Stainless Steel Deficiencies and Fixes
Below is a list of common deficiencies found in stainless steel work, likely causes and suggested fixes.
Parts, especially parts made from different thicknesses of material, look very different when viewed side by side on the jobsite
The polished surface is actually composed of parallel rows of tiny grooves whose sides reflect light. If the grooves on two adjacent parts are slightly different in depth, or in length, or have a different underbrushing (1st pass) they reflect light differently and so they look slightly brighter or darker from a given angle. As a result a ½” (12mm) baseplate will look different than a panel made from 16G sheet.
Two pieces of the same material from the same supplier don’t match when installed side by side
The brushing proceeds from one end of the coil or sheet to the other, so the sanding medium is duller at the end of a run than at the beginning, resulting in a variation in texture, especially when seen from a distance.
Specify that material be sequenced (in a process similar to the colour matching of wood veneers)
Brushing goes various ways on a group of related surfaces
The drawings did not indicate which direction the brushing should go, so the supplier saved material by rotating parts during the layout for cutting, or “nesting”.
Oil canning is visible on the face or on edges, especially from a distance
Just like glass, a sheet of stainless steel with a slight warp acts like a lens with a very long focal length. This means that what looks good close up can look awful from across the street. The solution is to use thicker material and ensure edges are “controlled”, meaning framing, brake-forming or hemming.
As a bonus, brakeforming tends to pillow thin sheet in the middle or unify it, producing a more appealing and uniform reflection. This principal is developed to an art with rigidized metals.
There is an obvious variation in appearance on the face opposite studs, at welded corners or along welded seams
Re-polishing after welding is difficult on #4, use XL Blend S instead. If welding is not actually required, it is best to substitute mechanical fasteners or adhesives.
Polishing is incomplete or the worker has used the wrong media and / or tools, so more work might fix the problem.
The polishing media has to be carefully chosen to match the brush on the sheet and applied in a sequence using tools with characteristics similar to the machine that originally applied the factory finish to the material.
There are dull or intermittent marks running the length of parts parallel to the edges
During processing steps such as brakeforming, no die protection was used and the tools on the machine compressed the fine ridges of the brushing. Use a hard urethane sheet over the dies.
Visible welds mar the appearance and / or blue / gray heat marks are visible
Eliminate welding if possible, or ensure welds are positioned in accessible locations so they can be ground and polished out.
Use pickling paste to eliminate the thermally-induced discolouration
Corner radii are too large or too small, or there are undesirable facets
Each brakeformed radius / sheet thickness combination requires a tool worth thousands of dollars. Very tight radii also require the sheet to be scored on the rear face with specialized equipment.
Large radii can be rolled on specialized intermittent rolls by skilled operators but are often “bumped”, resulting in facets.
Require samples in the contract and confirm the fabricator can deliver what you want.
Brown rust marks appear on the work a few weeks after installation, and / or the work corrodes badly to the point of failure
Alloy 304 is not actually “stainless” in an outdoor, curbside environment and will develop stains under normal urban exposure. To be truly stainless, the steel must have some Molybdenum, designated by the middle number, meaning that 316 is acceptable.
Another possibility is contamination by mild steel particles which occurred in processing or in the suppliers shop. Sources include forklifts used directly on plate without a pallet, nails in skids, grinders which had previously been used on steel, wire wheels, etc. Unfortunately, it is not possible to remove contamination that has worked in to the surface, but light contamination due to dust may disappear in a few months of steady rain.
From Anton Nelson of Dewhurst Macfarlane and Partners: The reason some stainless “rusts” is because the heat of the welding causes the chromium in the stainless to come out of solution and form chromium carbide. The chromium carbide is prone to corrosion which is why you sometimes find rust marks around welds in stainless steel parts. It is rust, but not iron oxide, the chemical which is the result of rust in non-stainless steels. I don’t think chromium carbide has the same detrimental structural effects as iron oxide, but it is unsightly. The way to avoid this problem is to use low carbon stainless such as 304L or 316L if possible when you are doing welded architectural connections.
Brushing does not extend in to corners
This is a design problem, as the tradesman must have clear “on and off” access to a surface to be polished. Inside corners etc, are impossible to reach, so parts requiring polish to the inside of corners must be mechanically fastened so parts can be polished before assembly.
The author would like to thank Bill Butt of Excelsior Metal Polishing, Anton Nelson of Dewhurst McFarlane and Partners and Davide Parisi of Vaughan Metal Polishing, all of whom generously shared their knowledge.