Hmmm the pros/cans of rondo and fiberglass. That's a good question, now if I miss anything anyone feel free to add it in.
Fiberglass
Pros
It can be strengthened using other materials (Kevlar, carbon fiber, window screening etc)
If you use the mat fiberglass you can shred it to get into tight places
Cons
Uses a lot of brushes
Uses a lot of resin
you need to have on hand at least 2 packs of fiberglass
Takes awhile to set
Have to do multiple layers for any real strength
Messy
Rondo
Pros
Easy to use
sets in a short amount of time
You only need one or two brushes
gets in all the small spaces
easier to make repairs
Cons
Uses bondo up faster
May not be as strong as fiberglass
Sets quickly
Fiberglass
Pros
It can be strengthened using other materials (Kevlar, carbon fiber, window screening etc)
If you use the mat fiberglass you can shred it to get into tight places
Cons
Uses a lot of brushes
Uses a lot of resin
you need to have on hand at least 2 packs of fiberglass
Takes awhile to set
Have to do multiple layers for any real strength
Messy
Rondo
Pros
Easy to use
sets in a short amount of time
You only need one or two brushes
gets in all the small spaces
easier to make repairs
Cons
Uses bondo up faster
May not be as strong as fiberglass
Sets quickly