A mill finish is typically the finish a material has as it exits the mill where it is processed. Most metals, such as aluminum, are polished after they leave the mill. This leaves the materials bright and shiny and removes any residue that might be present. A mill finish, however, will often be dull, grainy and without a certain luster. This is often referred to as an unfinished appearance.
Any aluminum with this type of finish will contain a slight amount of oxidation. This is a trait of all extruded or rolled aluminum. Steel will also contain a slight oxidation as it exits the rolling mill and typically undergoes a chemical wash to eliminate such an undesirable finish. Stainless steel, for example, will not be brightly polished as it exits the mill and will have a lackluster dull to even black appearance as it is processed.
Builders will often wish to incorporate a mill finish into a project that is traditionally completed with a chemically finished or a machined appearance. This offers a more rugged appearance and allows the builder to design an end product that is unique to the project. When planning to paint or seal the material, the finish will have a slightly irregular surface which will hold the finishing agent at different thicknesses across the surface, resulting in a unique appearance once dried.
By using this process, a builder can achieve differing appearances in a project without the bonding and coexisting difficulties associated with marrying different materials. A uniform method of attaching a product's pieces together may be used with several different finishes to the various parts by utilizing different levels of finish. While aluminum can be anodized, aluminum with a mill finish must not be sealed once anodized.
If planning to apply a sealant or a silicone base adhesive to a mill finished piece of aluminum, the surface must first be wiped clean of any residue from the extrusion or rolling process. This residue will only be removed by wiping with a light chromate or a phosphate etching material. Simply wiping the reside off with a finger will result in the residue re-forming on the surface of the material. In some applications, such as airplane or boat manufacturing, the raw materials are assembled into large sub-sections and then dipped into a phosphate bath to remove the reside left from the extrusion process.