To add to your post, only a 'gel coat' can totally seal it.
We use fiberglass to make our motorcycle trike bodies and it takes a lot of Bondo, primer and endless sanding with 800 wet/dry paper to get smooth it enough for a final paint job.
Fiberglass is, after all, made of tiny little sharp needles of fiber.
I once washed Himself's work clothes in the household washing machine by mistake and my clothes and all my linens were wrecked.
One of my dogs even got keratitis in both of her eyes because of the 'invisible' stray fibers embedded in the "clean" blankets on the sofa.