Custom & Standard Polishes on Sheet Metal
The mill finishes are applied to strip and sheet stock forms of the material. Plate and bars are available from the mill in a hot rolled condition only. Additional finishes are applied to these products by other processors. The initial, and sometimes final, surface finish on the stainless steel is applied at the stainless mill or secondary metal processor. A secondary metal processor is a plant that purchases No. 2 finish material from the steel mill and adds a mechanical or chemical finish through additional processes. This processor may be the end fabricator, a division of the mill, or an independent company specializing in the finishing of metals.
No. 1 Finish
A No. 1 finish is characterized by a rough, dull appearance. The dull surface is produced by hot rolling the stainless steel slab to the desired thickness, then annealing and descaling by pickling. This finish is available only on thick stainless steel plate, sheet, and bars, because it is a hot-rolled finish and not acquired by reduction on cold rolls. It is not typically an architectural surface, because the color and texture are not consistent and because the surface on large plates is mottled and uneven. This finish often contains streaks running in the direction of the rolling process, which are darker lines in contrast to the rest of the surface.
The No. 1 finish is also known as a hot-rolled and pickled surface. The minimum thickness available is 0.063 inches (1.6 mm). Because the sheet thickness is reduced by hot rolling, variations in thickness of as much as 10%, plus or minus, may occur. Small quantities are not normally stocked or available from the mill. For large orders, the surface finish can be improved and certain quality criteria can be established with the mill before any processing has begun.
No. 2D Finish
A No. 2D surface is a cold-rolled finish, applied to the surface by passing the coil of stainless steel through dull cold rolls after annealing and pickling. The result is a matte surface with low reflectivity. Care must he taken if this finish is to be the final architectural surface. The mill must control the annealing process to ensure color uniformity. The finish surface of the sheet must be protected with a paper or plastic coating soon after the final pass through the dull rolls.
No. 2B Finish
A No. 2B surface is a cold-rolled finish with a more reflective surface than a 2D finish. The No. 2B finish is obtained by passing the pickled sheet through a final set of highly polished rolls. This finish is a smoky, reflective surface, not often used for architectural applications, except as flashings or as in the case of the roof surface of the Chrysler Building. The No. 2B finish is commonly used as the mill sheet prior to the application of more polished surfaces by secondary finishing processes. Further passing through cold rolls improves the finish but sacrifices the ductility of the thin sheet.
No. 2BA (Bright Annealed) Finish
Bright Annealed finishes are highly reflective surfaces produced at the mill by final annealing the stainless steel in a controlled-atmosphere furnace. The purpose of the controlled atmosphere is to prevent the development of surface oxidation and scale. The Bright Annealed finish cannot he matched in a fabrication shop. Attention to detail and control of welding should be exercised when using metal with this finish.
The 2BA finish is a common ornamental surface used on reflective trim parts and strips and ornamental metal panels and fascia. It is not a true mirror finish, but has the greatest level of reflectivity of all mill products. The reflective image often has a slight "smoky" appearance or may be very clear and reflective, approaching a mirror-polished surface. Samples should always be required when specifying this surface finish. The lack of apparent polishing lines gives the 2BA finish a nondirectional appearance.
The most common architectural stainless steel surfaces are the mechanical surface finishes. Mechanical surface finishes are applied to sheet or strip stock from the mill by a secondary processor. A metal-polishing house or metal fabricator will start with a No. 2B mill finish. If plate or bar steel is to receive the final mechanical finish, then the fabricator would start with a hot-rolled mill surface.
The mechanical finishes are applied to the surface of the metal by successive passes of polishing wheels or belts across the surface. In the case of plate or bar steel, the rough mill surface must first be ground smooth to remove the scale or leveled to eliminate the inconsistent surface.
