Need a quick opinion on glassing over old thru hull

[vetting]

Trojan boat with an inner liner and 1" thick hull.
Removed a super old depth sounder that had a wood fairing block on the outside to make it level. The thru hull is 2". Owner wants to replace it with a 3/4" thru hull for a washdown. The hull is all glass/composite. I am going to cut back the inner liner to a total of 6" in order to glass down the 4.5" seacock mounting pad. Should I dish out the outside 12" in each direction or go less? Should I make a 1/2 thick disk and epoxy it in the existing hole so I'm not dishing the entire inside/outside down to a thin spot?

 

[DeepBlue2010]

I wouldn't do it this way. Cut a plug out of marine ply. Get it soaking wet with poly and secure it in place with 5200. Grind this down just enough for two layers of 1708 with the smallest first. Next, either drill and cut the same hole again and install your plates or relocate the new hole to a nearby suitable place if you are worried drill a patched hole. This is what I would do anyway. Take it for whatever it is worth

 

[vetting]

I wouldn't do it this way. Cut a plug out of marine ply. Get it soaking wet with poly and secure it in place with 5200. Grind this down just enough for two layers of 1708 with the smallest first. Next, either drill and cut the same hole again and install your plates or relocate the new hole to a nearby suitable place if you are worried drill a patched hole. This is what I would do anyway. Take it for whatever it is worth

Here is what I ended up doing. Made 1/2" thick plugs in various sizes out of layers glass and epoxy. Then with thickened epoxy, set the plug in the middle thickness of the old hole and braced it in place while the epoxy setup. The next day, took a flap disk and dished out a 1/4" on the inside and outside. Then laid 5 layers of glass with epoxy going small to big circles on the inside and the outside. Once cured, faired it out with TotalFair and covered with Interprotect 2000e. Then installed the backer plate with thickened epoxy and one layer of glass on the inside. Then drilled the new hole and installed the new strainer and seacock.