On Monday, 13 March 2017 21:11:42 UTC+11, Per Carlin wrote:
Has never heard of leaking integral tanks. They are leaking in the valves and/or fittings, not in the tank itself.

Sorry, I haven't done this myself (owned a 3 and used to maintain the club's 2). I have heard stories of washing the inside if there is porosity but it makes little sense - as Per says, more likely a fitting (or even more likely poor sealing of the outlets and dump valve for a pressure test). Plus anecdotally the washing process was not highly successful.
Has the current owner found water somewhere or has it failed the leakdown test?
It may be possible to hear a leak with a stethoscope if it is a crack somewhere (though would be very difficult given the low pressure specified in the manual for testing).
There is a vent tube that runs from the tip to the root that would be a candidate for issues (was suspected to be partially blocked in mine). Stating the bleeding obvious here but you would want to be really sure where the leak was before cutting holes!

Cheers Ben