As SD mentions this a hard subject and questioned often (Not just here).
A Marine Surveyor may be a good source. I am not an expert of wood or fiberglass. I know enough of both to fix and maintain to have a good boat. I try to explain how I question fiberglass.
I have worked on boats and have some confidence in old fiberglass (With reservations).
Looking for another sailboat, I too have looked at older boats. We know that Wood has proven itself as a boat building material with Centuries of examples under its belt. Fiberglass on the other hand is young and has not had the years to prove itself for longevity.
I tell friends, I feel I could buy a fiberglass sailboat from the 60's and it last the rest of my life.
Wood boats; I see boats 100+ years old that are in very good condition, because they were cared for properly. I also see wood boats from the 70's that are no more than fire wood now because of care.
Fiberglass boats; I read of some from the late 60's and early 70's and they mention soft spots on the deck. I question; "Where"? Are these around Sanctions etc, where screws/bolt holes allow water to penetrate to the core? Are the soft spots on the deck where water should not have a way in?
I see care of fiberglass may not be as stringent as wood. But, fiberglass does seem to show a need for some amount of care. Gelcoat is flexible to some extent and over time it may stiffen and crack. What I think; this allows for water to begin its work of erosion. Over time depending on weather (Freezing, extreme heat). Once the Gelcoat loses integrity it like a war. The first line of defense has failed and now you retreat to the next line which is the fiberglass itself.
If, the fiberglass fails there may be many reasons. Was sufficient epoxy used to saturate the cloth/fibers? How was the epoxy mixed? Too much hardener or not enough? Does the amount of hardener matter? Etc.
Boat owner forums can help about a particular manufacture. Also, seeing how many of an old boat that still around and being used may help (Walking around docks looking and talking with people).
What I look for first on fiberglass boats is the Gelcoat as that what we can see. Then for myself I tap around on the deck and hull (Sometimes I just use a Bolt Washer) for soft spots that may not show walking around on the area. Does it look like a road map in places? Why and how deep does this go? I know I can take cracked gelcoat and with proper waxing/buffing a person not see the cracks, but, sometimes they still there!
I guess I have found boats from the 60's I feel be a good buy and last the rest of my life. I have also found some I not too sure of. The condition of what I can see has a big bearing and then I test my assumption by what I can not see (Washer tapping, trying to see the Fiberglass from the inside best I can, etc.).
Everyone may have a different opinion and ways of checking. Which for me, it gives more ideas for making a better buying decision. I just give a little of how I know/do and not say it the best.
But, fiberglass not been around long enough to be fact, it more in Theory to me. As I said earlier I have seen both wood and fiberglass still going strong with care.
I have also seen both sent to the scrape yard.
