I have to build up an area where the the rear quartres have been replaced near the trunk. Can I put everglass short strand fiberglass body filler over the epoxy primer or do I have to take it to metal? Or should I just scuff it a little and lay the fiberglass body filler over it? What thickness is safe to build up with this fiberglass reinforced body filler? Thanks

Fiberglass is not the way to go, it will fall out or deep crack over time. The best way to fill & level the seam of the patch panel would be to have the metal work as level as possible, then etch prime the area and top prime with epoxy or 2K primer. Prep area then use a spray poly, it will and can fill up to 1/4 deep, that way you have enough poly film build to block (150grit to 220) down to get the arrow straight level your looking for. You will have to 2K prime (320 or 220 prep) over the poly as it tends to be very pourious. This will allow you to feather edge the poly edge also. I've used this system for years and it is the quickest way and best way to get any seam or large area straight. Point about spray poly is was made for hail damage & large area fill primeing, and can be used on the whole car but plan on many hours of blocking, But! The end result will be show car arrow straight and a very soild base to prime over. And will last for years won't crack shrink up or any of the other nasty things bondo and fiber glass tend to do.

If your going to re-do it do it right the first time. Straight Metal needs no Bondo work it straight first.
"Lost Art of metal workers"

Who makes Poly primers? Does PPG have one?

All Top shelf paint lines have a high build poly spray filler primer, But remember you will need at least a 2.0mm fluid tip to spray this high of a build filler. Most call it bodyman in a can, I've posted several items on this stuff and I've used it for years and by far it is the best stuff to use if your looking for that arrow straight body. But it all comes at the cost of many hours of blocking to get it that straight, it allows you to have the film build to block down to acheive the ultra straight body. It is very easy to use but remember to wear a mask even while sanding this stuff. Read all the MSDS on it and even have them at the store explain to you how to use it, as well as what PPE to wear.