Zinc is a sacrificial coating applied to steel.
Dichromate is a sacrificial coating on zinc.... kind of like phosphoric acid is with steel. Zinc itself is pretty fragile, especially to acids.
When you see the yellow fading that is the dichromate coating pretty much used up, when you see the part become whitish and hazy that is "zinc rust" which is kinda how it dissolves itself. You won't see steel rust until the zinc is gone. This is why it's such a neat coating. If the part was plated with lets say nickle, as soon as the coating cracked the steel would rust and lift off the remaining coating. For an engine part this would be a bad situation.
As such, using zinc to "build up" parts isn't a great idea, because by it's very nature, zinc is meant to dissolve, which if it was determining the size for a part as soon as age set in the part would leak or not fit right. You shouldn't really be coating anything thick enough to change the size appreciably.
If you need to redo something like a throttle shaft that doesn't fit, you either need to build up the base metal (steel) or you need to drill/bush/ream the throttle shaft hole (AED has a kit).
Anyway, none of this has to do with the original poster's question, so I'll wander off for a bit.