No. 3 Finish
A No. 3 polished finish is characterized by short parallel lines, running the length of the sheet. The lines, known as grit lines, are mechanically applied by abrading the surface with fine-grit abrasive belts or wheels. The No. 3 finish is the coarsest of the standard polished surfaces. The abrasive range used is from 80 to 150 grit, with 100 grit the standard for this finish. Typically, this finish is produced by passing the metal first through an 80-grit aluminum oxide belt, and then through a 100-grit aluminum oxide belt.
Polishing fabricated parts to this finish is difficult. Matching one polished surface provided from a fabricator or polishing house to that of another No. 3 polished surface provided from a different source is difficult to impossible. Depending on the lighting, the finishes may appear as different metals. The characteristic short, skipping grit lines are difficult to match or repair on large flat surfaces. When the short skips are not closely matched for depth and length, the rework will stand out. Weld repairs at corners and edges can be effectively matched when polishing is kept to a minimum by masking and blocking the weld.
The short, parallel grit lines tend to break up the reflected light, which assists in concealing minor scratches and imperfections on the surface of stainless steel. For large areas constructed from multiple sheets, it is critical to use sheets polished by new belts with matching grits.
No. 4 Finish
A No. 4 finish is similar to a No. 3 finish, except that the parallel grit lines are finer. This mechanical surface is developed by using 120 to 240-grit abrasive belts or wheels. Following the same steps for polishing the No. 3 finish, follow a pass through a 100-grit aluminum oxide belt with a pass through a 180 or 240-grit silicon carbide abrasive belt. The No. 4 finish is a common architectural surface because of the more lustrous appearance obtained by the finer grain. The fine lines can be repaired and reproduced to match welded or fabricated sections, as in the No. 3 finish. The matching of the finish is difficult, but achievable. Care should be taken when mixing different suppliers of this finish; slightly different grits will create differences in the final appearance.
No. 5 Finish
The No. 5 finish was never produced in the United States, and records of the true nature of this finish are scarce. It was said to be a European finish produced by polishing with a nonwoven abrasive belt-a rather broad description. A guess would be that this finish is what is now known as "hairline," "long-grain," "fincline," or any of a number of names relating to the finish produced by fine-line brushing with a Scotch-BriteR pad.
No. 6 Finish
A No. 6 finish has a soft satin appearance. It is produced by tampico-brush ing' a No. 4 finish in an abrasive and oil medium. This finish has very fine grit lines, but its reflectivity is lower than that of a No. 4 finish because of the many minute lines on the surface.
A common variation of this surface uses the fine range of the No. 4 finish applied by using Scotch-Brite pads or Scotch-Brite rolls. This finish is characterized by long parallel lines running the length of the sheet, which appear almost as fine grooves in the surface of the metal. Common in the Far East, this finish is known as "long-grain" or "hairline." The reflectivity is greater than a No. 4 finish, and the scratch lines are longer, running the length of the sheet or part.
This variation of the No. 6 finish can be repaired and matched with greater ease than the other directional polished finishes. Minor surface imperfections on the base stainless steel will disappear when viewed from certain angles, but may appear when viewed from others. It is important, when inspecting the sheet stainless steel, to view it from different lighting angles, preferably under the lighting conditions in which the final installation will be viewed.
The benefits of the directional finishes on stainless steels lie in the degree and form of light reflectance from the surface. The linear texture of the stainless surface created by the small ridges acts as a defraction grating. This property causes a perceived iridescence by scattering reflected light off the closely spaced ridges. The scattered light conceals minor surface imperfections on the stainless surface. The directional surfaces, such as the No. 3 and No. 4 finishes, reflect light at right angles to the grit line direction regardless of the light source.
No. 7 Finish
A No. 7 finish is a highly reflective surface. The grit lines are still apparent, but the surface starts to take on a mirrorlike aspect. This finish is produced by successive buffing operations on a finely ground surface. It must be polished through successive grits up to and including 320-grit silicon carbide. After the abrasive polish, a buffing operation is performed to develop the mirrorlike surface. Light is reflected from the No. 7 surface in intense hot spots, as in reflective glass or the more highly polished No. 8 finish.
