from my little experience....
 

I made some cowling for my scrathbuild project I have made and this is what I do:
The first layer are 1 oz cloth. You can have a very good finish with a low amount of resin. The second cloth are 5 oz I think. This layer add the strenght and I usually overlap the cloth to have in fact 2 layers. This procedure are probably too heavy for you but I think the idea can work. Sometime, I put only narrow band of cloth on specific aera where I want more strenght.
You can look to my website, I have some pics of my fiberglass parts.