Here is my experience. Since gel coats are porous the gel sucks up the polish oils and makes it look like there is improvement. With oxidation that bad I suggest wet sanding to remove all the dried gel coat layers. You can also use 3M super duty compound, but I think you need to sand with
Unigrit. A boat that bad I would find a air sander. 105 is a great follow up to sanding or 3M, but is whimpy to start with.
Cheers,
Greg