Typically none. In the vast majority of cases, D-G powered rockets do not need any fin reinforcement.
If it is needed, a single layer of 0.6-0.75oz would be perfectly adequate. Why do you want to use fiberglass for a D-G powered rocket though?
After all of the discussion about fiberglassing on another thread in the last couple of days, I realized that one of my dormant projects could probably benefit from tip-to-tip fiberglassing of the fins, and it is in that range, at least partly. It's more in the F to H (possibly I), range. I was building an upscaled Estes Super Flea that was based on 2.34" diameter tubing. If you don't quite recall what the Super Flea looked like, see
here. Note the long swept fins. For my upscale, I made them out of 1/4" basswood, which worked fine and wasn't at all heavy. Because I was also making the rocket with an interchangeable motor mount, the fins were surface-mounted.
Yeah, I know.
I thought that bonding them with Aeropoxy and giving them really good fillets would be sufficient, but I found out otherwise when I accidentally backed into the partially-completed model (I had it standing on the floor behind me, along with a whole bunch of other stuff) with my desk chair in the workshop awhile ago. Two fins broke off, taking chunks of the airframe with them. Sure enough, the bond was much stronger than the parts that were bonded, but that wasn't good enough in this case. A tip-to-tip veil of lightweight FG would seem to be indicated here, even though this is basically a mid-powered rocket. I plan on starting the build over from scratch with new components, because there are a few additional things that I want to execute better on it this time around as well. It will be my first experience with fiberglassing a fin can, but I think I know what I will need to do. Just in case, though, any tips?
Mark K.