You're right about that layer of resin. Mine was solid except for the self induced chisel spot that I caused.
I ground and filled that area. (see pic of hole above.)
I lost a lot of pics, so the exact sequence has some pics missing. These are pics that I scarfed off my phone.
I did the same "Thru Hull" 2x4 clamp method on the first board that you show in the pics. I ran one top to bottom through the 2 drain holes to clamp the upper and lower halves as one.
I did some wedge 2x4s from the floor to the transom also.
I pretty much took advantage of every Swiss cheese hole that caused the damage in the first place. I figured I was going to fill them with epoxy anyway.
I used West System Epoxy on the whole transom process.
I use Total Boat now because of this guy.
All the years I've been farting around boats, he'll forget more tricks than I will ever know.
Andy is wise beyond his years:
https://www.youtube.com/watch?v=Zka0i0SOGd8&t=2sBack to the transom
Two things that I've noticed with these boats that has been brought up by others (Besides being well built and great design)
1. Is the thin bottom at the bilge / transom area.
2. The rot is always the same on every replacement thread on relation to the bilge and well drains. - All the other holes depend on the due diligence or lack thereof on sealing the holes they made.
I noticed the thin areas when I had my head in the bilge and a trouble light was on the floor.
Now don't get me wrong I know you can have very strong fiberglass that you can read a newspaper through but this is 1975 technology.
If you've been in any "Boat Factory" it's not very impressive.
Given the growth that Glastron enjoyed during that decade, production was a priority and "Total Quality / Continuous Improvement" hadn't hit our shores yet. Maybe there were inconsistencies here and there. On the whole though,these are better built than some modern boats.
Boats that were not meant to last more than 10 years are going on their 5th decade.
On to the Transom fix:
The drain hole is up off the bilge floor to compensate for the bilge / transom transition.
Thin areas beneath where the new wood is located.
Epoxy / Kitty Hair /Cabosil /Wood Flour in different combos.
The 11 ply was sealed, then "glued" to the transom with slightly thickened epoxy / cab mix and 2x4 clamped for a couple of days.
Each layer was troweled with a 1/16" notched trowel epoxy cabo WF mix. (PB mix)
The area below the wood at the transom / bilge joint was slathered with the pb mix first and then the Kitty Hair mix was mashed in the area with a 1x1" stick, fingers (Ouch!) and a big ole screwdriver. Then I mashed the PB mix over top.
With the epoxy, the working times are longer so I could take my time.
I'll cover the drain later. enough talk.