I purchased a camper shell last year, I'm not sure what brand it is. Snug top made one, Just know that style of shell was popular back in the 80's and 90's with the mini trucks. It looks like fiberglass mat inside the shell but where I sanded it looks like pure resin. I also sanded to bare plastic due to a lot stress cracks on certain areas and very thick paint. I used a angle grinder with 60 grit and now I have couple low spot where I grinded the paint, How can I fix that? Ill post up some pictures of type of shell on a truck and my shell.

It's probably just an old style "open layup" piece which is just regular fiberglas. Tons of these camper shell makers popped up back then with simple one piece molds. What looked like resin was probably just the clear gel coat. Gel coat was blown into the mold first, then the rest of the mat, cloth, etc. Bad news here is the use of an angle grinder. You made more work for yourself. Cracks are always best opened up in a V shape with a Dremel tool. Sanding for paint removal should be done with a DA and even at that you have to keep it very level so it does not "dig in" to the surface. I never drop below 80 grit for removal. To fix your low spots you can just any Bondo type filler that is rated for use on fiberglas. Evevrcoat's or Marson's line of fillers is fine. If they aren't to low you could even use just a poly putty like USC's Body Icing.

Okay, good to know its fiberglass. Once I remove all the paint to bare glass can I add another layer of fiberglass to make it stronger and add filler to smooth it out? I have some fiberglass mat but just found out its for poly resin only. Also if I sand down to bare glass and live it alone is bad with out primer or sealer? Sorry for so many ?'s im new to this.

Yes, you can take fiberglas resin and add that to finely chopped up mat (pull it apart and chop it up with scissors, strands should be about 1/2 inch long) and apply a couple of coats that way. Just sand it level and your filler should lock on to that for final leveling. Bare fiberglas must be sealed, preferably with an epoxy primer, then your paint system.

I would like to run one layer of mat all over my shell, I have 3 yards of mat that weighs 3/4 oz and only recommends poly and vinyl ester resin. I would like to use epoxy since supposedly is flexible and stronger, is there a better mat that I can use with epoxy?

On large surface layups like you want to do you are better off doing "like" materials rather than using epoxy resin. Although I like epoxy resins for repair work on attachment points, surfacing, and such on fiberglas I prefer to do laminating work with the same type of material that is already present. Regular fiberglas will be plenty strong enough, so you just need to pick-up some fiberglas resin.