Not sure if the following helps in your case since you mentioned the comptip.
Here in the UK the H16 mast are all Aluminium from top to bottom.
However I had the strangest situation with regards to water in the mast.
Last season I had my first and only capsize, on holiday in France, fortunately I had a float on the end of the mast, well it took three of us to right the boat. Myself and two nice kayakers that came to my rescue, I thought hang-on thats not right.
Two days later I stripped the boat down ready to transport back to Blighty and on de-masting I noticed a swooshing about with in the mast.
The thing is no water would come out, in the end I drilled a pilot hole at the bottom so as to drain it, it took ages to drain so it must have been pretty full and that would explain the problem with righting the boat.
Two weeks ago I thought I would sort it out, so laid the mast under water let go and it kept floating, dunking under, no bubbles nothing.
No leak was detected, I was like how can this be?
The only assumption I have made is that the mast only leaks when under tension, when the mainsail is rigged, what the heck?
So I decided to pop the rivets holding the masthead, silkaflex'd around the rubber plug (that had deteriorated over the years and replace the rivets of the halyard swage plate. But before doing so I gave it a blast with some expanding foam through the lower rivet hole of the halyard swage plate in the hope it would expand and seal around 12 inches of the top of the mast.
Re-attached the halyard swage plate with coating of sikaflex sealant and re-riveted with SS rivets and the same with mast head.
The mast still floats have yet to test under tension.
Will let you know how it goes.
Hobie 14 Turbo (~1979)
Hobie 16 Carumba (1983)