I worked in an elctro plating shop when I was younger. The anodizing process is basically controlled rusting. There are three levels of anodizing. The first two are cosmetic and the last one is protective. Type I is very thin and mostly seen on cheap aluminum furniture and fixtures. It is usually clear. Type II is thicker and is what you generally see on automotive and motorcycle applications. It can be applied as a clear coat or dyed with a significant of colors. Type III is thicker and designed for components that will see significant wear. This is Military Specification grade and is more expensive. Any plating process may change the physical dimensions of the part being plated. Oxidation and Reduction - you can't have one without the other as my chemistry professor use to say. So you lose a little metal from the part being plated and hope you replace it with the same plating material. This is in microns and usually is not a problem except things like fork tubes or hydraulic cylinders which must be exact to work correctly. Any anodized surface can be scratched and any that have been dyed will eventually fade.