It depends where you're sanding and what kind of threads you are referring to. I used a combination of Dremeling and a sander.
Certain sanding can be done by hand.
The kind of fiberglass I was dealing with may not be the same as what you're dealing with. Mine were short and very course, and simple manual sanding would have taken days. A hand sander took minutes.
This may be helpful. I will be adding more content to the article:
http://www.aokforums.com/thepropden/thepropden-about567.html
If you can post a picture of what area you're dealing with and the type of fiberglass it's made of, perhaps we can advise better.