It may be, and probably is, osmosis.
What ever you do, get opinions from someone OTHER than a boatyard fiberglass
They make a fortune in scaring the daylights out of boat owners who then part with lots of money
, all to no avail. I have then seen the problem, re occur. Repairers will NEVER put a guarantee on there work repairing osmosis. WHY?
I have seen osmosis on one side of a hull and yet no sign of it on the other. Sailing in warm waters does seem to bring the "problem" out sooner.
Osmosis is very common in ALL fiberglass
boats as well as fiberglass swimming pools and spas, but it is cosmetic only. The boat will NEVER leak because of osmosis.
It does NOT lead to the boat loosing structural strength, EVER.
It will reduce the resale value of your boat, so fix it just before you sell the boat, and not before.
It is generally caused by having too much stryene in the fiberglass layup
. (which is impossible to control)
It is impossible to build a boat in fiberglass without the possibility of getting osmosis at some stage. Vinylester resin is not the answer either.
I compare osmosis to have fine cracks occur in concrete paving, yes it does occur, can you do anything about it, NO.
I know, I have built many hundreds of fiberglass boats swimming pools and spas.
Osmosis does not occur with epoxy
resin only Polyester resin.