There are a number of stainless steel finishing options that alter more than just the appearance of the material. Whatever the intended use, choosing the right finish option is essential.
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In projects when design is a primary consideration, an attractive finish will enhance the appeal of the end-product. For example, in architecture and the automotive industries, different finishes can be used to achieve a variety of visual effects. In retail products, particularly kitchen appliances, stainless steel No. 4 finish is one of the most popular finishes available.
The choice of surface finish is also important where fabrication processes will be applied. Rough surface finishes are appropriate when the steel will be ground prior to painting and gluing. Smooth surface finishes are better where the steel will be blended.
The choice of finish should always be clearly specified and properly defined by standard industry designations.
During the late 1970’s, British Steel scientists found that dull polished finishes on stainless steel showed a wide range of surface roughness. Further testing revealed that steel with high surface roughness was heavily damaged by the polishing operations, whereas steel with low surface roughness was relatively unscathed.
During the mid-1980’s dull polished finishes became widely used on projects such as high-profile architectural projects. However, it was soon discovered that some of these dull polished finishes had poor corrosion resistance, especially when exposed to seawater. Consequently a new surface finish description was introduced which remains in use to this day.
Three more common stainless steel finishing options are:
No. 2B is the mill finish, meaning it has not been processed further. Matte finishes are dull in appearance and are not ideal for aesthetic end uses. However, they’re a good choice where appearance is not important or when further finishing is intended. No. 2B Matte finishes are the least expensive of the stainless steel finishing options.
The finish is produced by ‘cold rolling’ stainless steel through special rolls or dies. The cold rolling produces a smoother, less pitted surface. Next it is softened and de-scaled in acid solution. The steel is given a final pass on polished rolls to further enhance its smoothness.
Common applications include:
The No. 4 Brushed finish can vary with different suppliers and even from batch to batch from the same supplier. The variations arise from differing manufacturing conditions, such as wearing of the abrasive belts used in these finishes. Some level of variation should be expected when ordering No. 4 Brushed finish. It can be helpful to request a sample of a few square inches to ensure the finish achieves the desired effect.
Brushing the stainless steel produces a distinctive look with a muted luster and a pattern of fine parallel lines. It has strong decorative appeal without being too reflective, as too much reflectiveness can be undesirable. For example, overly reflective stainless steel accents on a building could be blinding in bright sunlight. The drawbacks to this finish include reduced corrosion resistance, because the grooves of the finish are susceptible to rust.
The finish is created by sanding the stainless steel in one direction with a 120-180 grit belt, followed by softening with a 80-120 grit medium non-woven belt.
Common applications include:
The Gateway Arch in St Louis, Missouri is the world’s tallest arch and is clad in brushed stainless steel.
The DeLorean DMC-12 sports car, most famous for being featured in the Back to the Future films, is paneled in brushed stainless steel.
Mirror finishes are highly reflective and created by polishing the stainless steel. The polishing process enhances appearance and consistency, making cleaning easier. It also masks the after-effects of welding and hides surface damage.
No. 8 Mirror finish is created by mechanically treating the surface with a series of progressively finer abrasives. Alternatively a special rolling procedure is used which can simulate the appearance of mechanical abrasion. For this stage, it is essential to remove deep scratches as any surface defects will be very noticeable on the finished product. The final process involves buffing the surface for 5-10 minutes to create a mirror-like, highly reflective finish.
A benefit of No. 8 Mirror finishing is that it improves corrosion resistance. The polishing eradicates crevices where corrosive particles can lodge themselves.
Common applications include:
The stainless steel finish of a fabrication project is no less important than the steel’s grade or form, because the finish is most likely to impact aesthetics, performance and safety. Each finish leaves a unique look, and some also help the steel stay clean and hold up to environmental factors. Brushed finishes are more resistant to corrosion and are easier to sanitize, while matte finishes cost less but are more susceptible to moisture, salt and other sources of corrosion.
