I can only speak to what I had to do to my hulls. I picked up a 1980 18 that had been sitting stearn end in the drink for a long time, the rear transom and the first 2 feet of hull bottom were soft. What I had to do was drill 1/8" dia. holes approx, every 1 sq. inch in order to drain existing water and that took about 3 weeks and I do mean three weeks. Then I mixed West System epoxy with a small amount of filler and began to inject this into the holes. What I wanted to do was back fill behind the holes with material so as to build a solid surface inside the hull. Then for the hole fill stage I filled the holes with same mixture not to much powder and covered the holes with wax paper. After dry time sand then two step urea paint with primer.
That part of the hull is stronger that new without to much weight add on.
Remember let the hulls and holes dry out before any fill work.
This works however I had to break down the boat in order to handle the hulls movement required to do this work in stages and in small areas. The fill work needs to be done alittle bit at a time or it gets sloppy and the mixture can get warm if to thick.
Hope this helps
Harvey 1979 16. 1980 18( in mint shape )