My 1986 21 ft Mirage power boat had extensive rot through the complete substructure, including the transom.  All rotten wood was removed and replaced with Okumi marine plywood, and fiberglassed with cloth and MAS epoxy resin.  I used both Medium and slow curing systems.  Also used MAS wood flour to create all my fillets.  The resin system worked extremely well, and no sanding or cleaning of excess amine was required between cured layers (my layers were applied within a day of each other).  The fiberglass part of the project took me 2 months, and the finished product was actually better than new.  There is higher quality materials (Okumi plywood, MAS resin), and significantly better craftmanship than what came from the manufacture.  I will be able to keep this boat for another 20 years.  I am extremely proud of how my boat turned out.