The above is right that "Polyester fiberglass resin" is the first choice for automotive repair.
It is cheaper and requires less strength in the catalyst to kick it then the epoxy fiberglass resin.
Make no mistake about it there is a BIG difference as far as quality and strength between the epoxy fiberglass and the polyester fiberglass.
It really does not matter what one you use on an automobile
but try to GIVE a boat repair guy polyester he will never use it
as it will not hold up.