I have reservations about that product. It seems kind of gimmicky to me. Galvanizing is essentially plating a ferrous metal with zinc, and is normally accomplished in one of two ways. Hot dip is almost self explanatory. The part is literally dipped in a vat of molten zinc until the zinc is bonded to it, then removed and the excess runs back into the vat. This leaves a fairly substantial protective layer on the part. When people talk of cold galvanizing they're usually referring to electroplating, which generates very little heat but also leaves a much thinner protective layer.  I just can't see how this stuff in a spray can can stand in for either of these methods, but I've been wrong before. I'll be interested to hear how this goes for you.

I have a spare NOS agitator shaft for my A700, and in looking it over I can't see any evidence that it has anything but a coat of paint on it. In the few places where the paint is chipped away the underlying metal has the dark appearance of raw steel, not bright the way galvanizing would be. So was paint all the protection it needed, or am I overlooking something?

There are at least several ways to do this, we have had good luck with Two-Part Epoxy that are mixed and painted on, products like Por-15 are probably also good.

You can also get a length of heat-shrink tubing like is used to repair vacuum cleaner hoses and just heat-shrink it to the outside of a clean primed center-post [ just be sure to carefully seal the HST at the top and bottom with an RTV type sealant ].