Stainless steel finishes have more significance than just the shine or dullness of stainless steel. Proper surface preparation for your specific application can reduce or eliminate contamination between batches and facilitate more efficient cleaning of tanks. Ultimately, the choice of stainless steel finish for your tank depends on understanding your process requirements. It is also important to consider the needs both inside and outside the tanks. While a basic finish may suffice for the exterior in many cases, the wetted surfaces (where the process fluid comes into contact with stainless steel) require a highly polished finish. The term “RA (Roughness Average)” is used to measure the microscopic peaks and valleys on the surface, with higher numbers indicating larger allowable defects.
A mill finish is the default finish of stainless steel when it leaves the factory for the manufacturing floor. The surface retains mill scale from the rolling process, resulting in slight discoloration of the stainless steel. Weld beads are buffed to remove discoloration but are not smoothed or polished. This finish is commonly used in industrial applications where appearance or washability are less critical. A maximum tolerance of 250RA is accepted.
The 2B stainless steel finish is one of the most widely used finishes in various applications. It has a matte appearance, but the surface is milled to be smooth, and many of the larger pits are polished out. The 2B finish is cost-effective and versatile. All sharp edges are ground down and finished smoothly, but the welds remain exposed and rough.
A 2BG finish is slightly more refined than the 2B finish. It includes all the features of the 2B finish with the addition of ground and polished welds. This finish is suitable for applications that require cleaning between batches, as contaminants can be trapped in the bumps of an unsanded weld.
A #4 finish is a highly polished finish that incorporates ground and blended welds. It is commonly seen on household appliances and extensively used in food service and production. The entire surface of the product is polished in line with the grain of the stainless steel, typically using 180 grit sandpaper. This finish is measured by RA and does not exceed 32RA. Casting pits must be smaller than 0.016 inches. Although more expensive than the 2B and 2BG finishes, the #4 finish offers several advantages that make the investment worthwhile. It provides an attractive and uniform satin finish, cleans faster and easier than lower quality finishes, and is widely popular in sanitary applications.
A #4P finish adheres to the same standards as a #4 finish but has a lower tolerance for casting pit size, allowing pits no larger than 0.002 inches. This finish is commonly used in food and beverage production and pharmaceutical applications, where bacteria and contaminants can be trapped in microscopic pits.
The #6 finish starts with a #4P finish but undergoes further polishing with a minimum of 240 grit sandpaper, resulting in a smoother and less textured appearance compared to a #4P finish. The maximum allowed RA for this finish is 18RA. While the stainless steel grain is still visible, it becomes shorter and thinner and blends more seamlessly into the overall surface. This finish is less common and more expensive, but it provides an attractive satin sheen and a surface that is even easier to clean thoroughly.
The #7 finish is a reflective finish achieved by buffing the surface after fine sanding and polishing with 320 grit. The grain lines are still visible but the maximum accepted RA is 12RA, resulting in a smooth and shiny finish.
#8 Mirror Finish
A #8 finish is the highest quality finish available and is often referred to as a mirror finish. Stainless steel tanks with a #8 finish undergo polishing with fine 400 grit sandpaper and buffing with a polishing compound to create a brilliant high-luster surface. The grain lines are barely visible and can be further buffed out using 500+ grit sandpaper to achieve a “super-mirror finish.” The RA tolerance in these finishes becomes very small, typically ranging from 3-8RA depending on the desired level of mirror finish. While less common than the other finishes listed below, mirror finishes can be a worthwhile upfront investment. For those seeking an aesthetic option, this finish provides an attractive and durable surface.
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