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I can only speak for a 22' 1979 Revenge (AKA Outrage). I had a deep chip that looked like wood starting about a heavy 1/8th inch down. Working on other interior parts of the boat I was amazed of the thinness off gel coating for such a sturdy surface.

If it were my boat I would not assume the worst of the entire transom. If you have exterior gel coat damage by the rotted tube areas, I would gut out that area and see just how far the the damage extends.
Garris