Why barrier coat?

[CaptainRivet]

I restored my hulls after sandblasting to fiberglass with vinylester supervised by an fiberglass expert (over 3 generations), Eduardo Tous from Gran Canary.
He repaired inside (and I outside) the damage to a level better then new and outstanding quality due to my surveyor who hadn‘t seen such a quality in last 10years.

He hast best experience with Hempel Majestic Thick epoxy primer and using that since it exists. Due to thickness it’s sealing, barrier and protection in one and supereasy to apply, you only need 2 layers wet in wet (here 6-10h) dry time. I used it too and was cheaper then the other typically used one…a 20l tin was 138Euro over him. Needed 25l for 2 hulls of a 40ft cat.
If just hairline cracks without any structural issues I would just sand away the old antifouling, clean with >95% alcohol (aceton contains oil, maybe a reason too that aceton residues collect in cracks and epoxy doesn’t like that) and apply the Hempel thick primer, it will seal the hairline cracks in the first layer and 2nd will even it out.

But be careful the hairline cracks could be water pressure too that’s trapped in the hull and due to heat expanding and cracking the paint. Jedi explained with a foil to tape over to find out if water is there.
Or another issue could be they were there before and cracks filled with eg old antifouling where nothing stick to it so after drying completely through the paint in the cracks could not hold and ripped.