Yep, somebody tried to resin-coat it without the fiberglass reinforcement that the resin demands for strength and it did not work and never will. Judging by the water stains around some of the cracks, I suspect that it isn't even epoxy resin, but polyester resin. Unfortunately, that will generally mean that the resin is even more brittle than epoxy and less well bonded to the wood. I really doubt that you have enough coating thickness to be opening the cracks up to fill them with epoxy and do any real good. Even if you did, the root problem is that the existing coating is neither strong or stable and it is just going to continue to crack elsewhere.
That would seem to leave the options of either removing the existing resin and starting over (a nasty and very tedious job) or adding a better skin on top of what is there, be it a layer of fiberglass, or maybe even something like heat-shrink aircraft Dacron to keep the weight down.