I purchased a used boat from a rental fleet. It is in generally good shape, however, I have not yet flipped it, and would like to make sure the mast does not take on water. I sail with my two kids, and, thankfully, we get some strong winds on the lake. This weekend I want to pull the boat on its side and check the tip, and if needed, seal it up. I want to then take the boat out and flip it to see if I still have those righting skills (I was very proficient at righting my cat in my youth). I am not sure what sealant to use, nor if just dunking in water would show leaks (like a bubble test on a tire).
I'd recommend removing your stays from the adjusters and then carry your mast to the water. Wade in a shallow area with the mast and look for bubbles escaping. Check particularly around rivets and where the comptip meets the aluminum. No bubbles, you're golden. If you do find an area that leaks, buy a tube of marine grade silicon and force it into any leaking area. If it's around the comptip, make sure you get any silicone out of the sail track (use a paper towel). Let the silicon dry for at least 24 hours, put the mast back on the boat, and go get wet!!
I'm not a big fan personally of the "watch for bubbles" method, I did this once with Hobie 18 mast that I suspected wasn't sealed and ended up with water inside the mast that I couldn't get out. On Hobie masts there are thick foam plugs inside the mast at the hounds, comptip joint, and base.
I ended up having to take off the mast base and tear out the foam plugs to finally get the water out. Also it's a lot harder to force an entire mast under water than it sounds.
Make sure there is no water in the mast right now, then seal every joint and rivet penetration with with outdoor silicone (redo every year) or with something like 3M 4200 http://www.westmarine.com…ealant--P004_127_002_511 which will last a lot longer.