I worked with composite materials for over 20 years. I've never built fenders specifically but I can give you some advise about fiberglass in general. Plus: Lightweight, doesn't rust, resistant to chemicals, resists light impacts that would dent metal. Minus: Expensive to repair when damaged, total failure when damaged, in some cases the appearance is different because of the thickness of the material. If there's a specific issue you're concerned about e-mail or PM me.
