Downwinder - I was also a newbie at this about a couple months ago but no more. I am in the middle of replacing all of the balsa in my salon
First, you must find the source of the leaks
(if you have one, you likely have several) and stop them. See the excellent write-up in MaineSail's page on how to properly seal mounting holes. Some of the balsa had turned almost to powder, some just falling apart and some looked good but had lost
the adhesion to the outer wall. Be brutal, take anything questionable out. The balsa between the layers stayed wet for a long time. then I took a large wire wheel
and cleaned the surface as best I could.
Next (and I am not saying this is the best way), I laid out the new inner wall on a sheet of PE film on the salon
table, cut out the new balsa, glued them up and let them cure.
Then I mixed up some thickened epoxy
, spread it on the new balsa and clamped it up in the window openings, using longer 2x2s to keep the now quite flexible walls straight. After curing, they are quite strong and with the inside finished surface nice and smooth, turned out well.
The decks will be the last area I tackle, I wanted to get good at this first as all of my decks have a curve to them. I do not think there is any quick easy way to do this correctly!