RE: Fuel proofing and hinges
The bottom line is...
If you're only using it for fuel-proofing, it really doesn't matter if there is water in it.
I have used 70% Isopropyl many times to thin epoxy for fuel-proofing and there is absolutely no problem doing it.
It DOES make the epoxy a little weaker, but you don't need strength for fuel-proofing.