The totally anal retentive way to do it would be to remove all the hardware (including endcaps) and replace it one by one sealing all the rivets (including their centers if using standard rivets) with 3M 5200.

The more effecient way would be to go around everything on the mast with a smathering of 3M 5200 (it cleans up well with mineral spirits) without removing anything.

If you wish, you can lay the mast in water and push it under looking for bubbles to see where the leaks are and fix only those areas.