No. 8 Finish
The mirrorlike surface of a No. 8 finish is produced by polishing a finely ground sheet of material with successively finer grit abrasives up through 320-grit silicon carbide. After abrading, the surface is buffed repeatedly with very fine buffing rouges.
Typically, there is fine "angel hair" visible in the reflective surface, particularly when viewed from an angle. The angel hair is very small grit lines that were not removed in the buffing process. Removal of these grit lines is very difficult with conventional buffing equipment. To achieve a flat surface, the back side of the polished sheet should he roughly ground and polished too.
Highly Reflective Surfaces
Japanese stainless steel processors introduced a mirror-surface product they call a No. 8 finish. Because of the superior nature of the surface, it is sometimes called a "super No. 8" or "No. 9" finish. This finish shows no grit lines whatsoever and has a reflective surface indistinguishable from a glass mirror. Currently there are no U.S. firms producing this finish. Japanese, Canadian, and Chinese sources are producing this finish.
The mirror-finishing process uses special high-speed buffing equipment with special rouges. This is followed by a thorough inspection of the surface and repeated buffing of areas that show any signs of grit or surface marking.
There are many other custom or proprietary finishes used to provide special reflective properties to a stainless steel surface. Some of these finishes can be intermixed to produce interesting reflective effects.
Selective coating of mirror-finished stainless steel, then applying a directional finish over the unresisted regions, followed by removal of the resist will produce contrasting reflective surfaces. Selective finishing can produce very decorative surfaces on stainless steel. By application of special resists to the surface of stainless steel, exposed regions of the surface can receive textured polishes by blasting or brushing. The resulting surface is highlighted by the contrast created by the different reflectivity of the textures. Fine detail can be achieved by using photoresist materials. The resist materials used are often rubber films cut by hand and carefully applied to the stainless surface, or plastisols applied by silk screen or by photoresist techniques.
Certain finishes, through the use of grinding wheels and stainless wire brushes, can be created on a stainless surface. For mechanical brushed finishes, definition between mirror and brushed transition lines is critical in producing distinctive designs. Brushed lines can he counterpoised to present contrasting appearances caused by light reflecting at different angles.
Using a grinder in a random fashion will, in the hands of a craftsperson, produce elegant surfaces of alternating swirls. The swirls have random biases in relation to one another and reflect light in a similar fashion, producing an illusion of contour.
These are wholly custom finishes, usually dependent on the abilities of the applicator. It is necessary to begin with flat 2B or 2D stainless steel. A light directional polish is applied to the surface by passing the sheet through a polishing roll. A grinder is applied by hand or by using a CNC-assisted frame, to control the movement and bias of the grinding head. As the grinder passes across the surface, small areas receive the texture in a slightly curved direction. Alternating the grinder disc edge will apply the curved brushing in similar fashion to a painter using a small brush. Larger swirls can be produced as well by multiple grades of disc grits. Small areas and prewelded assemblies can also receive custom swirl ground patterns. It is important to have control samples to work from if any level of consistency is required.
The "engine turn" custom finish is characterized by small circular lines, either in rings or in overlapping complete circles, applied over a mirror or No. 2 finish. The circles can range in diameter from as little as 12 mm to as much as 300 mm. The circles are applied by CNC-operated machines, which control a bank of spinning motors. The motors have a disc of silicon carbide imbedded in rubber or resin or in a block of the same material. While the disc is spinning, the motors lift the spinning disc from the surface and move it to some preprogrammed location, each time raising and lowering the disc to apply the decorative finish in repeating swirls.
A distressed finish is produced by randomly applying a scratch or series of scratches to the metal surface using a small grinding wheel. The application is sometimes performed by hand, using a wheel to scratch the surface in random patterns.
- Zahner, L. Architectural Metal Surfaces. New York: John Wiley, 2004.
- Zahner, L. Architectural Metals: A Guide to the Selection, Specification and Performance. New York: John Wiley, 1995.