The inside of my fibreglass tank has minor imperfections in surface flatness so when i fitted my bulkhead i knew i'd need to extra tighten it to give me a proper seal, which isn't always a good thing long term when the threads are plastic. They can fail overtime when overtightened.
So i just hand tightened it as you usually would, with an extra quarter turn, but then added a bead of silicon around the rim where the bulkhead pressed against the fibreglass.
Not as much as a single drip has leaked out.