Most anodizing on machined parts are of two commonly specified types: Type II (or “regular”) sulfuric anodizing and Type III Hardcoat or “hard” sulfuric anodizing. The Type II & III designators come from the common Military specification, MIL-A-8625 (there is also a Type I anodizing, but this is done using environmentally-unfriendly chromic acid, and it is rarely specified these days).
Both Type II and Type III “hard” anodizing are very hard coatings. Type III “hard” anodizing is done at a lower temperature, is more expensive, and a little harder than Type II, but you need special equipment to tell the difference in practical terms. Type III, however, is much thicker than Type II, typically .002″ vs. .0006″ respectively, which makes it more resistant to scratching and heavy wear.
There is another type of anodizing, commonly called “architectural” anodizing. It is essentially the same as Type II anodizing above, but uses metallic ion dying which is completely colorfast in sunlight.
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