Pics. I contacted Gougeon Brothers, who make West Marine epoxies and talked to their technical department. I described what I was doing and sent photos. My question was that typically a 1:12 bevel is mentioned for filling a hole, but in this case the thickness varied from about 3/8 to 1/2" so a 1:12 would be 4-6" and I just didn't want to disturb an area that big if I didn't have to. The rep confirmed that I did not and that the approach I took would be appropriate. After treating it with Naval Jelly to neutralize the rust, rinsing and drying it I ground it back with an angle grinder.
After cleaning it and wiping it down with acetone I treated it with unthickened expoxy, as recommended, to ensure proper bonding of the additional laminates. I then laid it up with alternating layers of chopped and woven mat until I was level with there surface wetting each layer out with unthickened West 105 with 205 slow hardner. It was messy, but I had cut out all the layers first and it went quickly. The "laminate" photo shows the result. Note that I greased and re-inserted the bolts and they came out fine, although I did chase the holes with a tap when I was done to clean them out.
The hole in the deck you see is the enlarged hole for the VHF connection which was the original culprit for water entry. I ended up masking off at the start of the non-skid and sanding the top down to it with 100 grit. Then I thickened epoxy with cabosil to a peanut butter thickness to fair the plateau (filler photo). I overdrilled the VHF hole and cleaned it out as best I could with the old allen wrench in a drill trick. I also dumped some acetone down inside and let it dry for a few days as I had to wait a couple of days from laminate to filler and from filler to gelcoat. It takes quite a lot of the cabosil to thicken the epoxy and I quickly learned to make many small batches (~100mL of epoxy) or the exotherm would just harden it right up. I ended up dumping about a cup of thickened epoxy in the VHF hole.
I sanded the filler with 100 grit, per West's instructions to give the gelcoat plenty of tooth. That is necessary to use gelcoat over epoxy successfully. I bought a gelcoat matching kit and I can't recommend the boatworks youtube channel enough for what I learned, especially about gelcoat matching.
which takes just drops of each color to make a difference. Get your color right then add the hardener. I had it really close, but realized I needed another coat. Rain was threatening so I unfortunately rushed the second match and blew it. But I really needed a third coat. The shoulders of the plateau are damned easy to sand through, and in the final pic (IMG-1394) you can see where I went through a bit. As you can see, I didn't go crazy getting the area under the plate pretty.
One of the holes wasn't very straight from the factory and even the dimension of the plateau was a little off even from the factory so when I bolted the stainless plate back on (with a generous quantity of sealant under it) it looked a bit askew. Saving the original wires turned into more than a wasted effort. I ended up having to removed them anyway and doing it at the end was a pain. The original wiring was crap and not only that, I found butt connectors inside the wall cabinet in the head. So much for careful wiring by S2!
The great news is that with the mast back up the divot in my deck is gone. I am also now 100% sure of the integrity of the boat and how the mast is supported. I also found no other major damage, nor did the surveyor. The hole for the VHF was right in front of the bulkhead inside the cabin. I can only surmise that the small area of wet core was just in exactly the wrong place allowing about a half inch of compression right above the bulkhead.