I have sailed in and restored several fibreglass (GRP) yachts from small 18 footers to a 43 footer. The age and time in the sun can cause damage as you said to the gelcoat
or coating but rarely have I seen substantial damage or weakening of the structure.
There are two items in the construction you need to understand. The glass or fibreglass which is relatively inert and will not break down because of UV or salt water
contact. Then the resin which bonds it all together.
Resins vary in their ability to withstand both salt water
and UV. Polyester resins used by most production boats manufacturers vary from Orthophthalic resins to Iso and Vinyl ester resins. Ortho resins were used in the 60's and 70's a lot because they were cheap
but they are susceptible to osmosis
Iso and Vinyl Ester resins are much better at withstanding osmosis
and UV attack.
ALmost impossible to know what a builder
has used with out expensive testing.
resins are used by one off builders and some race
yacht builders. High bond strength and used when glassing over plywood
. Some are susceptible to heat distortion in climates with high temperatures.
Best way to determine what condition the fibreglass is in on a boat is to visually inspect it and use a small ball peen hammer and tap the hull
litening for any hollow spots. This indicates delamination
or osmosis. A solid hull
should have a sharp solid sound.