Hello.
I'm sure i'll be corrected if i'm wrong.
I believe that in most cases that zimmerit was a factory applied coat which would have been applied after the base coat of red oxide primer.
If the damage to the zimmerit was old then the tank may have been repainted and so the damage would be the same colour as the rest of the tank. (obviously).
If the damage was fresh or no new camufladge had been applied then the dmage would go through to the base primer coat.
I'm unsure how 'corrosive' the zimmerit would be to the primmer. I have seen quite a few kits where the damage zimmerit is left at the primer coat.
Post some pictures of your kit so we can see the progress! - good luck!