: I read in a book or magazine article (sorry, don't remember which one) that
: the fill coats do very little in the way of strength - they are primarily
: to fill the weave of the cloth to give a smooth finish. Without fill
: coats, you will not be able to sand the epoxy without cutting into the
: glass. And that will seriously weaken the boat. The book recommended not
: doing fill coats on the inside of the hull and deck to save weight,
: material, and time. On the inside, a real smooth surface isn't required,
: and you won't be sanding there.

The fill coat is not "structural" in that more epoxy does not make the boat less likely to break when it impacts something. It does provide some protection of the bottom when you end up riding over something abrasive. How much you "need" depends on how often you encounter abrasion situations. You certainly don't want to start abrading the fiberglass before you even put the boat in the water, so you don't want to sand into material that is needed for strength.

However, some people have been known to apply a layer of light cloth as the outer layer with the express intention of sanding into it. While the sanding does weaken the layup, if you add the cloth over and above what is "needed" for strength you will end up with a more abrasion resistant layer because fiberglass and epoxy is more abrasion resistant than epoxy only. Even if you break all the fibers in this layer, it is probably still stronger than a layer of epoxy with no glass in it.

The problem with this is you may be adding more thickness than necessary for abrasion resistance and it the sanded glass will never be quite as clear as the unsanded glass.

You can analyze this sort of thing to death, finding one layup system being slightly better than another, but in the end, all you will get is fractional improvements.