Peter, the time of etch in the Caustic bath is a key factor and then the time of replacing what has been taken off in the next bath, what process are you going to use?
If you just etch and then dye it that will not be any good. You have to use a process that is commercial.
Find a local Anodiser, makes it much easier. All he will do is add it to his next batch of heavy anodising.
Clive