What does 'mill finish' of aluminum mean?

"Mill Finish" is the natural appearance of the aluminum as it comes from the rolling mill or the extrusion mill. It is "as is" with no external mechanical or chemical finishing. Extruded metal is considered "mill finish". All aluminum has an oxide of some varying thickness. Anodizing is a very heavy controlled oxide. Anodizing is an electrolytically formed and controlled heavy oxide 0.0003 inches thick on up the 0.002 and on up. Mill Finish is a very lightly oxidized film and will wipe off with your bare finger and immediately form. Rolled sheet would probably have a thinner oxide than hot extruded aluminum. If you want to bond silicone adhesive then you want a phosphate etch, or a light chromate or a thin (.0004 anodize in either chromic, sulfuric, or phosphoric.) If you anodize, do not seal the coating and be sure your anodizer knows you do not want it sealed.