I just put the backing on a short kids bow using automotive repair fiberglass and some long cure kinda thin epoxy that came from a local hobby store. This is a very light weight bow so it would not compare to an adult draw weight, but it came out pretty nice and like said above, you can see the wood grain through it.
If I do it again I'll use a boat builders grade of fiberglass mat and a better epoxy.
In the late '90's I built a bow for a guy who specifically wanted it built with West Systems epoxy. It was a simple straight longbow from a Bingham kit with a draw weight of about 50#. I used it to shoot at a local indoor get together before sending it off to him and it seemed to shoot nicely. About a month after he got the bow he emailed me and said "don't make any more bows with it, it broke while my nephew was stringing it". Not sure if the break was due to the West systems epoxy or his Nephews stringing style!
More importantly, the West Systems stuff had a very short pot life. By the time I got it in the oven (and I rushed the lay up job) the epoxy remaining in the cup was very hot and way beyond being useable. The short pot life itself is enough to make it incompatible with bow building. The oven may not have been neccessary to the cure process but I used it anyway "cause that's the way it's done" :D