"Can hardcoat anodizing be reprocessed?"
The short answer is yes, however Type III anodizing can present a number of potential issues to consider before reprocessing parts that have been hardcoated:
Dimensional Change: Reprocessing hardcoat first requires stripping off the old hardcoat anodizing. Hardcoat anodizing causes dimensional growth on parts, typically .002 inches thickness. And, hardcoat creates a 50% outward growth and 50% inward substrate penetration. Removing hardcoat anodizing will take .002 inches or more off of each surface, or over .004 inches off a dimension. Reprocessing will add back .002 inches to the dimension but it will end up being .002 smaller than it was prior to stripping. For many parts and applications this is unacceptable. This means the hardcoat process has to run longer in order to build up to the previous required dimension and it is a more complicated process to control.
Surface Finish: In addition to dimensional changes, stripping off the old hardcoat will always degrade the surface finish to some extent. It will become noticeably more matte or dull from the etching process. The amount of surface degradation varies with the aluminum alloy. 6061 alloy shows the least amount of degradation. 7075 and 2024 alloys are typically the worst, sometimes even producing noticeable pits in the surface.
Hardness: The etching process used to strip the hardcoat anodizing on a part removes some of the magnesium, copper, or silicone alloying elements leaving the surface porous. This porous surface will be softer and less wear and scratch resistant.
Main Takeaway: Our general advice is to not strip and re-anodize hardcoat finishes if at all possible. However, there are some types of parts that will do well with reprocessing. We recommend consulting with our technical staff.
See also "Can .031" 3003-H14 sheet aluminum be hardcoat anodized?"