RE: How to Strengthen Fiberglass Seam in a Tight Enclosed Area?
I've built some really narrow fuses with a method that used a molded joggle (basically an offset, molded-in step). What we did was to lay up the mold as usual and once we had the two halves finished, we'd go back and insert the plug in one half. Wax up the whole assembly like you were going to mold up another half. You then lay up on the mold flange and slightly up the side of the plug (1/2" is fine). Do that to either the top or bottom (lets say bottom for this example). You'll repeat this process with the plug in the other mold half and lay up on the "other" seam side (top). Take the finished pieces and sand the top of the flange to 1/4-3/8" and this will be your "step". These mold "steps" will be used when laying up a new fuse, one bolted to opposite mold halves. You need to locate these mold steps with an offset to the fuse mold to allow for the fiberglass overlap (measure cloth stack up ie 3 layers of 4oz glass= .025 or something like that). Using clamps locate the steps and drill locating holes through both step mold and fuse mold flange. It's best to use the existing fuse mold locating holes/pins if possible.
When you go to lay up a fuselage, these steps are bolted on and are treated as an extension of the wall of the mold so wax the heck out of them. You will need some micro-balloon mix in the sharp crevis created by the offset and lay up the glass normally. Trim like normal when cured and pull off the step molds (be careful not to pull the glass off the wall of the fuse mold to create a potential spot for joining micro-b. filler to squish into). Sand the molded step rough to create good mechanical bond (mask off fuse mold flange so you don't scratch it). Lay up the halves with your micro-balloon joining filler around the edges and on flange and close mold.
This is not as ideal as a wet layup seam but it works when you have no choice but to close the mold without any way to press the seams.