every splice.
The stainless steel grate under the revolving doors was made of unsupported 1/8” stainless with wide spans in the slotted pattern, so the weight of a high heel depressed it until it was permanently bent.
Flat stainless column covers on the exterior appeared badly “oil canned”.
Curved column covers on the same column had a checkerboard appearance.
This article is intended to help designers avoid these issues and provide background to facilitate troubleshooting in the field.
Stainless steel has a problem in common with most high-end materials: Stone, stainless steel, bronze and glass seldom have a thick or opaque coating, so the effect of every process is visible, making it hard to hide bad workmanship. Worse, polished stainless steel sheet is shipped with masking, so defects are not visible until the masking is striped. Because the work is expensive and hard to fix, when bad workmanship makes it to a site despite a designer’s best efforts, suppliers are reluctant to admit fault. It then falls to the designer to “call BS” or to identify the source of a problem and suggest a fix.
The most common stainless specification is “304 Stainless Steel, #4 brushed”. Behind this spec lie pitfalls that can result in a job which complies, but looks rough, patchy and just plain old cheap. Workmanship aside, specifying exotic finishes which are subject to interpretation, like “angel hair” or “non-directional”, can produce even more variable results. A list of some things that can go wrong with stainless steel work is found below, together with the likely causes and suggested fixes, but first, a brief explanation of the stainless finishing process is required.
Both stainless steel sheet and plate are rolled in lengths and widths much greater than the material which eventually finds its way in to most fabrication shops. In a stainless mill, billets weighing 25,000 lbs (12,000 kg) or more are rolled hot, then the raw material enters two different streams; Plate and sheet. Material destined for sale as plate is further rolled to thicknesses from ¼” (6mm) to over 6” (150mm) and sizes from over 10’ x 20’ (~ 3m x 6m) to 4’ x 8’ (~ 1.3m x 2.8m).
Surface quality of plate varies enormously and most plate suitable for use in polished architectural applications comes from modern mills in North America or Europe. Low quality plate best suited to industrial applications can have significant warpage, roll marks and improperly alloyed iron, which causes staining of finished work and contamination of processing equipment. Even high quality plate can meet some specs with visible “rescue grinding” where defects were removed.
Stainless sheet suited to architectural applications is produced and stocked in coils 4’ (1.2m) or 5’ (1.5m) wide and come with a “B” or “Bright” finish which looks like a cloudy mirror (2B is most common). Coils are “sheeted” at service centres, meaning sheet lengths are only limited by minimum orders and the fabricator’s ability to handle the material.
This sheeting process is why it is expensive to specify a small number of parts 8’ long with a 1” return all around, because making the parts requires material 8’ 2” long, meaning almost 2’ will be lost from the next stock length up, which is a 10’ sheet. By contrast, if thousands of parts are involved, the fabricator can order the material sheeted to the length required, resulting in less waste.
If a polished finish is required on sheet (the industry uses the term “polished” for all finishes, including “#4 brushed” and “mirror polished”) the finishing is applied on the entire coil in a continuous process using a multi-headed belt sander, prior to cutting in to sheets. The masking is applied immediately after the polishing, before the sheets are stacked. If a finish is required on plate, the finish is applied by special-purpose sheet-fed machines, or by a manually-operated overhead gantry polishing machines. As a result of the manual processing, high-quality finishes on plate are much harder to achieve than on sheet and supplier qualification is paramount.
The key decision to make when specifying a finish is whether the stainless will be used as whole sheets, without the need for re-finishing, or whether it will be joined by welding or otherwise worked and must be refinished after fabrication. This choice is required because a #4 finish is cheap, but is created by machines whose characteristic long surface markings cannot be restored or blended by hand, making invisible re-work impossible.
Only finishes such as Excelsior Metal Polishing’s “XL Blend S” or Vaughn Metal Polishing’s equivalent can be duplicated with hand tools and should always be specified for work which requires blending. The same principals apply to round and square tube: Chose #4 if the parts will be cut and used, but upgrade to XL Blend S if polishing is required after fabrication.
Non-Directional or Angel Hair finish is commonly specified for stainless steel paneling used in high-traffic public environments, because it can be restored on site by semi-skilled workers using an orbital sander. Applying this finish directly on raw 2B sheet produces a dull result, so this finish looks best when applied over a #4 finish. To produce the 25 tonnes of 316 stainless plate with a non-directional finish we needed for a job in the Jamaica Airtrain Station, our polisher (Vaughan Metal Polishing, VMP) started with a series of progressively less aggressive grits culminating in a #4 pass, which removed the scale and leveled the surface.
We saved VMP an enormous amount of time applying the random final pass by rigging 6 heavy-duty air-powered random orbital sanders to a frame. The VMP polishers moved it by hand in a carefully timed pattern over the 20’ (6.5m) plates to apply the uniform and repeatable non-directional finish. We knew that since the oscillating radius of the sanders used was ¼” (6mm), a similar sander could be used for repairing damage on site.
Because plate can vary in surface quality and be damaged by handling, when writing the specification for polished stainless work that will be fabricated from plate, it is worth including a line requoring the fabricator to select the plates for their finish quality, so they cannot claim that finish defects are the result of industry norms. It is also advisable to suss out whether the supplier is accustomed to handling materials with delicate finishes, since a correct set up is time-consuming and expensive to implement.
Finally it is worth noting that architects led by Frank Gehry, have sometimes chosen to celebrate defects such as the wrinkled appearance that results from wrapping flat metal sheet around compound curves. Making a positive statement with a ”defect” in workmanship, is actually a very difficult stunt to pull off and is not recommended for amateurs!
The author would like to thank Bill Butt of Excelsior Metal Polishing and Davide Parisi of Vaughan Metal Polishing, both of whom generously shared their knowledge.