Fiberglass Matt / Cloth to use for structural bulkhead rebuild?

[er9]

I was taught that the reason for using matt has three purposes.

first...If you are laying up a laminate over gel coat working from the outside/in...the matt (thin angelhair) helps prevent the texture of the woven fiberglass from showing through the gelcoat.

the second was to build up thickness of a laminate quickly and cheaply.

the third was that a final layer of matt over the laminate of a deck for example, helped create a barrier against water intrusion into the woven layers.

[Singularity]

FWIW my understanding in blue.

Originally Posted by er9

I was taught that the reason for using matt has three purposes.

first...If you are laying up a laminate over gel coat working from the outside/in...the matt (thin angelhair) helps prevent the texture of the woven fiberglass from showing through the gelcoat.

I have no experience with this though it makes sense in new construction on decks/external hulls.

the second was to build up thickness of a laminate quickly and cheaply.

This is my understanding of what is the essence of 1960s-era fiberglass boat construction. A) people didn't know better at the time than "it needs to be as thick as wood" (or something), B) chemicals +/- glass was relatively less expensive back then, C) quicker to lay up sloppy.

the third was that a final layer of matt over the laminate of a deck for example, helped create a barrier against water intrusion into the woven layers.

On the one hand this sort of doesn't make sense as cured resin doesn't let water in, and appropriately saturated glass-resin doesn't like water anyway. But on the other hand, for intrusive processes like osmotic blisters or micro-cracks (in freezing climates) allowing water to enter creating freeze-thaw expansion...having a big fat layers of mat might keep intrusive processing closer to the surface. But when it comes to stuff like bulkheads not subjected to getting superficial fractures (like something banging on the hull or deck) and not subjected to immersion/freezing...I just don't see how mat has any utility apart from it's use as sort of semi-structural spackling or something.