There are many types of stainless steel finishes available, so it’s essential to be specific and use standard industry designations when outlining what your project needs. In this article, we’re exploring the most common stainless steel finishes in detail so you can make the best choice possible for your next stainless steel fabrication project.
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According to ASTM International Standards, there are eight stainless steel finishes. Numbers 1 and 2 are called “mill finishes” and can be used as-is or may be a preliminary step to polishing. Finishes 3 through 8 are achieved mechanically by polishing the metal with fine abrasive belts. The three most popular stainless steel finishes are No. 2B, No. 4, and No. 8.
No. 2B matte is the most broadly-used finish in stainless steel fabrication and forms the foundation for smoother finishes. Achieved through a cold-roll process, the finish resembles a hazy mirror with limited reflectivity. No. 2B finishes are frequently the most cost-effective option available and used in applications where aesthetics are less critical or additional finishing is planned.
To produce a No. 2B finish, the stainless steel is cold rolled through special rolls or dies to smooth out the pits of the metal’s surface. The steel is then annealed, and its surface is softened and descaled through pickling. Additional smoothness is achieved through a final, light cold rolling pass.
Parts specific to the food packaging industry, which may require a sanitary finish, are commonly produced with a No. 2B finish at BSMW. It is the standard finish for 201, 304, 304L and 316L steel grades.
No. 4 brushed finish is achieved by mechanically polishing the metal with increasingly-finer abrasives. Hallmarks of the finish are a distinctive subdued luster and a pattern of uniform parallel polishing lines.
Depending on the project’s requirements, the grit abrasive can be anywhere within the range of 120 and 320. Higher grit numbers indicate a finer polish and lead to a more reflective surface. At the higher end of No. 4 finish, the stainless steel is considered food grade.
The appearance of stainless steel with a No. 4 finish will differ between metal fabricators due to different working conditions, equipment, operating techniques and sanding belts. For this reason, it’s advisable for a customer to request a sample from the fabrication company to ensure the look of their No. 4 finish is acceptable for the project.
Both the DeLorean sports car (of Back to the Future fame) and Gateway Arch in St. Louis, Missouri feature No. 4 stainless steel. Parts specific to the architectural and food industries are commonly produced with a No. 4 finish at BSMW.
No. 8 mirror finish is the smoothest and most reflective finish that falls under ASTM standards. Every surface defect of the metal must be removed to attain this look. It’s achieved by polishing with gradually-finer abrasives, similar to the No. 4 finish, but its final process involves five to ten minutes of buffing with extremely fine polishing compounds.
No. 8 finishes are most commonly used in decorative and mirror applications as well as restaurant utensils and surgical instruments. Parts specific to the dairy and beverage industries are commonly produced with a No. 8 finish at BSMW.
Chicago’s famous Cloud Gate sculpture, colloquially known as “The Bean,” is also made up of stainless steel plates in No. 8 finish.
An entire room of the Badger Sheet Metal Works’ shop is devoted to stainless steel work, highlighted by the company’s LYNX Timesaver surface-finishing machine. It gives the team the ability to enhance the quality of a surface or prepare it for paint application. The LYNX machine is used to apply Scotch-Brite™ No. 4 polish finish and deburr the metal. In a single pass, the machine completes three steps using two sanding drums, a Scotch-Brite™ drum and a water filtration system to keep the parts cool.
The LYNX Timesaver, and everything else located in BSMW’s stainless steel room, are maintained at stringent 3-A Sanitary Standards that are required by the food, dairy and pharmaceutical industries.
Finishing stainless steel makes it more aesthetically appealing and improves its strength and corrosion resistance, making the application an intelligent choice for maximizing your project’s investment. BSMW offers a number of finishing options for stainless steel applications, ranging from No. 2B to No. 8 polish finish. If your project requires polishing, passivation, and/or bead blasting, its team is ready to assist with stainless steel finishing expertise that will take your next metal fabrication project to the next level. Contact BSMW to get started today.
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