One of the best ways to ensure that your mast is sealed is to simply put it in the water the next time you go sailing. Push it down a bit and if you see bubbles, note where they come from and seal those areas with silicon or 5200. Previous to this you can get a good cut at it by putting some sealant on and around all the rivets and joints on the mast. Not pretty, but then neither is trying to right a boat with a flooded mast!



Also, it is sometimes advantageous to drill a small hole (mine is near the base) and thread a screw into it. You can then remove the screw and the hole can act as a drain if water does get in somehow - put a dollop of sealant on the screw when you put it back in. It's amazing how, just from rain, you can pick up some water in a mast if there is a leak somewhere - being able to drain it means you won't start out at a disadvantage due the mast already having water in it.