When you start with delaminated material, you will have no idea how large the repair will be. Just start with what is visible. From there, you will just follow the damaged material until you get back to solid fiberglass. Here, the objective was to get to the fiberglass. First, I gave up making repairs with polyester resin decades ago. Everything is done with epoxy, but even epoxy won't adhere to soft filler materials. The cracks were followed with a Dremel tool until there was no further evidence of crazing or delamination.
The delaminated materials are ground out until we are again to solid fiberglass. If you notice, at the right end, there is a cove where the liner has chipped out. This is a clear sign you aren't done. I slid a knife under the liner and shipped away loose material, and continued grinding for several more inches.
This was a larger damaged area in the stern. You see the same abrupt edge at the right end, again showing that we're not done, and more loose material needs to be chipped and ground away.