The less resin you use, the less likely the fiberglass is to shift after you've got it on. Excess resin lests the cloth slide, shift, sag....

You can get a finish similar to fiberglassing, but lighter by using silk and yellow glue mixed 50-50 with water. Its cheaper, easier to prevent wrinkles and sags and... its easier to correct sags and wrinkles if they do occur. Since the TF kit doesn't need the strength added by glassing (and the silk adds almost as much strength anyway...) Its an option you should consider.

The filler for sanding... mix some corn starch into yellow glue, or Elmer's "Weather Tite Wood Glue" (which is a gel of yelow glue and finely ground wood fibers, and is sandable.)

www.thaisilk.com has good prices (it was well under $3 a yard {48 or 54 inch wide, not square} for "10 mm Habotai" {which is the right stuff} when I last bought some)