Those voids look all too familiar. The ones that didn't open with the 4 1/2" angle grinder got punctured with a dremel tool with a burr bit. The toerail cavity was attacked with a 4 or 5 inch diameter wire wheel that I mounted on a 6 inch length of all-thread and chucked the mighty power drill. That pretty much chewed out any unsaturated glass in the cavity. Before I taped the seam each of the voids were dabbed with straight epoxy and while still in a green state filled with thickened epoxy. Then the whole seam got another pass with the grinder with much less aggressive grit before washing. That helped the tape lay flat (as possible) to avoid replacing the original voids with more voids of my making. Hope that makes sense and helps you out a bit.