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Cadmium plating (Cad) is applied OVER the Zinc base. There is YELLOW cad (most brake boosters) and WHITE cad (Zinc appearance). As mentioned, Cad is applied to increase durability of the Zinc. Polish the zinc plated part before cad and you'll have shiny cad. Bead blast or wire wheel and you'll have a more subdued cad. Etc.
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Not many plater's apply Cad over zinc.
I think you are confusing the "YELLOW" Zinc (/or Cad).
Finish TYPE II (ASTM-B633)
Zinc plating with the yellow Tint is obtained via the conversion coating process by post plating treatment with (di)chromate acid. The length of time the item is immersed in the solution creates the color, in addition to the pre plate surface condition prior to plating as you stated. Think of the Yellow zinc (Cad) as a additional layer of protection from crossion and abrasion. (Example: The thick layer of skin on Pudding left in the fridge UNCOVERED!)
These additional treatments are intended to provide some corrosion protection or a better surface finish for additional coatings (Could be CAD?).
I had purchased a caswell set up and could only get smaller parts plated at home. I still prefer to send them out to vendors who are willing to run small batches of zinc.
I work near a plate shop for the NAVY.
To many fumes for TODAY!
Dave is Right Get your tanks set up soon!
Plate on!
Jim