The Type III is a far superior anodizing ...
Anodizing is a very misunderstood suface treatment for aluminum and aluminum alloys. Type I, II and III anodization processes all oxidize an aluminum surface to aluminum oxide to protect it from further oxidation. The ultimate difference between the three types of aluminum anodizing processes is the thickness of the protective aluminum oxide layer.
Type I anodization makes an oxide layer that is ~0.0001" thick; Type II anodization make an oxide layer that is ~0.0005"-0.0015" thick; and Type III anodization makes an oxide layer that is ~0.001"-0.005" thick. (The oxide layer thickness of a Type II and Type III anodization depends both on the alloy composition and the processing time.)
Type I anodization uses the lowest voltage and thus makes the thinnest oxide layer. It is primarily used when some surface conductivity is desired and little surface wear is expected. A good example is the exterior surface of spacecraft. The low process voltage limits the oxide layer growth to ~0.0001" which doesn't require changing machining dimensions to insure adequate thread clearances.
Type II and III anodizations uses a higher voltage than Type I and produces a thicker oxide layer that is an electrical insulator. The "as machined" dimensions of threaded parts must be modified (OD machined smaller, ID machined larger) to accomodate the expanded oxide layer.
The principal difference between Type II and III anodization is that a Type III process uses a higher voltage to obtain a thicker oxide layer and requires a refrigerated plating bath to accommodate the higher plating currents. Since aluminum oxide is not conductive, the oxide layer thickness is proportional to the applied voltage and to obtain a thicker Type III oxide layer the temperature must be lowered to prevent the finish from "burning".
Regardless of Type, the oxide layer is clear, however it is porous. To color the surface, the anodized part is soaked in a dye solution that fills the pores and immersed in boiling water to close the pores sealing in the dye. Type III processing is harder on dyes and thus is more restrictive in color choices than Type II anodization.
Also, the cost of the Type III is more expensive than a Type I.
Because it requires refrigerated plating tanks and uses higher voltages and currents, a Type III process requires more equipment and electrical power than a Type III process, and thus is more expensive than Type II precessing. You can reduce the Type III process time to obtain a Type II oxide layer thickness (which is what most rocket folks are doing), but why bother to use the more expensive Type III process to get an equivalent oxide thickness?
Bob