What does mill finish of aluminum mean 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