You may find that some of the fiberglass won't come off cleanly. Or if it does it means you haven't removed enough area to get back to solid core. I did a similar job from underneath on the side decks but used 1/2 inch balsa core material from Jamestown Distributors along with 1/2 G10 plates under deck fittings. As boat projects go it was technically easy, but quite messy. Line everything with plastic.glue the fiberglass layer again,
Yes, PPE and protecting the interior are very important. Having done this I don't envy you one bit.
I also used balsa since it seemed to have held up for at least 40 years. Mapping out each area with a 1/8" drill bit and a Sharpie. As mentioned above, it's doable but messy. I wouldn't get too hung up on saving the old skin, this could be a great time to learn about vacuum bagging. If you can do it upside down, you can do it anywhere!
I have no experience with injectadeck other than videos lamenting its use when it comes time to do the job over with a solid core. As far as expanding foam goes, again no experience but it seems to me that one could end up with a bigger project than what they started with if they weren't careful. If I had to do it again I'd use balsa, (there's a chance the right person could talk me into foam), and perhaps plywood or G10 in a few strategic places.
Mind you these are just opinions, I'm sure there's any number of cases where the alternatives have worked out just fine. Good luck!
yo convinced me to go with the proper repair.I used vacuum bagging on my port side repair, but decided not to use it on the starboard side. Vacuum bagging requires a perfect seal for it to work and you can only achieve that by sealing against the top skin (assuming you are going from below). If you try to seal against the bottom skin, the air will travel through the good core and escape through the staple holes. On the port side, I installed the bulk of the new core with vacuum, then filled in the space between the new core and old core using extendable drywall sanding poles to push up. At the end of the day, our boats aren't super light racers, so a little extra resin between the outer skin and new core isn't going to hurt anything. Here's a some more detail on my experience:
The first step with replacing the core was identifying justhow much core was rotten. As you canexpect, the affected area was greater than I initially expected. The void along the edge and between the topskin and balsa created by not spreading...ericsonyachts.org
Yes, I cut into the laminate while it was on the hard, but the rest of the job was in the water. The Mast was up the whole time.