Posted for "Bill Howie" <bhowie@optidisc.com>:

 I was concerned about the same thing and to ease my mind I used the balloon
 test with Helium as that is a smaller molecule than air and certainly water.
 
 Bill Howie
 IVP with 15 hours
 

"""
     I'm getting ready to undercoat my wings and thought it might be a
 good idea to check for potential fuel leaks before painting.
  ...snip...
 I've done the balloon test and maintained pressure well
 for 24 hours.  I'm reluctant to just fill up with fuel only to find I
 have a leak and have to deal with surface contamination in the
 fiberglass.  Using water could be a potential approach assuming I can
 dry things out later on.  Any and all suggestions will be greatly
 appreciated.
"""