Hm, are you talking about something like this? One of the varieties of body filler that has fiberglass strands in it?
That stuff is twice as strong as normal body filler but it is very, very heavy and still liable to crack. It's not really practical for the inside of armor. Now, the bondo without the strands is very good for smoothing the exterior if applied in moderation
Regardless, just to clarify, this is the stuff ya want
Fiberglass cloth + this resin/hardener combo will still be your best bet for time, effort, weight and durability. I was surprised by how easy it is to do. It's just a little messy to clean up after.