I am suprised that putty works. It is mostly clay and some have fiberglass or other stuff mixed in to keep it strong. I used it a lot in Vaccuum sealing applications. Also I would not expect the putty to release air bubbles, which could be a problem.
I am kind of confused after looking at your picture though... It seemed like your Fish Finder screen was all clutered? Is that right? Mine is perfectly clear except where I would expect to see noise or Hopefully marks...
Good Luck
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