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Engineering design

How to match an existing finish in
architectural stainless steel applications

By Nigel Willcock, Technical Specialist at the Abrasive Systems Division of diversified technology company 3M

The beauty, practicality and durability of stainless steel make it highly popular in architectural settings, for use in both decorative and functional applications. The ability of stainless steel to resist corrosion - due largely to its passive, self-healing surface layer of chromium oxide that protects the bulk metal beneath - is a key part of its appeal. Stainless steel is light, bright, easy to care for and, when correctly finished, hygienic. Another attraction is the fact that stainless steel can be polished to a wide range of standard and special finishes, making it ideal for use wherever aesthetic appeal is important.

Most of the polishes or finishes applied to stainless steel can be categorised under one of three groups, namely mill finishes (which are usually matt, dull and very rarely decorative), mechanically polished finishes and special finishes. Special finishes include electroplating, texturising, colouring (mostly using chemical surface treatments) metal spraying and electro polishing. Blast or dry etching and barrel/vibratory finishes are also used. In some architectural situations, such as health care, catering and sanitary environments, the finish of stainless steel is not only decorative, but also has implications for functionality, for example in terms of hygiene, friction, wear and corrosion resistance. However in any setting, an appropriately finished stainless steel surface is likely to exhibit optimal durability and resistance.

Although there have been attempts to standardise finishes throughout the stainless steel industry, variations inevitably occur, thus it is often helpful if those purchasing or commissioning stainless steel items can refer to a standard or trade name where possible.

An overview of these is available from the British Stainless Steel Association's website, at: www.bssa.org.uk

Finishing stainless steel for architectural applications
The architectural uses of stainless steel are far too numerous to list, but examples include cladding, decorative walls, kitchen fittings such as sinks and splashbacks, balustrades, signage, trims and accents, sanitary ware, lift doors, grilles and screens. In many of these applications, the aesthetic appeal of the stainless steel, and thus the way in which it is finished, is of primary importance.

When fabricators prepare stainless steel items for architectural use, they have several options. Fabricators may take pre-polished/pre-finished stainless steel and make the item from that, or they may polish/finish the item after fabrication. In practical terms, however, many fabricators use a combination of both.

Whichever approach is chosen, many manufacturing processes can affect the finish of stainless steel. Processes such as welding, bending and stamping can all damage or disrupt the existing finish, which obviously compromises the item's aesthetic qualities. Welding, for example, leaves an oxidised surface on stainless steel, which takes the form of a discoloured area. This must be cleaned and/or the weld removed. However, this in turn involves a degree of aggression - for example the use of strong acids or wire brushes - that is likely to further disrupt the existing finish. Such approaches can also, if not used carefully, damage the passive surface of the stainless steel to such an extent that it can no longer repair itself, and the item is thus much more likely to corrode or otherwise deteriorate.

Repairing the damage
Therefore, fabricators and others involved with the production of stainless steel for architectural use, must make good the damage caused by manufacturing processes, and finish the damaged areas in such a way that they become invisible. In other words, fabricators must re-finish these areas of stainless steel so that nobody can see where the original finishing ends and the new finishing begins. This generally involves the use of abrasives.

Exactly how those abrasives are used varies according to context and the form of finish involved, but broadly speaking there are three steps to seamlessly matching an existing finish to stainless steel. Those steps are:
1) Levelling the weld. 2) Setting the grain. 3) Final finishing.

Each of these steps brings its own challenges. When levelling the weld, for example, care must be taken not to use too coarse an abrasive, since fine grind lines are the easiest to remove. The grind lines should also follow the grain line direction of the original stock, because perpendicular lines are more difficult to erase.

Setting the grain requires the fabricator to abrade a directional scratch into the stainless steel, to prepare it for final finishing. The challenge here is to keep the work within the weld area, to minimise the need for blending when the final finish is applied.

Final finishing is the point at which the fabricator must finish the item in such a way that it appears seamless, without start/stop marks and with a scratch pattern that is without defects and that perfectly mimics the original. The key challenge here is matching that original finish. This is both an art and a science, and to do it well takes practice.

Fortunately, 3M can help stainless steel fabricators to apply that science to their working life with the Scotch-Brite Match and Finish System.  The Scotch-Brite Match and Finish System comprises an inline sander and an extensive range of Scotch-Brite Abrasive Discs, Belts and Flap Brushes, which can be used together to achieve a ideal result at all steps of the finishing process. Finishes suitable for virtually any use can be achieved with Match and Finish, and the entire system is versatile and user-friendly.

Looking forward
As the options for stainless steel finishing increase, and the material itself proves almost endlessly versatile and useful, it seems likely that fabricators will continue to need the skills, equipment, attention to detail and patience required to hand finish or polish many architectural items. Even though stainless steel of many forms can be purchased pre-polished and then protected during manufacture, the fabrication process inevitably leaves marks behind it and so weld dressing, blending and the polishing out of marks or defects can frequently only be addressed once the bulk of the fabrication process is complete. However, with  Scotch-Brite Match and Finish System by 3M, finishing becomes a much more convenient and user-friendly process than it may previously have been, and finally brings the ideal finish within easy reach of all stainless steel fabricators and manufacturers.

3M in profile
3M captures the spark of new ideas and transforms them into thousands of ingenious products.

Our culture of creative collaboration inspires a never-ending stream of powerful technologies that make life better. 3M is the innovation company that never stops inventing. With $31 billion in sales, 3M employs 89,000 people worldwide and has operations in more than 70 countries.

The UK and Ireland is home to one of the largest 3M subsidiaries outside the USA, employing 3,000 people across 17 locations, including nine manufacturing sites.

Products manufactured in the UK include coated abrasives, personal safety equipment, adhesive tapes, industrial microbiology products, drug delivery systems, high-performance coatings, secure documents, passport readers and imaging systems.

For further information, view website: www.3M.co.uk or follow on Twitter: www.twitter.com/@3M_UK also Twitter. www.twitter.com/@3M.co.uk/abrasives

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