Unlike paint from a paint can and baking ingredients the custard from the pie alone on a zombie's face does not temporarily disorient them and prevent them from attacking for around 15 seconds.
It is rather difficult to accurately hit several zombies consecutively in the face so the pie will attach. When Frank hits a zombie, the zombie becomes aware of Frank, where as the zombie may have been ignoring Frank before.
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Incapacitated zombies when Frank scores a direct hit