Just cos you have never heard of it....Models By Design work with Fibreglass products all the time. The product they have recommended has more of an industrial angle to its use, so wouldn't crop up much in hobby circles, but will still be a very good product.
As far as sanding this stuff down, I use a hobby drill with Permagrit sanding drums, usually the coarse ones. Not cheap, but works when you want it to. Once the worst is ground down, you can start on any finer sanding with aluminium oxide papers, starting at about 80 grade, then working down to more usual wet and dry papers. Maybe some filler over the top just to touch up any voids. To each his own I say, but try other people suggestions first. They are usually based on much knowledge gained from much experience.