The short version of my suggestion is to peel the skin, leaving the balsa intact.
The reasons that I am suggesting that the the kerf vacuum heat method be abandoned are as follows:
1. It's taking a lot of time and effort.
2. There will be a lot more scarfing with this method, on both sides of the cuts. While fairing back repairs consisting of 3 or 4 inch wide cuts beveled out to the approximate 12:1 ratio (4.75 degrees) is clearly easier than fairing in whole repair sections, this scarfing takes a lot of time and effort in itself. (Note: Scarfing is necessary whether or not the whole bottom is covered in additional layers of glass since to perform a proper repair and restore original strength, the resulting laminate should have approximately similar thickness and strength across the surface to prevent excessively stiff areas and nearby small weak areas / lines. See
EpoxyWorks.com for more on this.)
3. Even if you could get it dry enough (i.e. less than half scale on your meter), there is risk that lowering the pressure and adding heat has caused the lowered water's boiling point to produce expanding vapor which may have pushed the FRP away from the balsa. This would decrease the bond between the two and add small capillaries for future water intrusion. This could be why chapter 5 of the West Systems
002-550 Fiberglass Boat Repair & Maintenance Manual warns against vacuum combined with temperatures beyond 130 degrees F even though this temperature would seemingly be far below the glass transition temperature.
Now on methods to get this accomplished. Given that your sounding has revealed that you have little or no delam in general, and that you already have a decent heat source, you could try a peel with heat, wasting the skin in a small area. To go about this, I would suggest that a rectangle be created with saw kerfs, but not all the way to the corners. Use a multi-tool saw or similar to create additional kerf cuts to make the rectangle have rounded corners. Next, create a thin metal heat mask of about the same dimension as the skin you are trying to remove, place the heat close enough that it heats this rectangle just above the glass transition temperature, which will be far above the boiling point of water. Once the skin becomes gooey, the resulting steam will bubble the skin up in places, making it easy to remove with a paint scraper, but leaving the core intact. Once that is done, rough up the surface somewhat to break the resin seal, dry overnight with some light heat and airflow, replace any small sections of rotted balsa, scarf the outer edges and then laminate on a new skin. Luckily, this high heat method goes fast, forcing you to be present to prevent very possible fires if the FRP sags into the heater or the FRP gets too warm!
Alternatively, if you find areas of badly decomposed balsa, you could do the opposite - save the skin and replace the balsa. While texts on the subject talk about peeling the skin and reusing both, this is very hard to do if you don't already have significant delamination.
There is yet another alternative. This method is basically creating a sea of holes, then using vacuum and 130 degree heat to try to dry the core. This is also described in chapter 5 of the above. I hesitate to bring this up since I have now heard from two people who did this without really dropping the moisture level significantly. However, they both were comforted by the fact that all of the holes, once filled with thickened epoxy, seemingly acted as nails between the two skins to restore some of the strength in the area. It also has the added advantage that scarfing is not required since small holes don't create big weakness areas / lines.
Good luck!
-Chris
