A mill finish is the unfinished appearance of a material after machining, often dull and grainy. It contains slight oxidation and can be used for a unique look when painted or sealed. Cleaning is necessary before applying sealant or adhesive.
A mill finish is typically the finish a material has when it comes out of the mill where it is being machined. Most metals, such as aluminum, are polished after they leave the mill. This leaves the materials bright and shiny and removes any residue that may be present. A mill finish, however, will often be dull, grainy, and lack some sheen. This is often referred to as an unfinished look.
Any aluminum with this type of finish will contain a slight amount of oxidation. This is a feature of all extruded or rolled aluminum. The steel will also contain a slight oxidation when it comes out of the mill and typically undergoes a chemical scrubbing to get rid of that unwanted finish. Stainless steel, for example, will not be polished brilliantly when it comes out of the mill and will have a dull to even black dull appearance as it is machined.
Builders often wish to incorporate a mill finish into a design that is traditionally completed with a chemically finished or machined appearance. This offers a more robust look and allows the builder to design a unique end product for the project. When you plan to paint or seal the material, the finish will have a slightly uneven surface which will hold the finishing agent in varying thicknesses on the surface, giving a unique look when dried.
Using this process, a builder can achieve different looks in a design without the bonding and coexistence difficulties associated with joining different materials. A uniform method of attaching the pieces of a product together can be used with different finishes to the various parts using different levels of finish. While aluminum can be anodized, aluminum with a milled finish does not need to be sealed once it is anodized.
If you plan to apply a silicone-based sealant or adhesive to a finished milled piece of aluminum, the surface must first be cleaned of any residue from the extrusion or lamination process. This residue will only be removed by scrubbing with a mild chromate or phosphate etching material. Simply wiping the resin with a finger will result in the residue reforming on the surface of the material. In some applications, such as airplane or boat manufacturing, raw materials are assembled in large subsections and then immersed in a phosphate bath to remove residue left over from the extrusion process.
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