Tuning question

The last thing I do before I put my skis away is coat them with base
prep. I make sure to cover all the edges, it is done when the skis are
dried.
Every year there is rust in the front or top of the ski where the edges
do not make contact while running flat referred to as the shovel section
.. Normally you detune this area to prevent hooking.
It is surface rust but the skis were stored indoors and no other part of
edges has rust on it.
Any explanation?