RE: Prep for glass
220 is about as far as I ever go with balsa wood, especially if I'm glassing.
Then if I use epoxy resin, I'll give it a light going over with 100 just to get the bumps off. When it's primed, I hit that with 150 first time then 220 and 320 at the end. When I give it the first coat, I'll hit any runs that might have occurred with 320. After that I don't sand but I will use 000 steel wool to get any ridges off the markings and actually buff it up a little (I use satin finishes).
If I use polyester resin, I actually start with something like 60 grit, the polyester finish is harder than the epoxy. Give it a second (flow) coat and then use 60 to get the surface level finished off with 220. Then same as the epoxy for the primer/filler and paint coats.
Do yourself a favor and get a drafting brush. Use the drafting brush to smoothe out the glass cloth. It has just about the right "drag" to grip the cloth and move it a bit and it also develops a slight static change that heelps the cloth cling to the surface when you're laying on the resin.
Also, when you apply the resin, don't forget to squeegy off the excess and blot up any stubborn wet spots along the edges.
Dave Platt's Videos demonstrate this and it works quite well.