I finally got some time to open up the outer layer and see the core. It was pretty wet and rotted around the scar mark from the prior repair - see the first picture. I scraped away all of the delaminated core, mostly by hand since it was so loose. Then I let it dry a few days to see if any of the wet but non-rotted core was salvageable. After a few days I removed any remaining core that had no structural integrity left even after being dried out. I then sanded it down to the inner skin, exposed about 2 inches of further core just to get a good bonding surface. I ground out a 12:1 scarf as best I could with a battery operated grinder. My current state is shown in picture 2, its not perfect but it might be as good as I can get it. The repair area is 35" wide, by 17" high, but in an odd teardrop shape. As you can see, there are two pieces of foam on the two ends of the repair area, both from previous work done on the boat before I owned it.
I've been looking locally for some foam for the patch, anything like Divinycell, Core-cell, Airex, Klegecell and even Coosa board. I can't find any of the foams available, but I did find a seller of Coosa board which looks way to stiff for me to shape to the hull. I don't want to use balsa because I haven't done an extensive enough repair to be convinced that water will not find its way back to this particular area.
I visited West Marine and talked to a staff member about where I could find foam core material and he mentioned that I could potentially just epoxy/glass the entire area and skip the core altogether. His recommendation was to first mix some 105 and 404 to get the desired consistency (which is a wild guess for me), then break apart some fiberglass matting with my hands and mix that in, and once that is all done, add in the 206 hardener. Use that mixture for the entire repair area, and when I'm ready to fare it out, use the 410 silica.
The drawbacks of skipping the core I can think of are (1) it will be harder for me to follow the curve of the hull with an even thickness of patch work, (2) I have to spend more money on epoxy, and (3) the repaired area will have a different amount of flexibility than the rest of the hull.
The benefits are that (1) I don't need to buy core, (2) I don't have to worry about shearing, (3) epoxy and glass is probably stronger than a glass/foam/glass sandwich, and (4) water penetration is no longer an issue.
All of the West System manuals show core replaced with core, regardless of how large or small the repair area is. But many new boats seem to be all glass below the waterline.
So I'm stuck trying to figure out which route to take. Anyone have some advice? Thank you!