No one else has answered, so I guess I will.

The situation matters. I don't think that anyone's claimed that it makes the boat abrasion-proof, certainly not sandpaper-proof. Sandpaper is specifically engineered to abrade. It's usually made from some of the hardest materials known, short of diamond. There's almost nothing that will not be abraded to some extent by sandpaper.

But if the abrading agent is bushes, branches, waterlogged logs, actual sand, rounded river rocks and other hard natural substances, epoxy (which when fully cured is about as hard as a formica counter) has a pretty good chance of resisting.

As you say, adding a layer of glass, with the weave filled with epoxy, provides a thicker and more consistent layer of epoxy. There may even be some kind of an effect where the tensile strength of the epoxy-filled fibers can stop a sharp point from sliding across the panel and therefore reduce the amount of abrasion. It'd be a fun experiment for someone to see if that actually occurs.

But on the whole, I think that it mostly inproves the thickness and consistency of the coating and makes it more resistant to abrasion from most natural substances of the sort we tend to encounter when boating (except oyster shells).

Laszlo

Perhaps it is 6 of one and half a dozen of the other.

Perhaps it is the reason they stopped doing the decks of the C-17s with just epoxy and now recommend a layer of coth. 

I can verify that it is easier to scratch to the wood without the cloth.    

I agree with Lazlo on glass cloth. It has, however, been my experience that Dynel fabric, instead of glass cloth, does add abrasion resistance to the coating. However, when it comes to prolonged contact with barnacle encrusted rocks, epoxy empregnated cloth, of any type, will come out the loser.

Cheers,

Dick

Thanks, everyone, for your thoughts. Epoxy is a wonderful glue/coating but of course it has its limits. It's good to have the benefit of other people's experience while I figure out where those are.

Laszlo, I remember you used a carbon additive to give more abrasion resistance to the hull of your Wood Duck. Do you feel it made a significant difference?

Cameron 

   Whoops, I just came across the answer to that question a few posts down. It was not carbon but graphite additive.

Cameron