A Quick Guide To Stainless Steel Finishes For Process Tanks & Equipment

A Quick Guide To Stainless Steel Finishes For Process Tanks & Equipment

Stainless steel finishes are more than how shiny or dull the stainless steel appears to be. Having the appropriate finish for your application will reduce or eliminate contamination between batches and can facilitate more efficient cleaning out of tanks. Ultimately, knowing what your process demands will determine the stainless steel finish you choose for your tank. It is also important to consider your needs inside and outside of the tank. In many applications, the exterior only needs a basic finish while the wetted (where your process liquids make contact with the stainless steel) surface requires a highly polished finish. You will see referenced “RA (roughness average)” to measure the microscopic peaks and valleys of the surface. The higher the number, the larger the imperfections allowed. Below are the most common finishes we use on our stainless steel tanks and impellers. 

Mill Finish

A mill finish is how most stainless steel comes from the factory to the manufacturing floor. Mill scale remains on the surface from the rolling process, giving a slight discoloration to the stainless steel. Weld beads are buffed to remove discoloration, but not smoothed or polished. This finish is commonly used in industrial applications where appearance or wash-ability are less critical. A 250RA maximum is tolerated.

2B Finish

The 2B stainless steel finish is one of the most widely used finishes across the board. Its matte finish is dull in appearance but it is milled to be smooth and many of the larger pits are polished out. The 2B finish is inexpensive and multi-functional. All sharp edges are ground down and finished cleanly, but the welds are still exposed and rough. 

2BG Finish

A 2BG finish is a slightly more refined finish than 2B. The 2BG finish includes all the features of the 2B with the addition of ground and polished welds. This finish is used in applications that require cleaning between batches, as contaminants can be left behind in the bumps of an un-sanded weld. 

#4 Finish

A #4 finish is a highly polished finish with ground and blended welds. It is the finish you see commonly on household appliances and used extensively in food service and production. The entire surface of the product is polished with the grain of the stainless steel, typically using 180 grit sandpaper. This finish is measured by RA and will not exceed 32RA. Casting pits must be less than 0.016 inches. More expensive than the 2B & 2BG, the #4 finish comes with many advantages that make the investment worth it. It gives the stainless steel an attractive and uniform satin finish and comes cleaner faster and easier than lower quality finishes, making it widely popular in sanitary applications.

#4P Finish

A #4P finish has the same standards as a #4 finish, but has a lower tolerance for casting pit size. The maximum pit size allowed is 0.002 inches. This finish would be most commonly used for food and beverage production and pharmaceutical applications, as bacteria and contaminates can get left behind in the microscopic pits. 

#6 Finish

The #6 finish starts with a #4P finish but is polished again with a minimum of 240 grit sandpaper, giving it a smoother and less textured appearance than a #4P finish. The maximum allowed RA for this finish is 18RA. While the stainless steel grain is still visible, it is shorter and thinner and begins blending tighter into the whole surface. This finish is less common and more expensive, but results in an attractive satin sheen and an even easier surface to thoroughly clean.

#7 Finish

The #7 finish is a reflective finish that is created by buffing a surface after finely sanding and polishing with 320 grit. The grain lines are still visible with this finish, but the maximum RA accepted is 12RA, leaving a smooth and shiny finish. 

#8 Mirror Finish

A #8 finish is the highest quality finish we offer, and is considered a mirror finish. Stainless steel tanks with a #8 finish are polished with fine 400 grit sandpaper and buffed with a polishing compound to create a brilliant high lustre surface. The grain lines are barely visible and can be further buffed out to create a “super-mirror finish” using 500+ grit sandpaper. RA tolerance gets very small in these finishes, and typically range 3-8RA depending on how brightly a mirror finish you require. While less common than other finishes listed below, mirror finishes can be a worthy investment up front. For those in need of an aesthetic option, this finish makes for an attractive and durable finish.

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