Hello jserb:

I have used fiberglass extensively in my real J-24 sailboat and my Midwest Coast Guard model. There are two critical things.
1. Fine grade of fiberglass cloth.
2. West Marine Epoxy resin. You can get it from Home Depot. Do not use Bondo or Polyester resin, because they will absorb water and will ruin your wood bottom.

If you suspect voids inside the coated fiberglass after sanding, it is better to rough it up and recoat with another layer of epoxy. Use wet sand to start with 320 grit and all the way to 600 grit. This way you will not oversand the remove too much resin.

Over smoothing the bottom will actually slow down your boat speed due to excessive surface tension. A 600 grit finish will give you a surface like the shark skin that reduces friction.

Your epoxy fiberglass is the most important protection against water penetration (or Osmosis). Do it right and will save you tons of repair work later.

Enjoy

IDM